Engine/Transmission (1994 - 1998) 12v dies

[SLangegger]

I have had a stalling problem with my 12v. Sometimes the truck runs perfect with no issues. Other times, it acts up. The first symptom is a loping idle, soon after this happens, the truck acts like its running out of fuel and dies. After a long cranking time, the truck restarts and runs fine for a while. Sometimes a 50 miles, sometimes 100 ft. The truck has no cold starting issues, so I dont think its losing prime. Possibly the prefilter? Sucking air? I have replaced the overflow and changed the fuel filter. The only mods are 3k gsk and #6 plate. Any ideas? Suggestions?



Thanks in advance

Steve

 

[JFaughn]

Check the idle speed [ 850 RPM AC on ] , fuel pressure , new fuel lines/hose's [ pressurize the fuel tank - 3 lbs. - to look for leaks on the suction side of the fuel pump , hose's & lines ] , then throttle linkage /springs .

 

[Joe G.]

It's probably an air leak somewhere. Email me at -- email address removed -- for a copy of my article about the fuel supply system, how it works and how to fix it.

 

[SLangegger]

JFaughn,

Idle speed is good. I have replaced throttle return springs, all linkages seem to be working properly. Truck will die crusing at any rpm, not just at idle. Fuel pressure is next check, then lines. Thanks.



Joe G,

Email sent. Thanks



Steve



Edit: Installed F. P. gauge. 25psi at idle, 28-30 at 2000rpm. Im going to see if I can get it to act up later today. I will post back gauge reading.

 

[JFaughn]

With dying at any RPM , I would lean to electrical , fuel solenoid & or relays , solenoid linkage .
And still fuel lines & hoses .

 

[SLangegger]

JFaughn,



Thanks for the help. I will check those things out.

 

[bdcraig]

Does it make a pecking soung when it starts acting up? If it does it is probably the inj. pump.



Brian

 

[SLangegger]

BCraig,



It dosent make any strange noises. Just acts like its out of fuel.

 

[JFaughn]

Fuel pressure test .

 

[SLangegger]

I did a fuel pressure test. 25psi at wot. It gets pulled down around 15 for a few seconds right off idle when under normal acceration, but recovers quickly and holds a constant 22-25psi.



I replaced the rubber hoses going to the injection pump and the one from the return. They looked dry rotted, and I thought I found the problem... . it did not help. Im going to remove the fuel heater assembly, and replace the curved hose tonight.



If I had an air leak, what kind of pressures would I be seeing? What pressure is the minimum pressure for the P7100?



Thanks in advance,

Steve

 

[JFaughn]

I do not have the spec. s at hand , but that seems to be your problem , as best as I can remember 28-36 lbs , find a point on the return , slowly pinch off [ thats what the overflow valve does {weak spring}] , if pressure does not go high fast , then you could have a weak lift pump .
If both of these parts have not been replace , then do both , they last about the same .

 

[Joe G.]

Those fuel pressures are ok. Certainly will not cause dieing. When my overflow valve failed the engine ran fine down to 6 PSI. When I got home it would not start with that low of fuel pressure, but it ran fine. Even pulling the 9% grade on the way to my house. I NEVER advise throwing parts at a problem with the exception of the fuel hoses. If they are stock they will eventually fail. Find out what the problem is before you fix it.



If you have an air leak it will lower pressures. If the air leak is not too bad it will still run. Maybe die once in a while. A bad air leak will kill it so it won't start.



This could be a problem with a clogged pickup screen in the tank or a poor connection of the hoses to the fuel module in the tank.

 

[SLangegger]

Joe G,

I removed the fuel heater per your instructions. I cleaned the prefilter, and the bowl. I also replaced the curved hose. It made no difference. The next step I'm going to do is drop the tank, replace the rubber hoses, and inspect all the connections. Also, I noticed the flared connections (I think they are some sort of quick disconnect?) between the steel braided lines and the hard lines, near the engine, on the frame rail looked a little shaky. I asked Cummins about these, and they didn't know anything about them. Are these fittings known to leak too? I think you need a special tool to disconnect them?



Would there be any disadvantage to completely eliminating all the hard lines and all the steel braided lines, and replace with one length of rubber hose?



JFaughn,

I will try pinching the return line and watching the F. P. gauge tonight. I'm at the point now where I am HOPING its a bad lift pump, just so I can pin point the problem



Thanks again for all of your help. Your advice is much appreciated.

 

[Mhannink]

hes getting fuel so it should run fine. i would be looking at the fuel solenoid. if it cuts out at any rpm it may be losing voltage droping the arm and cutting fuel

 

[JFaughn]

The spec. s for fuel pressure are low , spec. s are spec. s [ a fuel pressure test is a test , below spec. is a time to fix or replace ] you may have more than one problem , if you ignore one it will make finding another harder , if there is another .
To check for leaks in lines or hoses , put 3 lbs. air pressure in tank then look closely with flash lite & mirror for fuel .

 

[SLangegger]

JFaugh,



What is your preferred method of pressurizing the tank? How do you control/monitor the pressure? I don't want to put in to much.

 

[CSchafer]

Mine did kind of the same thing on the highway yesterday just died. Poped the hood and found that the fuel solinoid needed to be pulled up with the key on. I tied it up and no more problems. FYI my overflow spring broke and gave me 5psi at idel and 9psi at 2000rpms and I didn't now anything was wrong I just happen to check my pressure. Tie up your solinoid and go drive it. It's a free test.

 

[Joe G.]

You can replace all the lines from the tank to the fuel heater/pre-filter assembly with a hose. Also the return line from the steel line on the IP to the tank. Others have done it.



As far as fuel pressure is concerned, the test is to see if there is good fuel pressure at the input fitting to the IP. If there is, then the problem is probably something wrong with the IP. If the pressure is low then the problem is in the fuel supply system. Not necessarily the LP or OF valve. Any small air leak will result in some loss of fuel pressure. The engine will run fine with lower than expected fuel pressure as long as there is no air in the fuel. It might starve at high PRM or something like that with low pressure, but just running down the road empty it will run ok. 300 miles of sweating low fuel pressure when my OF valve failed showed me that. Air in the fuel is a different story.

 

[JFaughn]

JFaugh,

What is your preferred method of pressurizing the tank? How do you control/monitor the pressure? I don't want to put in to much.

Get and old fuel filler cap & a shrader valve , auto parts store , maybe as AC part , drill a hole in center of cap , screw in seal and glue as needed , then fill & check with gauge , like filling a tire , if the leak is bad enough , use a regulator & leave air connected .

 

[SSkinner]

It would be worth checking to make sure the fuel shut off solenoid is pulling up all the way when the truck is running. This is rare be a commonly over looked item. When truck is running try to see if plunger will move further into solenoid. If it does, it's a good possibly it's the problem.