M931A1 - engine overspeed issues (RPM shoots up when put in neutral)

[zguinness]

I haven't had a chance to mess with this 931 for a while but I'm going to give it another shot. Here's the problem I'm having:

When I first start it up everything is fine. If I leave it in neutral the RPM stay steady. Now if I put her in gear at all and then drop it back into neutral the RPM shoot up rapidly. I have to apply the brakes and drop it back into gear to get the RPM to drop back down. Basically, once you've put the truck in gear it's impossible to put it back in neutral with the engine running or the RPM go crazy.

If I shut the truck down and attempt a restart soon after, I have the same problem. I start the truck in neutral and RPM shoot up unless I quickly drop it in gear. If I let the truck sit for a longer period of time and start it up the RPM stay normal.

Someone mentioned checking a spring on the throttle linkage... everything appears to be normal. I've had someone sit in the truck while I watched everything and I can see the linkage move when the truck is shifted to normal but the engine still revs up.

Is it possibly a fuel line leak... or something else that I'm not thinking of?

 

[doghead]

Unless you have recently replaced the fuel lines and rebuilt the tank selector valve, I would say you have an air leak.

 

[R Racing]

Unless you have recently replaced the fuel lines and rebuilt the tank selector valve, I would say you have an air leak.

Agreed !!! My 250 cummins use to do it till I replaced my bad fuel lines !

 

[zguinness]

Maybe I'm just dumb but why would it only happen with the truck in neutral?

I appreciate the responses!

 

[doghead]

In gear, there is drag.

 

[zguinness]

So check all lines from the tank to the engine?

 

[doghead]

Replace all rubber lines.

 

[zguinness]

Thanks for all your help. I really do appreciate it.

One last question (maybe): Is there a good place to buy the lines already made or do you have them made locally?

 

[doghead]

It's just rubber fuel line.

 

[zguinness]

Sounds easy enough. Hopefully everything will be right again if I replace the rubber lines.

Thanks again for the help!

 

[WillWagner]

Get a couple of lines for the supply and return, run them into a bucket of clean fuel and see what happens. If the issue goes bye-bye, the issue is in the delivery or supply circuit, the lines like recommended above. If it still happens, there is too much throttle leakage in the pump.