Engine/Transmission (1994 - 1998) Help, Low Fuel Pressure

[BVamvas]

I have a 94 5spd I am trying to find a problem with. The truck is all stock (last one?) with 224,00 one owner miles. Always over maintained and no real problems till now. The truck starts and runs fine when cold and at lower speed. If you hammer down on the highway it will start tapping and dropping cylinders and missing like it is running out of fuel. It will stall if you push in the clutch. It cranks a while and re-starts and may run normal for a while only to do the same thing all over. When it acts up the throttle may stick on a little.



I have re-sealed the fuel heater, replaced the transfer pump, replaced the fuel lines, (both rubber and hard) from the tank and checked all fittings and hoses. I hooked up a pressure gauge between the filter housing and inj. pump and show only 12psi at idle and a high of 20psi on a light cruise. The gauge line showed signs of a slight leak when I returned so may not be real accurate.



Where do I go from here other than the dealer for a new truck? I am not convinced yet with all the problems on the new diesels. Thanks in advance for any help I may receive.



Bryan

 

[Joe G.]

It sounds like a bad over flow valve. Your '94 may have the kind that you can stretch the spring. The OF valve looks like a banjo fitting on the engine side of the injector pump about even with the most forward delivery valve. If the spring is not broken it should be a little over a half inch (. 550") long. If it is not, stretch it a little as see if that affects your fuel pressure. Be easy with the spring. If it is stetched too much it will be ruined.

 

[BVamvas]

Hi Joe,



Thanks for replying. I forgot to mention the overflow valve. I started there by replaceing that. Do you or anyone else know if the filter on the sending unit in the tank is replaeable by itself? Thank you,



Bryan

 

[Joe G.]

I'm a little suspicious about re-sealing the fuel heater. Have you taken it out for a test? You can take it out by removing the large screw that holds it in. Then just screw the pre-filter where the fuel heater was. The large screw is just an adapter to put the fuel heater in the assembly and allow the pre-filter to bolt up to the bottom of it.

 

[ABorchard]

The screen on the tank module is replaceable, a little over $20 from the dealer but snaps off and is easy to clean. Unless your tank is really full of crud the filter is largely self cleaning due to the fuel sloshing around.



Your fuel pressure is too low and will cause the problems you speak of but before throwing parts at it I'd make sure you are getting correct readings. If you have a restrictor on the gauge line like a needle valve or snubber even a slight leak can cause low readings. Without some sort of restrictor fp readings on a 12 valve are worthless as the gauge will pulsate too much.



Hate to say it but if your fp checks out good my next suspect would be the injection pump needs rebuilding.

 

[JGK]

On the other hand, while not common, the lift pumps do occasionally fail on the 12-valves. At 225K miles, I would not be surprised if your's is ready for replacement.



They are not real difficult, except for getting the rear bolt started. Be careful removing and re-installing a new one as the plunger rides a cam lobe and you don't want to drop it into the engine on removal or re-installation.



Get one from Cummins or even a Kenworth shop (for a Cummins 5. 9 - Kenworth will actually sell them cheaper then Cummins!). They are about $150 bones +/-.



Do a search in this forum.



-Jay

 

[Howard Durand]

I didn't see any mention of the filter and housing. Did you check for any kind of problem there?

 

[BVamvas]

Hello all,

Thanks for the help so far. I don't think it is in the tank also, I am on my third sending unit now. Just changed the transfer pump last week. I did not leave out the fuel heater but will try that. I did check all lines and replaced the fuel filter also, fuel is clean enough to drink. I did make up a new line for the fuel gauge and will test pressure again next weekend. The gauge was reading fairly smooth, about 1-2 lb. fluctuation.



I too think it may be the pump but don't want to put that on and have the same problem on the test ride. Is Piers the best source for a nice pump? What is the HP limit for the stock clutch?



Thank you, Bryan

 

[ABorchard]

Before you mess with the fuel heater if you feel the problem may be an air leak the easy way to pinpoint it if you have an air compressor is to pressurize the fuel tank with a air nozzle wrapped in a rag. No need to get carried away and blow up your tank, 5psi is plenty. Do it with one person on the air and another under the hood or get crafty and use a hose extension. I use the method so often that I drilled and tapped an extra fuel cap and installed an a male air chuck. I never use the thumb wear-out primer button anymore even on a routine filter change, 15 seconds of air pressure fills the filter and bleeds out the air.

 

[BOverton]

What do you look for once you have the tank pressureized?