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Old 04-05-2006
vonTrapp vonTrapp is offline
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Default 12 Valve: 1994 thru 1998 Fuel System Operation and Common Repair Issues Part 1

This article was written and composed by Joe G.


1994 thru 1998 12 Valve Fuel System Overview PART ONE


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 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 supposed 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. 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.

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. If they are not bad they will be.

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. The next thing that may leak air is the fuel heater. If 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. If you live in an area where the fuel heater is not needed you can remove it.

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. If you remove the bottom you should use a u-joint and an extension to avoid a diesel fuel bath. The 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 screw. The pre-filter will then bolt up to the assembly body.

Below (FYI) is a shot of the adapter screw that is used to fasten the fuel heater to the assembly.

To read the next part of this article, you can click HERE to open up a new window, or...

you can click your "BACK" button to return to the "HOW TO" menu, and click on the thread titled 12 Valve: 1994 thru 1998 Fuel System Operation and Common Repair Issues Part 2 to read the next part of this writeup.
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