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Joe G's fuel system writeup

279246 Views 2 Replies 3 Participants Last post by  BBRam
1994 thru 1998 12 Valve Fuel System Operation and Repair

The fuel system discussed here is the supply and return system. It does not include the injection
pump and injectors. The details in this discussion are based on my 1995 truck. There may be
some differences in other year models. There are some illustrations here that are scanned images
from the 1995 Ram Truck Service Manual. This is a very simple system. It can be considered to
be in three parts. The first part is from the fuel tank module to the lift pump (fuel transfer pump).
In this part the fuel is being pulled by the lift pump so there is a suction condition. Any leaks in
this part of the system will be air leaks. The second part is from the lift pump to the injection
pump. In this part the fuel is pushed by the lift pump so there is a pressure condition. Any leaks
in the part of the system will be fuel leaks. The third part is from the injection pump to the fuel
tank. In this part the fuel is being pushed by the lift pump through the injection pump. There is
less pressure because the overflow valve on the injection pump restricts fuel flow to this part of
the system. Any leaks in this part will be fuel leaks when the engine is running. There may be
some air leaks when the engine is not running that may cause hard starting.

Fuel tank module

The gasoline version of the Dodge Ram has a fuel pump in this module. The diesel version
does not. However, there are some errors in the service manual in the diesel section that
mention the fuel pump and a fuel pump relay. These items do not exist in the diesel version. There
are a few things here that concern us. The gasket for the module may leak. If it does it
may be stopped by using a long screw driver and a hammer to tighten the large white plastic nut
that holds the module in the tank. If that does not work the module will have to be removed
from the tank and the gasket replaced. Any work other than tightening the module
hold down nut will require that the tank be lowered or the bed lifted to provide access
to the module. There are two fuel hoses to the module. One is supply and the other is return.
They are fastened to the module pipes with quick connect clips. Unfortunately they may not be so
quick to disconnect. The return hose is easy on my truck. The supply hose is a struggle to get the
clip to release. It is suppose to only be necessary to squeeze the tabs on the connector and pull the
hose off. For the supply hose on my truck it requires some needle nose pliers and a small
screwdriver to get it to release without damaging it. The pipes may have rusted bad enough to
leak. If the pipes leak, then the fuel module has to be replaced or modified so that hoses can be
connect to it.

There have been some reported problems with air leaks in the supply hose at the
tank. If there is an air leak here the truck will be hard or impossible to start. Some people have
replaced the quick connect fittings with clamps or threaded fittings and used hose to replace the
stock lines. There also have been some reported problems with alternate fuels causing the basket
on the fuel module to clog.

When removing the module from the tank have a bucket handy.
It will have some fuel in it that can make a big mess so lift it straight up and put the
bucket under it. One or both of the plastic connectors for the fuel hoses may be broken.
There are some ways to repair these, but I have not had to do that. There are slots in the fuel
tank module that allow the pick up fuel filter to move up or down. If these slots are worn
and still working leave them alone. If they are so bad that they are sticking you will have
to replace the tank module or figure out some way to fix them.

Do not use screws to freeze the up and down motion. If you do that the bottom of the
fuel module will not go down when the tank is full. The fuel gauge sending unit is a known
problem. In some cases they are repairable, but it is not usually worth the effort. This
sending unit has been repaired. The wear point is indicated in the picture. The retaining tab
wears so the moveable part can move up a little. This reduces contact pressure on the variable
resistor. It also allows the pin that the part rotates on to be below the top of the moveable part.
This lets the top edge of the pin to wear a grove so the part wobbles. The result is that the fuel
gauge may not work or it will be erratic. I have repaired this one by replacing the pin with a very
small screw and bending the follower arm a little to increase contact pressure. After all that I
found that the sending unit worked smoothly, but the reading was off on the fuel gauge. I
measured the resistance and found that it had little less than 12 ohms at full and about 113 ohms
at empty. That was incorrect, it should be full 3.8 ohms, empty 103.3 ohms. So, after all that
work, I replaced it.

And then what happens? Kirk Larson comes up with a simple very clever easy way to fix the
sending unit. Here is his explanation followed by a couple of pictures:

Since the fault is due to a lack of contact pressure I looked at it with a goal to apply pressure on
the arm where it would do the most good. There is a metal "cover" over the arm that is arched on
one side and straight on the other. I simply looked at the space between the metal cover and the
plastic part of the arm as I was holding the arm snug against the contacts. I got a piece of
sheetmetal (I had some scraps in the shop) that was that thickness and used the metal cover to
scribe a pattern onto the sheet metal. I cut this out leaving 3 bend tabs (two at the extremes on the
curved side and one in the middle of the flat side). I simply attached this sheetmetal to the
underside of the cover plate by bending the tabs tight with a pair of pliers. I filed the sheetmetal
smooth so there were no burrs, added a squirt of WD40 and the arm swung smoothly and the
contacts remained in full contact. The plastic part of the arm clip rubs on this plate.

Thanks Kirk.

Fuel supply and return lines.

There are supply and return lines from the tank to the engine area that go between the tank and
the frame. I have not had to repair those lines. Others have replaced those lines with fuel hose.
These lines on my truck appear out of the frame from behind the fuel tank as braided steel lines
that become hard steel lines. These steel lines connect to flexible “rubber” hoses. If these hoses
are the original stock hoses, replace them with diesel rated hose. If they are not bad they will be.
The best hose to use is marine grade. It’s not available where you are you can get it from
LarryB's Dodge starting problems solved here. in Tacoma, WA. When replacing hoses use a little
dab of Sil Glyde® from NAPA to lube the inside of the hose. That’s good stuff to have in your tool box.

Fuel supply before the lift pump.

This part of the fuel system is notorious for air leaks. Remember, you can’t see an air leak.
Everything may look good and be leaking air. There also may be more than one air leak. If the
rubber hose is the original it gets bad and does not look bad. The inside of the hose gets bad so it
sucks air from then ends of the hose. The exterior of the hose may look good in that case.
The fitting in the line from the fuel tank into the top of the fuel heater/pre-filter assembly may leak
air. It has a fiber washer in it that is kind of poor to say the least.

This is the aforementioned seal. It is Cummins #3923194. This part is not called out on Cummins Quickserve.

This is what the line looks like on a MY '98 truck.

Editor's note: the above two pictures and captions were added to this writeup upon request. It was originally supplied by user GAmes and added by dauntless89 on 9/7/16.

The next thing that may leak air
is the fuel heater. If the fuel relay or the fuel temperature sensor goes bad the fuel heater may be
stuck on and burn up. You usually can’t tell by looking at it if this happens.

These pictures illustrate this problem. The first picture shows a fuel heater with the rivets drilled
out so it can be opened. The second shows the burned heater element. The last shows the location
of the air leak which cannot be seen.

The fuel heater is actually worthless. Just a problem waiting to happen. It does not help to start in
cold weather. There is about a quart of fuel between the fuel heater and the injectors. The fuel
filter is full of cold fuel. So are all the lines, the lift pump, and the injection pump. So when the
truck is started it is started with cold fuel. It takes a lot of pumping with the starter to fill the fuel
filter when it is changed. It would take that much pumping plus some to get warm fuel from the
fuel heater to the injectors. I recommend removing it and throwing it away. The next picture
shows the fuel heater/pre-filter assembly with the heater removed. It also shows the short curved
hose between this assembly and the lift pump. This hose can also be bad and leak air. There are a
couple of gaskets in this assembly that may leak. The machined hex at the bottom of the
assembly may be turned with a 17mm socket. The two little screws on top of the assembly that
fasten the sensor in place fit a T15 torx bit. If you remove the bottom you should use a u-joint
and an extension to avoid a diesel fuel bath. The adapter screw that fastens the fuel heater to the
assembly body may be removed with an 8mm allen wrench. Use one of the gaskets and omit the
adapter screw. The pre-filter will then bolt up to the assembly body.

The above is the stock adapter screw that is used to fasten the fuel heater to the assembly. It is to
be removed and thrown away with the fuel heater.

Lift Pump

The lift pump is very simple and sturdy. When the camshaft rotates it pushes up
the piston. This stroke does not output fuel. It opens check valve A. As the
camshaft rotates the piston is pushed back down by the large spring. The
purpose of the small spring is to keep check valves A and C seated. During
this stroke check valve A closes. Check valves B and C open. This causes fuel to
move through those valves. B outputs fuel. C inputs fuel. The large spring is
what moves fuel, not the camshaft piston. The rubber primer pump button
pushes fuel through check valve B when pushed. When released it sucks fuel
through check valves A and C. A simple contraption to say the least.
The main thing to remember about the lift pump is that it cannot pump air. This means
that if there is an air leak in the system any place before the lift pump fuel pressure will drop. If
the air leak is bad enough there will be no fuel moved at all. The most common problem with the
lift pump is a fuel leak at O ring 3. The push button and push button valve are made of plastic.
Care must be used to remove the push button to avoid breaking it. This can be done with the lift
pump on the engine. Usually replacing the O ring will stop the leak. Removing the lift pump is
easy. Care must be used so that the piston rod does not come out and fall into the engine oil pan.
Reinstalling it is hard. The camshaft must be rotated so the low part of the lobe contacts the
piston in order to install it. The lift pump is hard to get to and apply enough pressure to compress
the large spring while starting one of the attachment screws. Some people have replaced the
screws with studs to help with this task. The size for studs is 8mm, 1.25tp (thread pitch). At least
40mm length. A little bit longer is better. Don’t forget the nuts. The input and output fittings are
easy to remove if the lift pump is out of the truck. Remember which way the check valves are
installed when you remove them. They must open toward the output. There are two O-rings on
the input and output fittings are exactly the same as the O-ring used for the push button. All three
should be replaced if the lift pump is disassembled. They cost about 50 cents each. If the springs
and check valves are ok and none of the moving parts are skinned up the lift pump is good. There
are no known sources for the springs and check valves.

Quick lift pump test.

If you suspect that the lift pump is bad it is easy to test. Remove the short curved hose that
connects the fuel heater/pre-filter assembly to the lift pump. See the picture above on page 5.
Connect a length of fuel hose to the input fitting of the lift pump. Put the other end in a bucket of
fuel. Start the engine. It may be a little hard to start because the hose will have some air in it.
After it starts check to see how it is running. If it’s ok, then the problem is not the lift pump.
Remember that the fuel from the bucket is returned by the fuel system to the tank so make sure
that the tank is not too full for it. It will empty the bucket of fuel rather quickly so watch the level
in the bucket and turn it off before the bucket is empty.

Finding an air leak.

You can’t see an air leak by just looking at the fuel system. Sometimes an air leak will drip a
little fuel if the engine is not running. The best way is to remove the lift pump and fuel
heater/pre-filter assembly. Include the input and output steel fuel lines. Block the output line.
Restrain the piston rod so it will not blow out. If it is not in the pump correctly, air will leak out
the weep hole. Then put about 20 PSI of air into the system. Dunk the whole thing into a bucket
of water. It will be obvious where the air leaks are.

Fuel supply after the lift pump

Editor's note: the above picture was added to this writeup upon request. It was originally supplied by user Cowboy303 and added by dauntless89 on 5/16/16.

If there is a leak in this part of the fuel system it will be a fuel leak. A steel line goes from the
output of the lift pump to the input to the fuel filter. This is done through a double banjo fitting as
shown in this picture on the right of the left picture below. The line from the lift pump is the
larger part of the fitting on the bottom. The smaller part of the fitting on top is the line from the
injector fuel return manifold which is hot fuel. If there is a leak at this fitting there will usually be
some fuel around the rim of the fuel filter. It may run down the fuel line and not appear on the
top of the fuel filter at all. Then the first indication of a fuel leak will be fuel dripping off the lift
pump. Note that the normal knee jerk response for a fuel problem is to change the fuel filter. This
is not a good idea. You want to find out what is wrong before replacing anything. You could
make an error of some kind and complicate the situation. In any case the fuel filter must be bled
which may be a problem since the lift pump push button tries to move air. The water drain valve
on the bottom of the fuel filter may leak. If it does it must be replaced. The steel line from the
output (line on the left in the left picture below) goes to the input banjo fitting on the injection
pump. The banjo fitting is the best place to install a fuel pressure gauge.

Fuel return system

The fuel return system starts at the overflow valve that is on the engine side of
the injection pump even with the first delivery valve. The overflow valve looks
like a banjo fitting, but it contains a pressure valve. The type shown in the
picture has a spring, a steel ball, and a washer shaped like a tiny derby hat to
regulate pressure in the injection pump. This is essential so there will be adequate
fuel flow into the injection pump plungers when the fill ports open. The line from this
valve goes to the rubber return line discussed in “Fuel supply and return lines”
above. If there is low fuel pressure that is caused by a weak or defective overflow valve it is easy
to check. While the engine is running have someone watch the fuel pressure gauge. Clamp the
rubber return line closed with some pliers. If the low pressure is caused by this valve the pressure
read by the pressure gauge will increase very quickly. Do not hold the return line closed very
long to prevent damage to the fuel system. If the overflow valve is the cause and is the type
shown in the picture it may be temporarily repaired. Remove it from the injection pump. When
you do this cram a shop rag below the valve to catch any washers you may drop. If they get down
there you may never see them again. Take the entire valve off. Don’t remove the small screw
until you have the valve out and on your work bench. The spring in the overflow valve is
supposed to be a little bit longer then ½ inch, 0.550". If it is shorter you can stretch it a little. Just
a little bit goes a long ways. If you stretch it more than a little it will wreck the spring so it will be
worthless. The steel line from the overflow valve may crack and leak.

Fuel pressure gauge

A fuel pressure gauge is an essential tool for trouble shooting a fuel system problem or getting a
warning that there is a problem before it gets bad. You want it to be installed in the truck so you
can see what the pressure is on the road. If the fuel pressure is bad you know that the problem is
in the fuel supply system somewhere. If fuel pressure is ok then the problem will probably
involve the injection system or maybe water in the fuel. Tap into the banjo fitting on the side of
the injection pump as mentioned above. Be especially clean when doing this because this fitting
is on the clean side of the fuel filter. Remove the screw part of the banjo fitting. Drill the hole
from the inside of the fitting because the way the screw is made it will keep the drill bit straight.
Tap it to 1/8 NPT from the outside. Clean up everything and reinstall the banjo fitting. I have
been advised that the banjo fitting sold by Geno’s Garage for 24 valve engines fits the P7100
pump so you can use that one instead of tapping the one on the pump. Don’t forget two new washers.
Use a needle valve. Use a grease gun hose from the needle valve to someplace to mount
the fuel pressure sensor that is not on the engine and subject to engine vibration. A grease gun
hose has male 1/8 NPT fittings on both ends. Close the needle valve all the way and then slowly
open it until the fuel pressure gauge barely works. That will dampen the pressure spikes from the
lift pump so the fuel pressure gauge or sending unit will not be damaged.

Fuel solenoid.

The fuel solenoid is actually not part of the fuel supply system. It pulls a lever on the side of the
injection pump. However, if there is a problem with it the symptoms are similar to problems with
the fuel supply system. Another thing to remember is that the fuel solenoid does not open or close
a valve. It parks the rack so that no fuel is sent to the injectors. Fuel will still flow through
the injection pump to the return line regardless of the fuel solenoid rod position.

The first picture shows the fuel solenoid. It is the round can with the two wires you can see.

The second picture shows the bottom of the fuel solenoid with the boot and rod. The boot should
not be damaged. If it is torn or rotten it must be replaced to avoid getting it tangled in the rod mechanism.

The usual problem with the fuel solenoid is that it will not pull up. When this happens it is probably NOT the
fuel solenoid. The most likely cause of the problem is the fuel solenoid relay. The next most likely cause is
the bluewire on the positive post of the driver’s side battery. I think that the ‘98 trucks use a fuse in the
PDC instead of the blue wire. In one case that I heard about, the rod end was rusted so bad to the
injection pump lever that it could not pull the lever up in spite of being ok. So if the rod is rusty,
clean it up and lube it. The next two pictures show the fuel solenoid relay (the larger relay in the
picture) and the blue wire (indicated by the red arrow) on the battery. The way to test to see if
one of these is the problem is to turn the key to start. If the truck does not start leave the key on.
Then try to pull up the fuel solenoid rod. It if pulls up and stays up twist the key to start. If the
truck starts then you need to check the relay and/or the blue wire. The relay is rated at 70 amps. A
30 amp relay will work for a while, but it will not last. The blue wire may be corroded, broken, or loose.

If you suspect that the fuel solenoid has failed, it is easy to test. The next picture shows the connector to the fuel
solenoid. The upper side in the picture is the wires from the truck. The lower side is the wires to the fuel solenoid.
Note that the wire colors do NOT match from the truck side to the solenoid side. The small wire to the left in the
upper side is directly connected to the ignition on circuit. It is low current for the hold coil in the fuel solenoid. The
black wire on the right with the light orange trace is to ground. The red wire in the center is from the fuel solenoid relay.
The red wire in the lower side is to the hold coil in the solenoid. The white wire is to the pull on
coil in the solenoid. The back wire is to the ground in the solenoid. In order to test you need three
test wires. Test using the bottom part of the connector. Separate the connector. Connect a wire
from the black ground terminal to a ground on the truck. Connect the red hold coil terminal to the
positive post of the battery. Then using a test wire connected to the positive post of the battery
touch the pull up coil terminal. Just touch it, don’t hold it on there because there is a lot of
current for the pull up coil. The fuel solenoid rod should pull up and stay up if the fuel solenoid is good.

If the starter contacts stick so the starter continues to run after the key has returned to the run
position the fuel solenoid may burn up because the pull up coil is held on by the stuck contacts.

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Very nice write up

Great job !!:hyper:

I had a question on the Kirk Larson Fuel gauge sender fix. My truck is the 99, but from the pictures the sender is the same. Can that fix be used for the 98.5 to 2000trucks?

My Fuel Guess gauge told me I was getting great fuel mileage then I ran out at just under 1/2 a tank, last night:CRY:

I should have known my XTZ was not giving me 425 miles on a half tank with all city driving. It was a nice fantasy:thud:
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Moved to the 94-98 Tech articles forum.

Thank you Joe G & ykdave. :thumbsup

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