idling problem

My boss has a 1978 jag xjs v12. He says that at idle it will rev up then drop back down to idle speed then rev again. It does this continuously. Note: I'm not doing this for my boss (he left the poor girl in a barn until the interior was covered in mold) But he might sell the car to me cheap and I want to know what will fix it. hee hee hee /ubbthreads/images/graemlins/devilgrin.gif

It might be as simple as the butterflies being out of adjustment. Disconnect the rods, clean the throttle bodies, and set them to .002.

This cycling at idle speed is caused by the of the ECU being signaled that the car is coasting at slow speeds and the logic unit then cuts off the gasoline to the fuel injectors in an effort to save gas during coasting in gear. when the idle speed drops to normal the cycle then starts again and the idle speed again goes up. Either your butterfly valves are out of adjustment causing a fast idle or the micro switch at the throttle cable is out of adjustment therefore signaling the ECU that the car is coasting. The units should be checked for adjustment. Also throttle linkages on either side of the engine having loose joints or connections might cause this response. I think that the XJS collection of technial tips, etc., under the Jaguar Lovers Forum covers this phenomenum and the adjustment remedies. Hope this steers you to the solution.

I've been dealing with my 1984 XJ-6 for the last 14 years; The same "idle hunting" symptom can be caused by the air intake hoses leaking air in any of the clamped connections as Jack mentioned, especially at the 90 degree hose to the throtle body/butterfly. a bad vacuum connection (vacuum leak) can also cause the "hunting" up and down. Sure, mine is not a 12 cylinder, but the symptom is the same. 12 cylinder engines are known to require new vacuum hoses frequently.

thanx guys I'll tell him but he probably will sell it. He doesn't seem to want the cool cars he has anymore. Lucky me.