idle

[lmchugh1]

I've been trying to figure out what's wrong with a 1985 6.2L truck. When the accelerator is pressed slightly the engine revs at an uncontrollable high rpm. To stop it I have to back off of the pedal. The truck idles fine but it surges when driven. Anyone have any ideas?
Thanks..Leon McHugh

[john8662]

Is this only when cold? If so, it could be the cold idle solenoid is way out of adjustment (I've had it happen). Just for test you could remove the green wire going to the solenoid on the injection pump. There will be two green wire connections on the pump. The one closest to the front of the truck is the fast idle (hooked to the solenoid).

Otherwise it can be several other things. Do you detect diesel fuel leaking on the injection pump, if you do this is also a source for air getting into the pump causing it to surge, and rev up, and then down.

[gavio]

I know this is gonna sound pretty bizarre, but maybe a fuel filter?

I had similar symptoms last year and went through everything - tore down the top of the IP, idle solenoid, shut off solenoid, throttle cable, the works and finally tracked it down by running the engine from a dish of fuel with various parts of the fuel system disconnected and lo and behold it was the fuel filter. I had ruled out a fuel filter 'cuz the one in there only had about 800 miles on it and on top of that it wasn't behaving like it did the last time the filter was bad.

Turns out that I got a batch of algae in the tank and it had made my filter get gummed up. Apparently the surging was caused by my overcompensating for the lack of power by pressing harder on the pedal whereupon a glob of fuel would finally squeeze through and it'd take off like it's tail was on fire.

Anyway, the moral of the story for me was "They really mean it when they say ALWAYS try the filter first!"

Maybe doesn't apply to you, but just thought I'd offer it up.

Good luck!

[DmaxMaverick]

Good advice on the filter. Always check that first, even if you just did it.

Is the problem only when out of gear? When you put it in gear, does it have power, or does it just fall on its face unable to reach converter stall?

It could be an air leak on the suction side of the lift pump, or the lift pump itself. Air in the system will throw off the timing enough to run high rpm's, but won't have much power. Did you change the filter lately? Could also be a leak on the fuel level sender/pick up line in the tank, or a bad connection or corroded line between the tank and lift pump.

If it idles normal when in gear and you can accelerate with normal full throttle power, then something has to be broken inside the injector pump.

Come back with more info and perhaps we can get it figured out.

[Robyn]

The most common reason for a runaway as you describe is a broken flex ring in the pump.
Take the IP off and take it to your favorite diesel pump rebuilder and have them bless it with some new parts and a good cleaning and a recalibrate and your little 6.2 will pur like a kitten again. This was common on the early pumps and can be a tad unverving if you are not sure what is going on. First one I had do it got real whizzeeeee and it was in town traffic. WHOAAAA thought I had a Bronco there for a minute "BIG SMILE" :0)
Good luck with your truck
\RC