1990 6.2 hmmwv idle issues

[FreedomDiesel]

i am very green on these vehicles first off. my problem is that im having an idle issue. if i barely push on the throttle the RPMs will rise and stay there even after i let my foot off. if i put it in gear the RPMs come down but as soon as i go back to neutral the RPMs jump right back up. seems to get worst the warmer the motor gets. If i unplug the cold idle switch from the pump the motor runs great.

i have learned there are two coolant sensors in the engine, one for the gauge (driver side head) and one for the fan/idle in the coolant cross tube on the front of the motor. if i unplug the upper sensor the RPMs skyrocket and the fan comes on so im thinking its not the temp sensor.

any ideas on why my idle does this? if i have missed a thread somewhere please post a link.

thanks in advance!
Andrew

 

[papakb]

A little more information here would be helpful. You didn't say whether this was a new truck or a new problem on an older one. The early 6.2s used a glow plug controller (GPC) on the crossover manifold, after the introduction of the S3 control boxes that changed to a thermal sensor (THS) feeding back information to the controller. The GPC and THS are not interchangable and need to be matched to the control box you have installed. The cold start switch is a fairly common failure item that controls the amount of fuel being sent to the engine and can also cause this problem if it isn't working.

Kurt

 

[FreedomDiesel]

this is a 1990 that was picked up from ft hood last week. it had this problem when we got it. could the GPC or THS cause a high idle issue even when the motor is hot?

 

[papakb]

If the THS is telling the engine that it's always cold it could. The GPC probably wouldn't because all it does is tell the PCB the engine is cold and fire up the glow plugs when starting. See if you can determine which control box is installed in your truck and make sure things are properly matched up. On the older trucks it isn't uncommon to see the wires on the sensors starting to crack so you might consider replacing them preemptively before they fail. The neutral starter switch on the shifter falls into the came category. BTW, that switch is a commercial $15 part available at most auto parts stores. The only difference is the one you get there won't have the wires attached and you'll need to add them. Search around the various boards here and on the G503 site, there are plenty of cross references out there for military to civy part numbers.

Kurt

 

[FreedomDiesel]

which switch are u saying is 15? the cold idle switch?

 

[porkysplace]

You will find some help in these
[h=3]TM's for the M998 series[/h]

 

[FreedomDiesel]

thank you!

 

[Retiredwarhorses]

Cold advance will do nothing of the sort, possible a sticking accel pedal, or pump armeture. Change your fuel filter as well.
very very possible a bad IJ pump...disconnect the accel cable at the pump via the C- clip. Manually operate it from there.

 

[Travis23]

thank you!

Did you ever find the problem freedom? I'm having the exact same issue except the problem perisists even while in gear. The truck starts up and idles correctly (low rpm) but if you press the accelerator, the idle rpms jump to 1200+ and stay there until I kill the truck. It doesn't seem to be a stuck accelerator pump at this point, my guess is the cold start advance is staying on all the time somehow.

Anyone know of a way to check this?

Sent from my SM-G930V using Tapatalk

 

[Retiredwarhorses]

Did you ever find the problem freedom? I'm having the exact same issue except the problem perisists even while in gear. The truck starts up and idles correctly (low rpm) but if you press the accelerator, the idle rpms jump to 1200+ and stay there until I kill the truck. It doesn't seem to be a stuck accelerator pump at this point, my guess is the cold start advance is staying on all the time somehow.

Anyone know of a way to check this?

Sent from my SM-G930V using Tapatalk

cold advance does not work that way...it advances the timing. Do what I said above, disconnect the accelerator cable at the pump.

 

[Travis23]

Ended up being a bad throttle return spring. It didn't have enough life in it to pull the idle back all the way down every time. New spring and she's good as new

Sent from my SM-G930V using Tapatalk