My 12 cyl. 89 xjs when warmed up, engine idle speed races up and down.When I put in gear,idle speed stabilizes.Anybody out there have this problem?Thanks,HarryK

 

The USA market cars do have some emission items that can cause this. I know little to nothing of that stuff as our cars are emissions "clean" of that junk mostly.

Common V12 issues that cayse what you have are:

1) AAV is NOT closing 100%.

2) You got a/many vacuam leak/s.

3) The TPS is set out of range.

4) The throttle discs and the rods etc are NOT set in synch, and the discs inside the throttle housing are clogged with with the V12 black goo.

Being as its a Marelli car (or should be), there may be some oddity with the engine management system causing this problem, but I seriously doubt it. The usual cause is the simple things I mentioned above.

 

My 89 did that when I first got it running after sitting for quite some while. I took the auxillary air valve off, put it in a pot of boiling water, watched it work, then cooled it, to watch it react. Sprayed it with penetrating fluid, worked it in the water several times, put it back on, and all seemed well. I checked for vacuum leaks using carb cleaner. The only one I found, was the little plastic elbow that is in the line that leads from the AAV hose to the air cleaner on the left side. That little elbow was part of the vacuum that keeps the air purged in the distributor.
It won't, can't really effect idle, but sure did make quite a hissing sound.
A easy test of the AAV, is to take the cover off of the left air cleaner, and put something over the hole where the AAV feeds, see if the idle stablizes.

 

The idle speed hunting can also be caused by the adjustment of the microswitch mounted to the bracket on the accelerator cable housing where the cable mounts to the bracket for the "turntable"

 

I have attached a photo of the switch. Not quite where I described it. With the engine off, grab the turntable and just start to rotate as if the throttle cable were pulling it. Listen for the switch to click. When correctly adjusted, it should click as soon as the turntable starts to turn.


If not, loosen the jam nut and adjust the screw a 1/4 turn at a time until the switch just clicks as the trurn table starts to open the throttle body.