1987 XJ-S V12, transmission recently rebuilt and working fine. Original cooling system removed, replaced by an electric water pump and two electric fans.

I removed the supplemental air valve several years ago and am using it as a cold-start assist. When it's cold, I flip a switch and the valve opens to let filtered air into the crossover pipe, this really helps when it's cold out. After the engine gets reasonably warm I close the valve, usually when the coolant temp is 110 degrees F or so. If the ambient air temperature is about 50 degrees, it starts fine without opening the "cold start assist".

I've also removed the auxiliary air valve several years ago.

Anyway, I've had an ongoing issue with idle when warm. When I have my foot on the brake all is well, but if I shift into neutral at a stop light, the engine with jump back and forth between 500 and 1500 RPM. It doesn't do this when cold, but only when the coolant temperature passes 140 degrees F or so. I can't remember if it did this before I removed the AAV and SAV, but I think not.

Thoughts?

Thanks,
John
1987 XJ-S V12
75,000 miles

 

That sounds like an air leak. At idle, the engine is not supposed to exceed 1200 rpm. If it does the ECU cuts fuel, causing the revs to drop. Then the excess air starts raising the engine speed and the whole process repeats.

I don't know why exactly, but engine speed in gear is lower than in park or neutral. I suspect that the driveline holds it down somehow, but whatever the reason, that's will be why you're only seeing this out of gear.

 

How are you controlling the idle speed without an AAV?

 

I control the idle speed by adjusting the throttle stops and the throttle linkage arms using the method suggestion by Palm. When my AAV broke I couldn't find a NOS replacement, so I had little choice but to go this way.

I suppose the easiest solution is to not put the car in neutral, but there are times when I want to leave the engine running for a brief period with the car in Park, where it does the same thing as it does (of course) in neutral.

As I sit here thinking, I begin to wonder if my cold start hack might be the issue, maybe the supplemental air valve isn't really closed when it's supposed to be. I'll try blocking that off and see what happens, but willing to look for other solutions if anyone has ideas.

Thanks,

John
1987 XJ-S V12
75,000 miles

 

I agree that you have extra air coming in from.....somewhere.

Or your idle adjust method has one throttle blade opened more than the other

Cheers
DD

 

I see that one way to search for an air leak is to use either small amounts of propane or carb cleaner and listen for an engine speed change.

Is this considered a safe method?

Thanks,

John