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Jaguar V12 AAV/Idle Adjustment Screw

Jaguar V12 AAV/Idle Adjustment Screw

The picture below, provided by Pascal Doxarve, shows the Auxiliary Air Valve on the Jaguar XJ-S V12.  The AAV contains a piston that moves under the influence of a wax bulb, much the same way an automotive thermostat works.  The AAV is mounted on top of the left rear coolant manifold, putting it directly in contact with coolant coming out of the left bank head.

The movement of the piston uncovers an opening that permits air coming in via the hose at left center of this picture (from the left air filter housing) directly into the left intake manifold via the hose at top center of this picture, thereby bypassing the butterflies.  The balance pipe across the top rear of the engine allows air to the right bank so the car runs on all 12 at idle.  As the engine warms up, the piston moves to cover the opening and reduce air flow and bring the idle back down to the normal warm idle speed.

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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