My 1988 3.6L has recently began the following:
At start up, engine idles at 600-700 rpms in Park, for about 10 seconds, then jumps up to 1000-1200 (still in park). Once I shift into drive it drops back down to normal. At stop lights, normal rpms. But once I shift back into Park, it bumps up again. On occasion it will fall back down into the ‘normal’ range when I have put it back in park and I am about to shut it down, but it usually creeps back up to the 1000 range if I wait too long to kill the ignition.
I know there are a thousands paths to go down. No recent changes under the hood, or anywhere for that matter.
I would start by spraying some brake clean fluid around the inlet manifold to see if a leak has developed.
I was planning to use my propane torch, unlit of course, and see if I get an uptick.
Start from disconnecting supplementary air valve or putting the mole grips on the hose originating from it. Other thing - disconnect aircon compressor plug and aircon fuse and try again. Have you got drag during the ride? (Car not slowing down with foot off the acc. pedal).
I’ll give your suggestion a try this weekend. The high idle is sporadic. It comes and goes. Sometimes once I start the car, with nothing running (no headlights, no blowers…) it will take it 30 seconds and it eventually drop down to 600-700. Warm start, cold start, there doesn’t seem to be any rhyme or reason.
Now that sounds like idle control valve, it takes over only on neutral/parked. Nothing will bump up your revs that way apart from control valve really. The other thing could be fuel pressure, buth that’s rare…