Jaguar V12 AAV/Idle Adjustment Screw
Members of the discussion list will confirm that the AAV is a common source of trouble, with the piston getting sticky or jammed. It's not intended to be rebuildable, but several owners have found a way.
There are also some indications that there are differences between AAV's fitted to early vs. late cars and UK- vs. US-spec. Some may provide more air when cold than others. Clearly, if you want your car to idle when cold as intended, you need to get the correct part for your car.
The bolt head protruding downward from the AAV is the infamous idle
adjustment screw. The bolt itself obstructs an passage within the
AAV, separate from the thermally-controlled piston. The farther this
bolt is screwed in, the more the bolt obstructs the passage, and the slower
the engine idles.
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