After several months of blissful "no issues", one has developed.

When the engine is warm the idle will drop to about 300 RPM then back up to 1000 over and over again. This only happen when the car is sitting in park.

I understand that some will suspect the AAV, but I took the AAV out (because it was broke) several years ago. After removing it I did the engine setup and very very carefully adjusted the throttle stop screws on each side. I know that's not ideal, but I didn't want to fork out the money for a NOS AAV.

So here is how things worked until this issue cropped up...

I replaced the AAV with an electric solenoid valve that I can control the the pax compartment. When the engine is cold (first start of the day in cool or cold weather). I open that valve. That lets filtered air into the balance pipe at the back of the engine. This stops the engine from dying after a cold start. When the engine temp gets to around 100 degrees F, I flip the switch back off and all is well. If I were to forget to flip the switch back off, the engine would remind me because (when driving) it would start to run unevenly. When it reminded me, I'd flip the switch back off and all was good. I've run the car in this way for three or four years.

Now, the engine will do the RPM jumping-up-and-down thing while sitting in park at idle.

My first thought is vacuum leak. My first plan of action will be to see if my "cold air start" valve is somehow leaking. But I thought I'd fish for additional input also.

Thanks!

John
1987 XJ-S V12
70,000 miles

 

I would definitely look for a vacuum leak. You can use the old tried and true method of using carb cleaner with a tube attached, or a propane torch with a rubber hose attached to a piece of brake line peened down to a small opening. Spray wherever you think there might be a leak. If there is a leak, the idle will change when you spray that area.

 

John,

Same Cold start as all mine.

That AAV went West on the HE's 20+ years ago, as did the balance pipe etc, still seem happy.
The PreHE was way before that, and still tearing up the Black Stuff.

I also suspect a Vac leak.
Inlet manifold nuts might need a touch up
That solenoid may not be closing 100%, doubtful, but what the heck
The throttle discs themselves may be coated in that V12 Goo, especially the B Bank if you are still running a PCV system, I dont.
TPS is out of range, again doubtful, but its on the list.
CTS is have a "moment", they can and do, so $20 for a new one (generic), and check the wiring inside the plug/boot.

There's a start, have fun.