Idle dropping low

[Dukeman]

Hey guys! Haven't been around in a while, mostly because everything has been running great and have been able to take care of what ever the big truck has needed but having a problem I'm hoping I can get some ideas.

Basically the idle drops down, sometimes to almost a stall. Starts up fine, idles great for a while then even when warmed up the idles drops to almost a stall. Doesn't stall, lopes for a couple seconds then idles back smooth for a while.

I noticed the fuel feed tube from the primary filter to the injector pump was old and cracked and dripping slightly so I had a new one made and replaced it. Replaced the main filter and the small metal filter in the top of the injector pump. Really thought that was going to do the trick, but no luck. I know there could be a ton of things that could be causing this and the truck is 160 miles away so I can't just go out and play with it.

Any ideas or thoughts would be appreciated.

 

[doghead]

Have you changed the fuel filter lately?

What about the other rubber fuel lines from for tank selector valve?

Have you ever resealed your tank selector valve?

 

[73m819]

Yep, sounds like still sucking air from a small leak (a $5 suck)

 

[M35A2-AZ]

Sounds like you still may have a little air leak. You could bump the idle up a little on the pump and see if that helps. Is this a NHC250 or 8.3?

 

[74M35A2]

Looks like a 250 truck.

 

[doghead]

Actually it looks like an M939 series truck.

Specifically an M931

NHC-250 motor.

 

[74M35A2]

Cute

 

[Dukeman]

Oh, and yes NHC-250 motor.

 

[Dukeman]

My first reply didn't post, hmmm, what did I do. Anyway, in response to doghead:

Yes, M-931, modified with a trailer bed and extended frame to accommodate the RV 5th wheel hitch.

Yes, changed the main fuel filter and the small filter inside the injection pump. I have not looked at the rubber lines going to the selector valve, that is a good idea. I have never resealed the selector valve, tell me about that please. Are there gaskets, o-rings that need to be replaced?

Thanks for the replies

 

[Dukeman]

Found the sticky on the selector valve rebuild. I will report back when I have a chance to tackle that project.

Thanks guys!

 

[WillWagner]

In pump screen filter is ONLY in case the pump grenades. It stops metal from getting to the injectors...it filters backwards, inside out and has a magnet in it, it has no effect on the way the engine performs. Run it out of a bucket to eliminate a chassis issue because a pump can cause the same issue.