T27 Torx Driver
Screwdriver or 6mm socket
Auxiliary Air Valve Gasket (Part Number EBC 8329)
Auxiliary Air Valve Hose (Part Number EAC 2655)
Auxiliary Air Valve Vacuum Hose Connector (90o)
Pot of water/stove/thermometer
Vice Grips or Vice
The car is supposed to idle at about 750 RPM when completely warm (coolant flowing through thermostats) and in “Park”. Mine was idling at about 850-900 RPM. The adjustment is a 13mm bolt into the side of the “Auxiliary Air Valve” (AAV) which sits on top of the left-side coolant rail, has a hose going into its top from the rear of the left-side intake manifold, and a hose into its “front” from the rear of the left-side Air Cleaner housing. Tighten the bolt to reduce idle speed, loosen it to increase idle.
Unfortunately for me, the bolt was all the way in, I was unable to reduce idle any further – time to check the AAV (also known as the “Extra Air Valve” which should not be confused with what is also sometimes called an AAV, but more properly called the Supplementary Air Valve or Solenoid Air Switch – that component feeds into the rear of the right-side Air Cleaner housing).
The AAV is expensive – about $400 for my vintage XJ-S at a couple of on-line parts houses, so it is worth attempting to fix it.
Use a screwdriver or 6mm socket to loosen the hose clamps on the hoses going to the AAV and carefully pry the hoses off (my top one was brittle and cracked). Remove the two Torxs bolts holding the AAV to the coolant rail. I did not have to drain any coolant, only a little came out of the rail.
You can read all about the AAV and how to test it in Kirby Palm’s “Book” (search for “Auxiliary Air Valve”). Once you thoroughly clean it up, do the boiling water test to make sure the valve is not stuck and closes all the way. Mine didn’t close all the way, allowing air to pass through to the throttle even with the idle adjustment bolt all the way in. When at room temperature, measure from the lip of the top opening of the AAV down to the top of the internal piston - it should be about 70mm. Mine was about 72.5mm. As per a suggestion in the Book, I carefully squeezed the bulb using a vice until it measured about 70.5mm. I did the boiling water test again and sure enough it closed all the way (it needs to).
When I took the AAV off, the hose going from the AAV to the Intake Manifold cracked. Motorcarsltd.com had one (their part number IN102126) for $13.17. Also, most of the gasket stuck to the coolant rail. Again to Motorcarsltd.com (their part number IN101388) for $1.16. I used a single-edge razor to scrape off the old bits and pieces of gasket, and reassembled things. Also, the hose going to the Intake Manifold has a tap in it under the manifold to connect to a vacuum hose. The connector is a standard white-plastic “L” (90o) vacuum connector. Apparently these break often – the one on my ’90 Coupe was fine, but sure enough the one on my ’91 Coupe was broken clean off. You should be able to get one from your local Part store, but if ordering from Motorcarsltd.com anyway, their part number is IN102278 for $1.53.
Be very careful when reinstalling the AAV Torx bolts – I snapped the one towards the front of the car seemingly because it had “bottomed out”. Fortunately, it snapped about an 1/8” above the gasket so it was easy to get it out. I stole a bolt from the ’91 Coupe (and now have got to find a suitable replacement) and added a lock washer. No leaks.
I now have hefty cold idle (about 1,800 RPM) with a smooth drop to warm idle at 750 RPM when the adjustment bolt is a good ways out. Next project…