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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.
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.
