Surging while warm: Cold Advance Switch?

[TonyS.]

Upon startup my 6.2l 1993 GMC G3500 has been idling high and surging every now and then lately. We haven't had any cool nights here yet. After a few minutes the idle comes back down to normal and everything runs fine. Also, possibly connected?, sometimes the gp relay(and light) doesn't come on when turning the key. After sitting for a little while I would think the gp relay would come on for starting.... I doubt the engine's still warm enough where it wouldn't be needed. Could these be related to a bad HPCA sensor or cold advance switch?

[TonyS.]

Ok, I finally found the answer. I'm going to t/s this weekend. This is a great site and I thank the owners and contributing members for their time and effort. Cheers!

[More Power]

Upon startup my 6.2l 1993 GMC G3500 has been idling high and surging every now and then lately. We haven't had any cool nights here yet. After a few minutes the idle comes back down to normal and everything runs fine. Also, possibly connected?, sometimes the gp relay(and light) doesn't come on when turning the key. After sitting for a little while I would think the gp relay would come on for starting.... I doubt the engine's still warm enough where it wouldn't be needed. Could these be related to a bad HPCA sensor or cold advance switch?

For the benefit of others...

The "cold advance" switch is screwed into the lower rear corner of the passenger side cylinder head. It's supposed to activate when the engine is 95 degrees F or colder. When activated, it passes 12v power to the HPCA solenoid in the injection pump, which raises housing pressure and advances the injection timing 3-5 degrees. As a consequence of advancing the timing, and the activation of the high-idle solenoid, the engine rpm should elevate 100 or so RPM above a typical warm idle. The combination of more timing advance and a faster idle help reduce the amount of white smoke at a cold start.

The glow plug controller incorporates a couple of thermistors (temperature sensitive devices), which control activation. If the glow controller is above about 125 degrees F, there might not be any glow cycles. A normal and healthy 6.2/6.5 engine and fuel injection systemm don't require glow if the engine has some retained heat.

Those engines that won't start when warm without glow may need a new fuel injection pump or a fresh set of injectors. The fuel isn't atomizing correctly, and the engine won't start well without glow.

Jim