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Discussion Starter · #1 ·
Reading the streams on this and other forums, it seems that about the only two things we know for sure about the BOSCH VP44 is that they cost a bunch of money and, if you don’t run a FASS Titanium, they’re gonna burn up. For some folks, it seems that is enough; maybe it is. The VP44 sticky on this forum gives a lot of information about what the VP44 does, but precious little on how it does it. Several years ago, I decided there must be more to them than this.

Here are my conclusions after studying the Bosch VP44 Service Manual (Rev 2000), the Bosch Distributor Type Diesel Fuel Injection Pumps publication (Issue 2003), the 2001 Dodge Ram Service Manual, and a VP44 Video CD that I bought on-line several years ago. I have also carefully read and filtered information from a number of forums. Information taken from Mopar1973man’s actual tear down of a VP44 and his accompanying photos and discussions were very helpful. There is an outstanding, detailed description of VP44 internal operation on YouTube. Unfortunately, the audio is in Spanish, but it pretty well backs up my description of its operation.

Although the VP44 is a precision device manufactured to extremely close tolerances, it’s not a magic box and the basic mechanics that allow it to accomplish its job are easily understood; it’s the control system that makes it more like rocket science.

There is a lot more detail to the operation of VP44 than I cover here, but here are the basics:



The Bosch VP44 is an electronically actuated, solenoid needle valve controlled radial piston Distributor Fuel-Injection pump. Basically it consists of:

1. Low Pressure Stage: Internal feed pump (vane pump), Pressure Regulator, Accumulation Chamber, and Overflow Valve.

2. High Pressure Stage: Radial Piston (plunger) High Pressure Pump, Distributor Shaft, Solenoid Valve, and Delivery Valve.

3. Timing Control: Angular encoder, Timing Ring, Timing Piston and Timing Control Valve (TCV)

4. Electronic Control Unit (also called the PCM (Pump Control Module)). This unit controls the volume and timing of each fuel injection by the VP44

If treated right, it will last for a long time. I bought my truck new in the autumn of 2002 and it is going on 187000 miles and 13 years; others have gone a lot more than that.

Here we go:



The High-Pressure Pump:
The rotating parts of the VP44 are gear driven at camshaft speed (1/2 engine crankshaft speed). It is mechanically ‘fused’ so that anything that locks up the distribution head will not destroy the drive gears. The VP44 uses a 3 roller (cam followers)/6 lobe cam ring (one lobe for each cylinder) cam-driven radial piston high- pressure pump to compress fuel to a pressure sufficient to open a fuel injector. All three rollers (arranged 120 degrees apart radially) are cammed in at the same time, compressing fuel by use of cam followers and pistons (Plungers) in a central compression chamber (Plunger Chamber). This pump can raise pressure to as high as 1500 bar (21,750 PSI). The fuel is distributed to each cylinder in the desired sequence by the rotation of a shaft in the distributor head. This distributor shaft contains passages that determine where the fuel comes and goes in the distributer head and which cylinder will be fueled. The sequence of the VP44 outlet is consecutive in a rotating manner (A-B-C-D-E-F); the sequence of the cylinders that the fuel goes to is determined by the piping arrangement from the VP44 output (1–5–3–6–2–4). One revolution of the distributor shaft delivers fuel to each of the six injectors (two revolutions of the engine crankshaft (four stroke engine)). The rollers run inside a movable ring containing the lobes and whose angular position is controlled by the timing plunger (piston). The ring is moved angularly to advance or retard the timing, depending on the desired engine operation (fully retarded for starting and advancing as needed to provide desired timing for proper engine operation).

The angular position of the ring (and thus the timing) is also affected somewhat by the feed pressure to the VP because the feed pump (lift pump) pressure connects to one side of the timing plunger and timing solenoid that control the position of the cam ring. Because of the low RPM during cranking, it is desirable to have the timing fully retarded (thus low feed pressure). This is why a properly controlled feed pump runs for a second or so to prime at key on, shuts off, and then runs at reduced (25 percent) pressure during cranking, finally going to full pressure following startup. It’s also why some folks with higher pressure relay-controlled aftermarket fuel pumps sometimes experience hard starting when the engine is hot. I believe this unintended consequence occurs because the relay which controls the aftermarket pump cannot respond to the ECM method of command to reduce pump pressure and the pump stays fully energized, thereby keeping fuel pressure up and not allowing the timing to properly retard. By design, the ECM pulses the voltage to the fuel pump during cranking rather than supplying a solid 12 volts. I think the OEM pump can see this as a reduced voltage, but the relay associated with the aftermarket pump doesn’t. Some folks believe that, even with reduced pressure, the aftermarket pumps produce too much pressure during cranking. Take your pick (maybe it’s both).

But I digress.

Low Pressure System:
Now that we have seen how the high pressure pump works; all we have to do is get fuel to the high pressure compression chamber (Plunger Chamber) to be pressurized by the roller cam followers and plungers.

Although the VP44 has an internal feed pump, in the case of our Dodge Ram/Cummins Diesel, fuel is supplied to the VP44 input by an external feed pump (lift pump). There will always be an argument on how much lift pump pressure you must have for proper VP44 operation. The Bosch manual states “Cars, light/medium duty trucks and tractors, where the fuel tank is near the VP, may not need any more flow than the internal fuel pump supplies. Equipment where the fuel tank is a longer distance from the VP may need an external feed pump. Stationary equipment and tractors may have a head tank supplying pressure.” I really don’t want to get back into this argument; just stating what Bosch says (I’ve got to believe they know something about it). My thoughts are that the Dodge Ram/Cummins, because of its sometimes unique uses and high horsepower requirements, may throw additional variables into the equation and thus require added input pressure to ensure the required volume is available. For instance, on a 45 degree incline, the internal Feed Pump would have to overcome about 4 PSI because of the location of the fuel tank and the weight of the fuel in the lines. I’m sure there are other variables also, hence the Ram Service Manual requirements for a minimum of 10 PSI lift pump pressure (minimum of 7 PSI while cranking (although one manual states maximum fuel pressure of 7 PSI while cranking—which, because of the effect on timing, really makes more sense)).



Onward:
The first thing the incoming fuel sees in the VP44 is a vane-type fuel pump (called by Bosch the ‘Internal Fuel Pump’) which raises the pressure to “8 – 22 bar (116-319 PSI) depending on the application and RPM” (direct quote from VP44 Service Manual and Bosch Fuel Injection Pump Manual). A pressure control valve at the output of the vane pump bypasses fuel back to its input to control the desired vane pump output fuel pressure and varies depending on RPM. This is a pure spring-loaded mechanical devise; it only depends on vane pump pressure, it doesn’t depend on the VP input pressure nor any electrical signal. The amount the pressure control valve opens depends on how much pressure the vane pump is producing. The vane pump is a positive-displacement pump; therefor its output flow increases pressure as RPM increases. From here on, as long as there is a sufficient volume of fuel available at the VP44 input, it doesn’t matter how much pressure the external feed pump (lift pump) is producing, the VP44’s internal pressure will be controlled at design pressure by the vane pump and the pressure regulator. Since the vane pump output pressure increases with RPM, the amount of fuel bypassed varies to allow a desired, controlled pressure which is dependent on engine RPM (the higher the RPM, the higher the pressure). This higher pressure is necessary in a radial piston solenoid valve controlled injection pump (VP44) because the same cavity of the high pressure pump (the Plunger Chamber) must be filled to a design pressure for each injection, unlike the in-line injection pump which fills each of six cavities in sequence. The higher the RPM, the less time there is to fill the Plunger Chamber, therefore the higher the pressure must be to assure proper filling of it prior to the next injection.



The vane pump output is fed to an Accumulation Chamber which is isolated from the VP44 inlet pressure by a diaphragm. The purpose of the diaphragm is to absorb (or dampen) high pressure fuel spill pulses (we’ll talk about that later). Some folks say this diaphragm doesn’t exist; I say it does; so does Bosch. Mopar1973man’s tear down of a VP44 doesn’t show a diaphragm looking exactly like we thought it should be, but does show a similar steel-backed plastic-like unit located in exactly the same position as Bosch drawings show the diaphragm. I say this is the diaphragm; so does Bosch. It performs the same duty as the old style diaphragm (else, why the plastic-like material). I do say that, from the way it is now constructed, it isn’t likely to rupture (as the original designed diaphragms were prone to do).

There has been a lot of confusion about this diaphragm. The VP44 has been around only since 1996; Dodge started using them two years later. It appears that, in the early VP44s, this diaphragm was under-engineered and prone to rupture and was blamed for many failures of the VP44 (mostly galling of the timing piston and liner by feeding pulsating pressure back to the piston chamber whose liner was also under-engineered (wrong material)). Later revisions changed its design, giving the diaphragm a solid steel backing which makes later VP44s much more dependable and has pretty much eliminated diaphragm failures. The diaphragm material, however, still has to absorb the energy from the high pressure fuel spill pulses (again, we’ll cover these later). The pressure spike it must absorb is initially very high (injection pressure), but the volume is very small and the diaphragm has lots of surface area. I’m not saying that the steel backing will flex; I am saying that the diaphragm material attached to it will still dampen the high pressure pulses. And it’s gotta do it very fast to get ready for the next event (less than 0.02 seconds at 2000 engine RPM). Later revisions to the VP44 also changed the lining material of the timing piston cylinder. These two major revisions plus the use of better solder in the ECU and closer attention to surface finishing are probably the main reasons newly re-manufactured VP44s are much more reliable than the early ones.



Let us continue:
The fuel in the accumulation chamber supplies fuel to the distributor shaft of the VP44. Inside this rotating distributor shaft is a cavity and passage to the high pressure pump Plunger Chamber. Each time a fuel injection event to the engine is desired the Plunger Chamber must be filled from the accumulation chamber and then isolated from it to allow the fuel to be pressurized. This filling of the Plunger Chamber and isolation from the Accumulation Chamber is accomplished by use of a solenoid controlled needle valve in the center of the Distributor Shaft which is controlled by the ECU (Electronic Control Unit) (Sometimes called the Fuel Injector Pump Control Module (FPCM) or just the Pump Control Module (PCM) (not to be confused with the Power Train Control Module (also called PCM) which controls things like the Air Conditioning, etc.))), located on top of the VP44, which, in turn, is controlled by the block-mounted ECM (Engine Control Module) that is measuring all of the variables needed to make the decision on just how much fuel should be injected and when.

Let us run it through an injection cycle (engine is running; the Distribution Shaft is rotating at camshaft speed):

1. In the beginning, the solenoid valve is spring-loaded open (de-energized) allowing fuel from the Accumulation Chamber to fill the Plunger Chamber. The high pressure pump rollers are off the lobes (in the valleys), so the plungers are not compressing the plunger chamber.



2. At a signal from the ECU (PCM) the Solenoid Valve energizes and isolates the Plunger Chamber from the Accumulation Chamber. The valve only moves about 0.3 mm (0.012 inches).



3. The high pressure pump cams start compressing the fuel in the plunger Chamber and delivers the fuel to the selected (by the angular position of the distributor shaft) VP44 delivery valve and on to the desired fuel injector.

4. When the desired amount of fuel for the current engine needs has been delivered, the ECU (PCM) tells the solenoid valve to open and the Plunger Chamber is once again connected to the accumulation chamber, the pressure to the injector collapses and the injector closes.

5. Note, though, that although the solenoid valve opens, the high pressure pump is mechanical and must cam up the full stroke (about 3 mm) so it must continue and complete that stroke. Until the end of its stroke, it is still pumping fuel but now is connected back to the accumulation chamber. Thus a spike of what was injection pressure fuel is spilled back to the accumulation chamber from the Plunger Chamber.

6. The accumulator diaphragm absorbs this spike and, as the high pressure pump completes its excursion, the pressure in the accumulation chamber returns to its normal value and again fills the plunger chamber in preparation for the next cycle. This happens about 60 times a second at 2000 engine RPM.

Continuing:
The Plunger Chamber needs to fill to a designed uniform pressure prior to compression. How is that accomplished?

You must remember that the ports to the Plunger Chamber are rotating rapidly (at one-half the engine speed) and the passages in it are very small; the amount of fuel it gets depends on how much pressure is in the accumulation chamber and how much time is available to fill the plunger chamber. As RPM increases, the time available to fill the plunger chamber decreases, so the pressure in the accumulation chamber must be increased by the vane pump and pressure regulator. By design, there will always be more than enough fuel supplied.

And that brings us to the Overflow Valve (perhaps the simplest and most misunderstood piece in the VP44).

The overflow valve is fed from a separate passageway in the distributor shaft but senses the actual pressure to which the plunger chamber is filled prior to being isolated from the accumulation chamber by the solenoid valve (prior to being pressurized by the high pressure pump). Any excess fuel above the overflow valve set point (about 14 PSI) is returned to the fuel tank by way of the Overflow Valve. The vane pump will always provide more than enough fuel to fill the plunger chamber, therefore there will always be flow back through the overflow valve and, therefore, always cooling flow through the VP44 as long as the engine is running and sufficient fuel volume is available at the vane pump inlet. By design, approximately 70% of the fuel provided by the vane pump will be returned to the fuel tank by the Overflow Valve. Some fuel is also returned to the tank from the injectors. The manual states that “A small amount of fuel is returned from the fuel injectors, while a large amount (about 70% of supplied fuel) is returned from the fuel injection pump”.

From this you can see that the Overflow Valve doesn’t really care how much pressure your external feed pump (lift pump) is putting out as long as there is sufficient fuel volume available at the vane pump input; it only cares about how much the internal vane pump is producing (regulated by the pressure regulator) and how much pressure is actually filling the Plunger Chamber while it is filling. There are other things in the VP44 that may care about what feed pump pressure you have; one that comes to mind is the timing piston. I have no idea how low or high feed pressure will affect engine performance after the engine starts; probably not much. We have seen that high feed pressure does affect the timing during starting with a hot engine sometimes causing hard hot starts but that has nothing to do with the overflow valve.

The Overflow Valve accomplishes two things: It limits filling of the Plunger Chamber to uniform value of about 14 PSI and the extra fuel (returned to the fuel tank) cools and lubes the VP44. The Overflow Valve also has a tiny orifice in it that is always open and aids self-bleeding of the VP44.

What does all of this mean to me?
Obviously, it is important to have enough fuel available at the internal feed pump (Vane Pump) input. What does that take? After all is said and done, I like to see fuel pressure above 10 PSI under all conditions for a couple of reasons: (1) The Dodge Ram maintenance manual states minimum feed pressure is 10 PSI and (2) It’s possible that the feed pressure does affect the timing slightly, although I don’t think you would ever see it. I do admit that I’m happiest when my feed pressure above 15 PSI (but below 20 PSI), but that’s only because I have been following these forums for thirteen years and it has left the inner part of my already weak brain a little paranoid. The original Carter pumps put out about 12 PSI but were very much prone to failure. There were a lot of VP44 failures blamed—rightly or wrongly—on low feed pressure. The later in-tank pump mods put out about 8 PSI but were fairly high volume pumps (it’s interesting to note that the in-tank pump is actually a relay controlled factory-installed after-market pump and they chose to go with a lower pressure, higher volume pump whose pressure is less than the FSM requirement of 10 PSI). The only thing I’ve heard bad about them is that nobody likes the low pressure they put out (they don’t put out the magical 15 PSI). The 15 PSI seems to be a number which was established on these forums in the early days because it takes 14 PSI to open the Overflow Valve and, with less than 14 PSI, it was believed there would be no cooling flow (some folks still believe this). As we now know (or at least I believe), there is no correlation (or very little) between feed pump (lift pump) pressure and overflow valve operation. Pressure inside the VP is controlled by the pressure regulator and the Vane Pump. If the engine is running normally, there will always be fuel flow for cooling through the overflow valve. My own truck’s fuel pressure measured at the VP input—runs at about 18 PSI idle, 15 PSI cruising at 65 empty, and about 13 PSI running WOT up the Ashton grade pulling a 28 ft. fiver (usually fourth gear at about 50 by the top (somewhat limited by EGT)). I consider this about right.

I've got a little more to say; I'll add it as a comment.
 

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Discussion Starter · #2 ·
Since I got cut off because I was too wordy, I'll add this as a comment:

My thoughts on keeping the VP44 Happy?

1. Good, clean fuel. Whenever you can, buy your fuel from a trusted dealer. Locations that have a high turnover of fuel are best. Find one in your location and stick with him. Stay away from the low-volume stations. Also, when you fuel your truck, make damn sure you’re putting in diesel fuel. Brain fade at the fueling station will cause immediate severe heartburn.

2. Assume that even the most conscientious dealers do not add any more lubricant than the code requires and that mistakes can be made. Knowing the extremely close clearance tolerances built into the VP44, I’m a lot more concerned about lubrication than I am about cooling. If the engine is running normally, you’re getting cooling through the overflow valve and injector return. Add some lubricant of your own at each fill-up. My personal favorite is TCW3 two-cycle motor oil because it’s cheap and so am I; and it’s always available. There are others. Do a little research.

3. Change the fuel filter on a fairly frequent time schedule. Time and your own experience will tell what that schedule should be. Changing out a clean filter is a waste of money, but not changing a dirty one restricts fuel flow.

4. Run on the top half of your tank, especially during hot weather. More volume in the tank makes a better heatsink for heated fuel returned from the injection pump, injectors, and most aftermarket fuel pumps (most high volume/high pressure aftermarket fuel pumps recirculate a lot of fuel to the fuel tank). More fuel tank volume in the winter means less volume of air for condensate to accumulate. Besides that, it does away with the one-quarter-tank/out-of-fuel phenomenon that confounds some of us (including me).

5. Don’t let your truck run out of fuel. Getting it running again can be a real PITA and usually requires bleeding the fuel lines, and, while you’re attempting to start it, the VP is not getting proper lubrication. That’s probably not a good thing.

6. And, yes, get an in-cab pressure gauge, so you can watch and worry about your fuel pressure. If you truck stops running, the first thing you will want to know is if you have fuel flow, and the first thing the forum members will want to know is what the fuel pressure was. Run your pressure above 15 PSI because everybody says to and it’s a good safe number and will give you a warm, fuzzy feeling. But if it gets down below that, don’t turn off your truck and walk home. If the engine is running, you are still getting enough fuel for cooling and lubrication. It will get you home. Also, if you are going to run your fuel pressure above 20 PSI, remember that any fuel that leaks past the front seal of the VP is going indirectly right into the engine pan; diesel fuel is not meant to lubricate the engine.

7. Don’t panic if your fuel pressure drops below the magical 15 PSI during a hard load. Pressure does not cool the VP44; flow does. The pressure at the VP44 inlet drops during a hard load because flow increases. Any flow going through the VP44 cools and lubes it, whether it is going to the injectors or back through the overflow valve. Increased flow means increased cooling. Think about it.

8. Buy a decent fuel pump. Sufficient volume is essential, as is enough pressure to give you that volume, but there are limits to what you really need. Running a 1000 GPH lift pump in a daily driver doesn’t do anything but heat up fuel in the tank; it doesn’t provide any extra cooling to the VP44. Reliability is what you are after. Contrary to popular belief, you don’t have to invest in a $650 fuel pump to get something that works; there are other options; again, do some research.

9. If you are running a lift pump which has no pre-filter arrangement, install an in-line pre-filter to protect it.

10. Fuel (lift) pumps are devices engineered and manufactured by man; even the best can fail, but I believe that any pump that is mounted on the hot, shaky engine block is going to fail sooner than one mounted near the fuel tank on the frame.

11. Don’t immediately shut down your engine after a hard pull. Let the engine idle a few minutes to get the engine compartment temperature down and allow a little further cooling of the V44 and the turbocharger. An exhaust gas temperature gage is handy for this. I like to have my exhaust temperature below 350 degrees before shutting off the engine.

12. And I guess the last one is be careful on your modifications that throw the VP44 outside of its design parameters, especially the electronics. The VP44 design specifications rate the max horsepower at 60 HP per cylinder. I won’t say any more about that.

As always, I say that everything stated here is my personal opinion. I know a lot of what you have read here disagrees with what has been put out as gospel, but these are the facts. Take it for what you may think it is worth. Others (who are, no doubt, smarter than me) may have more knowledge of VP44 operation than I do and I anxiously await your input. Maybe we can all learn something. Some of you have been around the VP44 for a long time; so have I (since 2002). Some have been around this forum for a long time; so have I. I’m not here to change your mind about anything (your opinions are just as important as mine.
 

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Discussion Starter · #4 ·
Looks like a question of terminology to me; what is the purpose of the plastic stuff on your picture?
 

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Looks like a question of terminology to me; what is the purpose of the plastic stuff on your picture?
Hold the o-ring on the back. But the rubber diaphragm is no longer use and doesn't exists. Little different when you have a up to date pump Rev027 and live picture to go with the article.
 

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Discussion Starter · #6 ·
I agree that your pics show that the original diaphragm is no longer used. The function of the original diaphragm still needs to be addressed and I believe the plastic, the o-ring, and the unique shape of the plastics accomplishes that function. Something has to absorb that pulse.
 

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Nope that plastic is extremely hard plastic and and doesn't flex. The steel plate is harden and doesn't flex. So the whole diaphragm no longer exist.
 
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great presentation J.J.!! after gettin lost trying to read, let alone understand the tech info mentioned, this puts it all out there in an understandable way, for me at least... thank you!
 
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GREAT JOB Jeeper Jimmy…..!

I’d love to have a link for the Bosch VP44 Service Manual (Rev 2000), the Bosch Distributor Type Diesel Fuel Injection Pumps publication (Issue 2003), and the detailed Spanish description of VP44 internal operation you mentioned on YouTube. Also you should post a YouTube video of that VP44 CD you have so that others can maybe benefit from it as well. Yes, Mopar1973man’s did do a tear down of a VP44 which was very helpful, and there’s another similar one on TDR from years ago which you might want to check out if you’re interested or maybe you already know about it.

The angular position of the ring (and thus the timing) is also affected somewhat by the feed pressure to the VP because the feed pump (lift pump) pressure connects to one side of the timing plunger and timing solenoid that control the position of the cam ring. Because of the low RPM during cranking, it is desirable to have the timing fully retarded (thus low feed pressure). This is why a properly controlled feed pump runs for a second or so to prime at key on, shuts off, and then runs at reduced (25 percent) pressure during cranking, finally going to full pressure following startup. It’s also why some folks with higher pressure relay-controlled aftermarket fuel pumps sometimes experience hard starting when the engine is hot. I believe this unintended consequence occurs because the relay which controls the aftermarket pump cannot respond to the ECM method of command to reduce pump pressure and the pump stays fully energized, thereby keeping fuel pressure up and not allowing the timing to properly retard. By design, the ECM pulses the voltage to the fuel pump during cranking rather than supplying a solid 12 volts. I think the OEM pump can see this as a reduced voltage, but the relay associated with the aftermarket pump doesn’t. Some folks believe that, even with reduced pressure, the aftermarket pumps produce too much pressure during cranking. Take your pick (maybe it’s both).
I like the explanation of the ring in conjunction with hard hot starts but the way I’ve understood it was through a couple different theories. One being that the timing solenoid requires minimum 9 volts to function properly. Working alongside a hot engine and during hot weather where everything is easily heat soaked and toasty, the voltage drops due to resistance and “potentially” the voltage to the solenoid may momentarily dip below 9 volts when cranking over a hot diesel engine. During that combination of variables the pressure against the solenoid and the lacking voltage could cause its inability to move temporarily. Or, the second theory is that the tolerances in a new remanufactured VP’s are expanded (potentially under different rates due to different materials) when everything is hot as so just described. During the heat soaked hot start the ring is locked up against the feed pressure and the solenoid isnt able to overcome the resistance. In saying that, the longer the VP turns during the extra cranking, the feed pressure purges, and proper internal movement regains. This is why we've discovered that the VP actually starts much easier with zero fuel pressure against it as long as the prime is maintained. Actually I'll just throw in that I also think the transducer is affected by voltage dips during hot starts which also plays a detrimental role in PSG failures. Maybe the solenoid not fighting that pressure spike is more helpful than we realize.

As for the ECM alternating voltage to the fuel pump. I can attest that the aftermarket pump does get the same signal as the OEM fuel pump, but my opinion is that those aftermarket pumps are so much more capable of generating higher pressures more rapidly and more forcefully than the OEM fuel pump that no matter if the voltage is alternated, the pressure will be too high for cranking. I’ve even confirmed with a test light on the open pin of the delay relay during cranking as the test light flickers rapidly. Just pointing out too that the hard hot start was even an early on issue enough for Dodge to create a TSB which reflashes the ECM to do basically what we do by installing the delay relay. Yeah, that’s funny to think that any OEM Carter lift pump could be that on top of its game but remember, those fuel pumps were regulated at 15 psi.

Equipment where the fuel tank is a longer distance from the VP may need an external feed pump. Stationary equipment and tractors may have a head tank supplying pressure.” I really don’t want to get back into this argument; just stating what Bosch says (I’ve got to believe they know something about it). My thoughts are that the Dodge Ram/Cummins, because of its sometimes unique uses and high horsepower requirements, may throw additional variables into the equation and thus require added input pressure to ensure the required volume is available. For instance, on a 45 degree incline, the internal Feed Pump would have to overcome about 4 PSI because of the location of the fuel tank and the weight of the fuel in the lines. I’m sure there are other variables also, hence the Ram Service Manual requirements for a minimum of 10 PSI lift pump pressure (minimum of 7 PSI while cranking (although one manual states maximum fuel pressure of 7 PSI while cranking—which, because of the effect on timing, really makes more sense)).
I agree that Bosch should know more about their pumps than anyone. But it certainly does beg the question as to why all the problems if they knew what was up? Especially since the VP is found in other vehicles. And addressing that further, if supply pumps aren’t needed then why the obvious failures in conjunction with little to no fuel supply. It would seem that maybe the VP44 on the Cummins may have needed a bit more R&D than what Bosch thought they could simply hand over to Cummins with the intent of only satisfying the following 4 years of production until the Common Rail was ready for production. No matter how brilliant the Bosch engineers are, I simply think they dropped the ball on this whole retrofit project and had no idea it would cause so much drama. I blame most attention on the aftermarket industry too. Because I believe that if the aftermarket industry didn’t bring it to the attention of the consumer world that ISB engines could so easily add immense gobs of plug and play power, these VP engines would be talked about as much as the VE engines. Something I don’t think Bosch or Cummins had in mind either.

Lastly, with all the other information out there about fuel pressure requirements, I’ve always believed the conflicting data which indicates a “minimum” of 7 PSI while cranking is simply a misprint. It should have said “maximum” 7 psi just like everything else does.


It appears that, in the early VP44s, this diaphragm was under-engineered and prone to rupture and was blamed for many failures of the VP44 (mostly galling of the timing piston and liner by feeding pulsating pressure back to the piston chamber whose liner was also under-engineered (wrong material)). Later revisions changed its design, giving the diaphragm a solid steel backing which makes later VP44s much more dependable and has pretty much eliminated diaphragm failures. The diaphragm material, however, still has to absorb the energy from the high pressure fuel spill pulses (again, we’ll cover these later). The pressure spike it must absorb is initially very high (injection pressure), but the volume is very small and the diaphragm has lots of surface area.
There are other things in the VP44 that may care about what feed pump pressure you have; one that comes to mind is the timing piston. I have no idea how low or high feed pressure will affect engine performance after the engine starts; probably not much. We have seen that high feed pressure does affect the timing during starting with a hot engine sometimes causing hard hot starts but that has nothing to do with the overflow valve.
I cant confirm anymore than by simply sharing the information and feedback I’ve heard and read over the years as to whether or not the timing piston is truly feeling the pressure from the feed pump. But I will say that if it doesn’t then there would be no reason why one of the things people will notice is a small increase in power after adding a higher pressure / higher volume aftermarket fuel pump. Its been recognized so much so that “if” you do happen to gain any unknown loss in HP after installing a new fuel pump then the odds are that the timing piston has become worn and weak in the sleeve and is not achieving optimal fuel timing. Power lost over time but likely gone unnoticed. Coincidentally though not long after installing those aftermarket fuel pumps do people find their factory VP completely has quit mechanically. So in noting that dilemma, is the timing piston affected by feed pressure? I think so but again, I’m not professing to be more knowledgeable by any means.

In addressing that further, when the old designed weak diaphragm would be compromised then it would greatly affect the timing pistons ability to do its job, and eventually result in that timing piston to violently vibrating in the sleeve until it failed completely. Basically self destructing mechanically.

In regards to the diaphragms true job, I understood its purpose is to dampen the pulses to and fro the timing piston. And the only component in the variable which would stop diaphragm damage was the pressure being delivered from the feed supply pump which would in essence cancel the spike pulses generated. But if the supply pressure dropped too low then the diaphragms movement would become too great while trying to dampen pressure spikes and diaphragm rupture would be eminent. I think today we understand that by running a better more reliable higher pressure fuel pump, those pulse spikes are no longer damaging. And by redesigning the diaphragm to a more solid material, failure from those spikes are no longer and issue.

The Overflow Valve accomplishes two things: It limits filling of the Plunger Chamber to uniform value of about 14 PSI and the extra fuel (returned to the fuel tank) cools and lubes the VP44. The Overflow Valve also has a tiny orifice in it that is always open and aids self-bleeding of the VP44.

The later in-tank pump mods put out about 8 PSI but were fairly high volume pumps (it’s interesting to note that the in-tank pump is actually a relay controlled factory-installed after-market pump and they chose to go with a lower pressure, higher volume pump whose pressure is less than the FSM requirement of 10 PSI).

The 15 PSI seems to be a number which was established on these forums in the early days because it takes 14 PSI to open the Overflow Valve and, with less than 14 PSI, it was believed there would be no cooling flow (some folks still believe this). As we now know (or at least I believe), there is no correlation (or very little) between feed pump (lift pump) pressure and overflow valve operation. Pressure inside the VP is controlled by the pressure regulator and the Vane Pump. If the engine is running normally, there will always be fuel flow for cooling through the overflow valve. My own truck’s fuel pressure measured at the VP input—runs at about 18 PSI idle, 15 PSI cruising at 65 empty, and about 13 PSI running WOT up the Ashton grade pulling a 28 ft. fiver (usually fourth gear at about 50 by the top (somewhat limited by EGT)). I consider this about right.
Don’t forget that the OEM lift pump was regulated not at 12 psi but 15 psi. Even if it was too incapable to reach that goal very often the Carter was/is a higher pressure lower volume pump. Nonethless, I’ve always found it coincidentally interesting that the overflow bleed off valve specs are very close to the OEM fuel pumps regulated specs. So then why the small orifice?..... Well that’s really interesting because it would seem that Bosch may have done that to assure a constant fuel flow though the VP. But.....if the pressure inside the chamber isnt being affected by or regulated by the feed pump pressure then in theory there should be zero reason for a bypass orifice as the checkball and spring regulated at “X” pressure would be all it would take to maintain an internal pressure for fuel distribution. Especially if the vane is the source. Again, sooooo many unknowns.

In regards to the in-tank pump….. Well I think that was Bosch telling Cummins or Cummins telling Dodge or who knows who telling who, that the fuel pressure wasn’t the problem but the volume was. Even though this logic went directly against their own specification requirements with minimum fuel pressures, and what reasons Dodge used to differentiate whether or not a fuel pump needed replacement. That there is a direct example going against your statement of “just stating what Bosch says (I’ve got to believe they know something about it”. If Bosch really knew better than anyone then what happened with that goofy retrofit pump? But sadly I think they were all wrong as its not simply pressure and its not really volume…..its BOTH. Too bad the in-tank pumps didn’t solve the problem either since all it did was make more of a mockery of the whole situation making Dodge look really bad(er).

But back to the overflow valve. I personally I feel the overflow is a direct correlation to both cooling and feed. I think the higher pressure feed supply, within reason, maintains a “charged” aspect to the fuel distribution inside the pressure chambers of the VP. Not so much just for cooling. And even though I cant confirm my own theories yet, from what I’ve seen I believe that the VP flows a constant amount of fuel out the overflow and to the injectors as long as fuel is supplied and the shaft is turning. I think the overflows intention is to maintain a static level of pressure in the low pressure chamber for proper further distribution, and also I think the VP passes fuel through to the injectors from the high pressure side at all times. Meaning, if there is no engine load then the fuel pressure to the injectors isnt enough to pop the injector so the fuel reaching the injectors merely passes on by through the injector housing, in the cylinder head and out the rear banjo. This would only make sense to me as otherwise the VP would become immensely hot inside the distributor as soon as the fuel has nowhere to go, like coasting in gear down a long hill. Especially as the fuel sits in there absorbing heat through constant friction and pressure, and fuel isnt coming back though the vane and into the low chamber. My impression is that this would instantly create fuel temperatures well above the maximum 160* regulated temp by the VP internal thermometer, before tossing everything into limp mode.

Also, if you are going to run your fuel pressure above 20 PSI, remember that any fuel that leaks past the front seal of the VP is going indirectly right into the engine pan; diesel fuel is not meant to lubricate the engine.
This is something I’ve never been able to confirm as I’ve never had anyone state that their front seal was compromised by too high of feed pressure. Matter of fact I dont think anyone has ever confirmed a pressure limit to the VP. That’s not me defending running 30+ psi to the VP either…..even through there’s people who do it, but I am going to step out there and risk stating my opinion that I don’t believe the shaft seal is limited by any amount of pressure generated by the supply pump. That would really be taking away from the engineer’s capacity to design something which houses a weak seal just barely capable of holding back the pressures generated by a weak supply pump. 15 psi is nothing by seal requirements. Sorry but the front seal would be the LEAST of my concerns. Look at the supply pressure numbers to the P-pump…..

Pressure does not cool the VP44; flow does.

Don’t panic if your fuel pressure drops below the magical 15 PSI during a hard load. Pressure does not cool the VP44; flow does. The pressure at the VP44 inlet drops during a hard load because flow increases. Any flow going through the VP44 cools and lubes it, whether it is going to the injectors or back through the overflow valve. Increased flow means increased cooling. Think about it.

8. Buy a decent fuel pump. Sufficient volume is essential, as is enough pressure to give you that volume, but there are limits to what you really need. Running a 1000 GPH lift pump in a daily driver doesn’t do anything but heat up fuel in the tank; it doesn’t provide any extra cooling to the VP44. Reliability is what you are after. Contrary to popular belief, you don’t have to invest in a $650 fuel pump to get something that works; there are other options; again, do some research.
I’m certainly not trying to pick apart your hard work and time putting this all together since I fully enjoyed the read. We’ve all talked about this pump so much over the years that I’m surprised I’m even investing this much effort right now. The monotony does get to ya at some point of the game….. But let me just point out real quick that your crudely extreme example of a 1000 GPH pump doing no good other than heating up the fuel tank is a contradiction to your very thinking of how the VP functions. If…..If you state that the supply pump is nothing more than merely a means to give the VP enough fuel to supply the vane which takes everything over from there…..even stating that the overflow isnt affected by the supply pressure, then how could a 1000 GPH pump ever heat up the fuel tank? Where would all this hot fuel be coming from? Its certainly not circulating any more than any OEM lift pump would be if by the previous statements are indeed accurate. If the supply pump isnt what we imagine it is then it will do no more to heat any fuel in the tank other than just be expensive, sit there, and look cool. Sure there’s a few people out there who still run the OEM fuel pump and/or still have the factory VP, or run dangerously low fuel pressures, and I’m glad to hear things are working out great for them. But statistics are statistics and we cant set aside and deny the facts that the amount of Cummins trucks which poured into the dealers during the late 90's and early 2000's for lift pump and/or VP replacements were nothing short of destructive to Dodge’s and Cummins reputation. I’m sure there was possibly even a fine line between a total recall and simply riding that wave for Dodge and Cummins until the CR motors came out and eased the consumer pain. Pure speculation there on my part too.....

But again, I’m totally impressed with your research investment effort. And I’d love to see those data sources you collected too. The more information, the better. I have no real position in this fight either other than simply making sure anything considered or discovered which isnt fully understood isnt preached to everyone as “fact”. Because even as much as we do know about this silly game changing revolutionary fuel injection pump…..we really don’t know much other than keeping good quality lubricated fuel supplied to the VP under reasonable fuel pressures and with ample volume keeps it happier. :thumbsup:
 

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Discussion Starter · #10 ·
Whoa, Katoom. Don't get the idea that I've put this on here looking for a fight.

From following these forums for these many years, it's painfully obvious that very few folks have any idea just how the VP44 functions. My input is merely an attempt to point out it's physical operation. If somebody else has a different idea of it's operation, as you have, then I say again: "Your opinion is just as important as mine".

I’m here to learn~~ not to fight. If I wanted a fight, I can get one without leaving the house.
 

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:hehe: Sorry Jeeper Jimmy. I wasnt trying to fight either but merely pointing things out. I thought I threw in enough praise to make that clear. :thumbsup:
 
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The only thing I want to kill off before the old myth starts again about the rubber diaphragm that is no longer. Other than that I think it cool some dug up the information explaining how these pumps work. :thumbsup:
 

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Discussion Starter · #14 ·
As far as the rubber diaphragm goes, it looks like that has been replaced by a solid partition in later revs of the VP. The high pressure pulse is still going to happen, but its volume is small and the accumulation chamber volume it is spilling back to is quite large by comparison and controlled by the pressure regulator (although I don’t think the pressure regulator can respond as quickly as it would have to absorb the pulse). Maybe it was decided a diaphragm wasn’t necessary. Lots of ‘maybes’ here.
 

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As for the high lift pump pressure. Return on the left and supply on the right.



There is only some much you can return anyways. Then your vane pump is at the shaft end of the pump. it's not a sealed unit so excessive pressure "could" cause front seal damage or start leaking on a old harden seal.


Also remember the VP44 is turning at exactly half the RPM rate. So at idle its only spinning 400 RPM.
 

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Discussion Starter · #17 ·
Yeah, Mountaineer; imagine how mine feels!
 

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As for the high lift pump pressure. Return on the left and supply on the right.

There is only some much you can return anyways. Then your vane pump is at the shaft end of the pump. it's not a sealed unit so excessive pressure "could" cause front seal damage or start leaking on a old harden seal.


Also remember the VP44 is turning at exactly half the RPM rate. So at idle its only spinning 400 RPM.
Also important to point out in that inlet / overflow outlet picture is that even though those two ports are right next to each other, they dont directly touch.

And in regards to overly high lift pump pressures..... The reasons I dont see this being a problem, nor a problem I dont think anyone has experienced, is because the feed pump is supplying the vane pump at approximately 10-20 psi. Then the vane pump takes that and increases it to anywhere between 150-350 psi. To the best of my understanding there is no other seal which retains the feed pumps pressure separated from the vane pump pressure. This is why we see the high pressure spikes travel back down the supply line. So in saying that, that forward shaft seal is holding back a heck of a lot more pressure than the supply pump could ever generate. :thumbsup:
 
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