M820 idle creeps higher once engine warms up...

[ksev62]

Ok, I've read soo many threads about NHC 250 idle or rpm issues but I've not seen the one I'm having. I'm a 30 year aircraft mechanic so I'm pretty handy and have decent deductive skills, so using all my resources available I'm still stumped. I resurrected an M820 that had been sitting a long time, fuel tank drain, cleaned, repaired and red coat, fuel filter replaced, lines between filter and tank cleaned, hoses replaced, prime pump seals replaced, plastic tubing replaced, verified return at tank is unrestricted and free flowing (initially put return into a bucket to catch bad fuel) etc. Truck fires right up EVERY TIME and goes to idle, regardless of how long it sits...it holds prime. Here's the big issue. Once the engine starts to warm up at sustained normal idle the idle will start creeping higher until I have to shut it down around 1800 because it's still climbing. If I fire it right back up immediately, the truck stays at idle initially for a few seconds but starts to creep again. I did find that by lifting on the toe of the accelerator pedal with a bit of pull, it will return the idle to normal immediately. Once I release the toe of the pedal, throttle creeps higher again after a few seconds. Keep in mind, all of this applies only when the engine is warm, when it's cold there is zero issue. I have not checked throttle linkage rig yet but have made sure the hand throttle is not causing the issue. What baffles me is, if it's a linkage issue why doesn't it happen until the engine is warm? Could the throttle return spring have issues when hot? Is there an interal pump issue? Sorry this is soo wordy but I was hoping to give all the details I can up front so I can hopefully rule out the sucking air etc type suggestions, I'm very confident in the fuel supply system integrity. Anyone ever see this issue before or have any troubleshooting ideas? I'd truly appreciate the help in figuring this out! Thanks!

 

[KRedd]

I have had the same problem on a M939 series 5ton. The M939 and M820 both have the NHC 250 engines. My problem was the spring weights in the injector pump being too weak and with time allowing the idle to increase almost to explosion. Hope this helps.

 

[WillWagner]

First, remove the throttle linkage from the arm on the fuel pump, leave the return springs on the arm, and start it. Let it run to see if the issue is still there. If not, problem solved, you need to adjust the linkage. If it's still there it is time for a pump. The most common cause for this issue is high throttle leakage OR, someone has messed with the throttle travel adjustments, which also leads to a trip to a pump shop. What was stated above will not cause a slow rpm increase as the engine warms, it makes the engine start and run at an rpm where the governor spring lets it go, the fly weights do not have springs, they contact the governor spring.