Just recently bought a V12 XJS
.Drives really well but once warmed up you wouldn't want to be waiting at the traffic lights or do small manoeuvres. The idle becomes very low as you touch pedal and sometimes stalls.
Just looking for any help.

I was told possible aux sir valve but the idle drops when warm not cold. I adjusted screw anyway and is fully out but didn't make any difference.

 

AAV only governs idke at cold...unless it leaks then you'd see it spike to 1500rpm then cut back.

Sounds like an imbalance in air intake, probably other issues, throttle position sensir out of whack, O2 sensors gone, or other temp sensor not right.

First things would be setting butterfly plates properly and checking TPS output. See sticky notes at top of forum for walk throughs.

 

I would make sure someone has not plugged/restricted the AAV intake from the backing plate of the B bank air cleaner casing.

Even if that AAV is jammed closed, adjusting that Bolt should alter the idle speed.

Once the engine is AT operating temp, that AAV should be closed, THEN that bolt adjuster is used to set teh HOT idle. Turning that adjuster when the engine is COLD to make any difference is masking other issues.

My thoughts:
AAV
Throttle bodies full of "V12 Goo"
TPS out of range, or dying.
Throttle rod adjustment incorrect
Timing is retarded
Maybe? a few vac leaks, small I would suggest.

 

What year is your XJS?

 

1988 model I bought it as a barn find. Not been driven for 10 years.

So I was told.

 

OOPS, the mud appears somewhat.

Still stands what I said.

BUT

NEVER start that engine unless you are prepared to let it get to near operating temp each time. After all that "sitting" there will be issues, and start, and stop, will compound things.

Obviously, if you start it, and the "brown stuff hits the fan", as in fuel pissing, coolant gushing, oil in strange areas, all of a sudden, SHUT IT DOWN, otherwise, let it run, smooth out, get back to what it was built for.