Another high idle question - 1988 3.6L

XJ40
1

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.

Thoughts?

2

I would start by spraying some brake clean fluid around the inlet manifold to see if a leak has developed.

3

I was planning to use my propane torch, unlit of course, and see if I get an uptick.

1 Like
4

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).

5

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.

6

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…