ANOTHER "my '96 GMC idles high" thread...

[cam.man67]

Man, I'm getting fed up with this truck. After chasing high idles issues for two years now...the high idle is back (it does periodically go away)!!! Or I guess I should say, never really left! @&$#!

Here's the symptoms:

After warm-up, the truck will idle at 900 in P, 750 in D. Not terrible, but this only happens after the truck gets shut off (after warm-up) for a period of time. In other words, if just left running, the choke never comes off and the truck will idle at 1500 all day long.

In addition, after shutting the truck off and restarting it, the truck immediately zooms to a 1500rpm idle, and will stay there for about 10 seconds before coming down to 900. If I put it in D before the idle comes down, it will not come down at all for the length of my trip.

This is all without a/c. With the a/c on, however (no matter if the compressor is cycles on or not), the truck idles erratically between 1000 and 1200. If I then go to put it in D, I can feel the truck accelerate against the brake. The truck will, if I let off the brakes, accelerate to 25mph without me ever touching the gas.

Trippy, right?

Here's what I've done in the past two years, all to basically no effect):
-new timing chain
-new plugs (2x)
-new cap, rotor (2x)
-new plug wires
-new thermostat
-new radiator
-new TPS (2x)
-new IAC
-shop-checked for vacuum leaks (2x)

All this has done nothing but lighten my wallet. The truck has no codes, no warning lights, and runs fine other than the stupid idle. No loss of power or anything.


I'm honestly wondering whether I'm going to have to replace the ECU at this point. I can't figure out what else could possibly cause this weirdness. Is there anything else you'd recommend I do before that point?