Another family XJ has a problem. When the A/C cycles, the idle speed jumps to around 2-2.5K. This is especially troublesome when ones foot is on the brake. At idle when the A/C compressor is not engaged, the idle speed is normal, just under 1000. To the best of my knowledge, it makes no difference while driving. It is a 4.0 HO, everything is stock. A little old lady drives it, and it was supposedly well taken care of by the PO.

Can anyone offer an opinion on what might cause this?

 

So does anyone have any ideas on this one?

 

Maybe check and clean the idle air controller? Its the only way the motor could get more air when you don't touch the pedal.

 

That makes sense, I'll try that tomorrow. Thanks.

 

Does the IAC open up a bit when the AC kicks on to keep the idle speed up with the additional load of the AC? If that is the case, either the IAC is messed up or the AC is not loading down the engine.

 

I haven't had the IAC out yet to look at it, that will happen this morning some time. Is it supposed to open up when the A/C compressor comes on? And please define "loading down the engine" for me, I'm confused about that.

 

It increases the rpm by giving the engine more air. When the ac kicks on it will "bog" down the motor the iac opens up the throttle threw a bypass port bringing the rpms up. If this part is defective or extremly dirty it can cause all kinds of problems. The iac is basicly an electric motor with a plunger on the end that moves in or out to control air flow.

 

Thanks for the clarification, makes sense now. I removed the IAC, and it was indeed filthy. I cleaned it and the throttle body up. Is there any way to test the controller without putting it back in? Perhaps some kind of ohm range on a multimeter? Is the plunger movable, or should it be, when the unit is out of the TB? I guess the acid test is putting it back in and seeing if the same symptoms occur. Or, if all else fails, go buy a new one.

Eagerly awaiting replies...

 

Npt sure about a bench test but the plunger is supposed to spin.

 

It did spin without too much effort. I reassembled everything, that seemed to partially take care of the problem. I also found out that the shaft on the auxiliary fan is bent. That explains a vibration that I could not track down.

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