(Solved) M939 (M934) NHC-250 855 Starts, dies, sits, then eventually runs fine?

[PyroJoe]

M939 (M934) NHC-250 855 1985-ish <9,000 miles on the odometer

Drove home 4 hours without any issue whatsoever, even towed a load behind it. Had normal power for the gutless NHC 250. Putted it around the yard for 2-3 years after that. Never drove it more than 30 miles a year since then. Today, it will fire right up, pur like a kitten, drive fine, but eventually, it dies. I can crank it all day long and it will cough with ether but refused to fire back up. I let it sit long enough and it fires right back up like nothing ever happened. I'm guessing I'm sucking air somewhere and the time I wait is the time it takes for that air to dissipate. Either that or the IP is getting hot and it has to cool down. I am finally ready to start working on this truck and this is the first issue to tackle.

On another M939 truck, I had to replace the rubber hose going from the IP to the fuel filter housing. My M934 hose looks fine, though, and doesn't show the same symptoms to that, at all.

I've re-brazed the pickup tube in the fuel tank on other trucks. Haven't done it here. Could that be the culprit?

The fact that time is the only factor in getting it to start normally again tells me it's either air or temperature related (i.e., it's not a shutoff cable being stuck, because that has nothing to do with time).

Edit: I eventually replaced the main feed hydraulic hose from the fuel pump to the fuel filter. The old line looked perfectly fine to me. After replacing, the problem persisted. By dumb luck, I noticed the adapter fitting between the pump and hose was a swivel type. It was just barely loose. I tightened it and no more problems!

 

[M35A2-AZ]

I would have to say it is getting air from some place.

 

[Trailboss]

Actually, it sounds like it's starving for fuel since it will run awhile, die, then not restart until hours later. Check for a clogged fuel filter or pickup tube first.

 

[PyroJoe]

Actually, it sounds like it's starving for fuel since it will run awhile, die, then not restart until hours later. Check for a clogged fuel filter or pickup tube first.

Forgot to mention, new filter throughout.

 

[Sephirothq]

did you try to bypass the shut off solenoid? I had one go bad when it was hot.

 

[PyroJoe]

did you try to bypass the shut off solenoid? I had one go bad when it was hot.

Would that just be unplugging the solenoid's electrical connector?

 

[Sephirothq]

no there is a screw to turn in on the side that will bypass the electronics. The only way to shut the engine down then is to use the emergency handle inside the cab to turn off a valve.

 

[PyroJoe]

no there is a screw to turn in on the side that will bypass the electronics. The only way to shut the engine down then is to use the emergency handle inside the cab to turn off a valve.

Oh ya... Been so long since I've touched one of these that I forgot about that. I had to mess with those to get some of them off the yard after auction.

 

[WillWagner]

If there is an issue with the FSOV, by passing will not isolate the hard starting issue ( screwing the thumb screw in ). Usually if the FSOV is FUBAR, it does not loose prime, it will just not start, unless there is a piece of debris under the disc. If that happens, there is a filtration problem.

If possible, hook a line directly from a clean container of diesel gas to the pump. Fire it, let it idle and goose the throttle a few times, then shut it off. After time, try and re start it. If it lights and stays running, the issue is in the supply, if it starts and dies, the issue is in the engine somewhere

 

[SSLT1KID]

I had a motorcycle that did something similar. The fuel tank was not properly venting, which was causing a vacume in the tank. Try driving it with the fuel cap off.

 

[WillWagner]

Plugged vent issues usually cause the opposite of what is being described. It will run fine cold and as the fuel is sucked, returned and heated in the tank when driving, a vacuum is created and actually creates a restriction in the system.
I have seen a plugged vent cause a truck and trailer to be towed and there was so much suction in the tank that even after the wait for the hook, traveling to the shop, there was still such a vacuum in the tank that it took a big pair of channel locks to remove the fill cap. When it came off, the tank popped!

 

[PyroJoe]

Funny. I was looking at the injection pump today and noticed the emergency fuel shut-off lever was roughly half-way up, rather than all the way forward towards the headlights. I pushed it all the way forward and now I haven't been able to replicate the problem. I should've left it alone till the next time it acted up. Not sure that was really the culprit, though, because why would it start fine - then die and not re-start. I'd think it wouldn't start, period. Maybe it's temperature related - who knows... I'll keep this thread updated.

I'll keep the cap in mind, too.

Edit: After disassembling the emergency shut-off solenoid to rebuild it, I discovered there is a spring and helical cut inside, which is supposed to force the ball valve to the Open position. You'll have no idea where "Open" is when looking at the valve from the outside, fully assembled. Where it "wants" to rest is where Open should be. The ball valve is very restrictive and overly complicated, and I'll be replacing mine with something better.

 

[PyroJoe]

I eventually replaced the main feed hydraulic hose from the fuel pump to the fuel filter. The old line looked perfectly fine to me. After replacing, the problem persisted. By dumb luck, I noticed the adapter fitting between the pump and hose was a swivel type. It was just barely loose. I tightened it and no more problems!