Jaguar Repair: Idle adjustment

Question
I have an 84 Jag XJ6. How in heck do I adjust the idle on this thing? I have been unable to find the adjustment.  It idles at about 1300. I think it should be around 750 or so.

Answer
Hi Hillary,

Several things can make one idle too high so we need to start from the beginning.

First confirm that the throttle is closing all the way. It has whet looks like a idle adjustment screw on the throttle but that is only there to take the load of the throttle spring off of the throttle shaft. The throttle plate closes all the way except of .002" of the throttle butterfly. Just confirm that the throttle is closing all the way and the cables are not holding it open any.

The idle air is through a bypass port around the throttle plate. The air comes in to the throttle assembly from the front and is the larger of the hoses attached at the base. The air enters from an aluminum unit mounted on the intake manifold just forward of the throttle assembly. This unit is called the "air distribution block" which sits on the "over-run" valve.
Down in this unit is an Allen screw that is the Idle speed adjusting screw. Clockwise slows the idle down and counter clockwise raises the RPM.

To ID the hole and screw find the idle mixture screw in the air flow meter and then look over the edge toward the engine and down below.

If adjusting the idle speed screw does not correct the problem you need to start looking for some of the common causes that can make it idle fast.

1. Any vacuum leak (Start pinching off hoses to see if it is a vacuum leak from a hose including the large hose to the vacuum booster.
2. The over-run valve could be leaking and the only way to test it is to remove the hose from the air distribution block to the Aux air unit further forward and put your finger over the hole in the air distribution block to see if it shuts the engine down. If it shuts the engine down it is not the over-run valve leaking causing the problem.
3. The Aux air unit is next and it is controlled by a Thermal/time switch in the coolant manifold over the intake manifold. It allows extra air (RPM) for cold starting.
4. Check the ignition timing to see that something has not caused an over advanced timing that might raise the RPM at idle.

Those are the most common causes of a high idle. Let me know.

Howard