Heres a new one for you guys

When I was driving home from work tonite, My XJ started idling really high, And I was wondering what was up. For some reason I then turned off the defroster. The idle went back down, Does anyone know what controls the slight engine speed up when you turn on the Defrost or A/C on? Whatever it is I need to fix it.


Also it sounds like it is missing a fire or two when it idles...Ideas?


Thanks

when you turn on your Defrost or A/C, the computer automatically raises the idle to compensate for the extra compressor load ( the A/C compressor runs in the Defrost mode to draw the moisture from the defrost air so you don't get moisture on the windshield in the winter!)

Well its giving it too much to compensate. Sticky IAC motor maybe?

Could be an unplugged/cut vacum line, that is actuated when the AC is turned on. The vacume plug to the top of my heater A/C controls came a little loose (after/during a radio installation). Ran my idle up pretty high and messed with the air duct switching.
Made for a rough idle also. Cut a vacume line to the vacume resivoir, same as above. Messed with the cruise control also.
Mine idles up some when the A/C is turned on, 100-200 RPM, not much really. But idled up something to over a 1000 with a vacume leak, the 02 valve usually compensated and brought the idfle back down somewhat, but made for a rough idle.

sounds like ti could definately be a cut/disconnected vacuum line, perhaps even the one right one the intake manifold

(depending where the break in the line is) you should be able to hear a hising/suction noise where the break occurs

Alright, I'll get out the can of choke cleaner and check it out.

Listening for a hiss, a visual inspection or the old suck test on likely vacume lines, will probably work better than the spray. Most of the vacume line leaks, I´ve found, were with my eyes or my fingures, during a systematic check of most all of the lines. A finger run along the bottom of the vacume lines, will let you feel chaffs, cuts or rotten rubber. Some day I´m gonna buy a hand vacume pump. I´d pull a vacum test (suck test) on the vacume lines running into the passengers compartment, with the heater controls in the defrost position, will probably help you narrow down the problem. If it´s affecting engine idle much, it´s a fairly large leak.
Most of the vacum leaks, I´ve found were at the plug for the vacume lines at the heater controls. The vacume lines running to the vacume resivoir/canisiter (down the right side under the battery) or along the firewall. Sometimes the plastic lines on the motor itself.
I´ve seen numerous intake manifold leaks, but they rarley affected the heater controls and seemed to change with motor temperature.
Most of the vacume leaks I´ve found, didn´t cause the motor to remain at a high idle, but it would periodically idle up high and the RPM´s would change some at idle (up and down).