It's been going on for a bit...at idle, several minutes after start, the engine will rev and drop back (kind of what one would do at a stoplight to attract attention many years ago). The RPM's will peak at about 1800 or so, then will drop way back down, and this will go on for 30 or 45 seconds.

Now, if when this is happening I put it in gear and drive, it runs just fine. If I just let it idle for a minute or so, this behavior goes away.

Fuel pump is about a year old, sump has been cleaned out, fuel filter replaced.

1987 XJ-S V12, about 68,000 miles.

Thanks,

John

 

The AAV (Automatic Air Valve) on an XJS sometimes the Piston Sticks which causes the Engine to 'Hunt' and Rev up and down.

Hi JCR

Its sounds like it could be to do with your AAV (automatic air valve) if the piston inside gets stuck then that can cause the Engine to Start

'Hunting' Revving up and dropping back again.

If you haven't used the Car much, it 'may' just cure itself or you could try a drop of penetrating oil or some WD40.

If you unclip the Air Box filter on the side it is on and then take out the Filter, you will see the pipe that leads into it (up at the Windscreen end of the filter box)

But don't mess around with the rubber pipe that leads into it as they are Fragile and might break.

I will see if I can find a Photo but if all else fails 'Grant' has a Fix.

 

You can have it rebuilt, purchase a new one at SnG Barratt...$200, or go with a deletion like Grants....I still service the AAV units, but the price for the new one is not much farther from mine. Best, JW

 

A piece of coat hanger bent at a sharp 90 degree angle and the angled bit cut very very short can be inserted into the tiny breather holes in the piston to lift and exercise it. Combined with a bit of lubricant can get it working properly sometimes.

Pull the rear intake cap when trying trying to remove the hose. Treat that hose like its glass.

 

When I first got the car, about 5 years ago, I put a ton of work into it, including taking the AAV off, cleaning it, testing it and so on. Of course, that was 5 years ago and it's got to break eventually I guess.

So...when it warms up...I'll have another try at it.

Thanks!

John
1987 XJ-S V12

 

Make sure BOTH throttle housings a free of the infamous V12 Black Goo, that will mess with idle when at operating temp, mainly.

5 years without a clitch, frame the thing, that is special.

 

Got the same issue. Thanks for the ideas.