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M939 NHC-250 Fuel Issue

spartan_185

spartan_185

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Kiowa,Oklahoma
Hello Everybody,
I have a M925 truck that has a NHC-250 cummins in it that I am experiencing a fueling issue with. Whenever I press the pedal to accelerate and then let off, the engine continues to go up in rpms and/or is very slow to come back down to idle. This problem gets more severe as the engine gets warmer in temperature, almost to the point I have thought the engine was going to run away. I have fully pressure tested the fuel system and have found no leaks. I have even gone as far as to completely bypass the trucks fuel system and run a line from the injector pump to an auxiliary tank and I still get the same results. The truck has fresh fuel filter and new rubber hose throughout the fuel system. I am beginning to think that the pump is shot but I wanted to consult with the experts here before I took that leap. The truck has plenty of power and only smokes a little bit while under load. I know the usual problem in this case is a air leak in the fuel system, but for the life of me I cant find one, even when pressurized. Any help is appreciated.

UPDATE: I solved the problem. Check post #10 in this thread to see how I found the solution. It was not a air leak issue.
 
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Vintage iron

Vintage iron

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sounds like you have done your homework. good to see your checked all of those things first. As a diesel mechanic I would say the Injector pump is the next step. Buy a new one that was never run or have yours rebuilt. The cost will be about the same, but you can have your pump turned up when you have it rebuilt. Be careful about buying pumps online, some people buy them out of government auction and don't know what they have. It could be used, it could be for 400 Cummins. good luck and let me know if I can help.
 
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162tcat

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Did you check to make sure the throttle lever on the PT pump is returning to the Idle position? It can stick on the transmission modulator cable or the hand throttle cable. Try to grab it and pull it all the way back to see what happens.
 
Trailboss

Trailboss

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I've seen a lot of the M939 trucks with the hand throttle cable disconnected. I just assumed it was to eliminate the redneck cruise control, but maybe it was a field fix to a common problem? I'll have to reconnect the cables and see what happens. Sorry for the sidetrack.
 
spartan_185

spartan_185

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Kiowa,Oklahoma
I have checked all cables and linkages. They all moved freely and I even disconnected the hand throttle. Symptoms are still the same. I should add that the idle seems to go up slightly with the engine temperature. Does anyone know a good rebuilder with experience wit PT pumps ?
 
spartan_185

spartan_185

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Kiowa,Oklahoma
I adjusted the idle back down, but the truck still exhibits the same behavior. I guess Im gonna swap the pump. I have a pretty knowledgeable mechanic in the family that worked on PT pumped cummins back when they seen heavy road use. He seem to think its the pump internals also. I will update whenever I find a suitable pump and swap it in.
 
spartan_185

spartan_185

Member
246
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Location
Kiowa,Oklahoma
Alright folks, I got a good take off pump from jcappeljr and swapped it in. Unfortunately, the engine still produced the same behavior. I was pretty discouraged but I continued to go through my head for any more potential problems. I had also noticed the truck was taking longer than usual to shut down, and had a fuel knock upon start up. I had a pretty good feel for the theory of operation on a PT pumped Cummins, so I started to ask myself what might cause the rail to remain pressurized after the throttle was released or when the fuel was shut off to the rail after the cab switches were turned off. I came to the conclusion that there might be a restriction in the return line. I started with the return line check valve and that's where I found my problem. Its a little round cylindrical shaped check valve that is plumbed in with the return system on the side of the block. It turns out that it was sticking and causing a restriction in the fuel system. This in turn was causing the injection system to stay pressurized longer than it should upon acceleration or when you shut the engine off. I took the valve off and blew air into it. The valve is supposed to be a 2 psi check valve and it took much more pressure to open it. I took the fittings loose on it and worked the inner valve until it released pressure as easily as I thought it should. I put the valve back on and hooked everything else back up like it should be. I fired the engine up and I instantly noticed a difference in the idle and start up of the engine. No more fuel knock on start up! I let the engine build oil pressure and run long enough to get the air out of the system. I gave it some throttle and it increased rpm and instantly came back down as it should when I let off. I was one happy camper. I double checked everything again and we went out for a test drive. It made the thirty mile trip without a hitch and operated normally when we went back to idle. I guess the good news now is that I have a spare injector pump for future use. Sorry for the long post, but I would hope that this would help someone solve the same issue in the future.
 

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charlietango

charlietango

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Winnipeg
the exact same issue.. started today for me this afternoon seemingly out of nowhere. Truck had been sitting a few weeks, started it up and it ran fine most of the day. All of a sudden erratic idle and it would rev up slow, slow to return to idle. (been home a few days and gone for weeks at a time..sorry if people were waiting for me to get back to them btw. I will catch up shortly)
I will be checking this Monday before I leave again lol !. Thanks Spartan
 
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