I've searched the forums, and can't find anything specificly like this. My car idles high (1300rpm) even though I have the air screw fully closed and the idle screw out. Once the car is warmed up, the idle surges between 900 and 1300 rpms unless I turn on the vent or turn on the headlights. When I use an accessory (vent fan on setting 3 or 4 or the headlights), it idles back to 1300 rpm without surging. My boost guage reads -19psi at idle and usually (when warmed up well) folllows the 10-8-10 boost pattern, so I don't believe I have a boost leak. All of the forums I've read surge when you do use accessories, but mine is opposite. Any ideas?

 

I would check your idle speed control valve (ISC) to see if it is hung open for some reason or malfunctioning. This will cause the idle to surge.

 

Where is my ISC idle speed control valve and how do I check to see if it is working?

 

The ISC is located on the underside of the UIM near the fire wall. You can remove it without removing the UIM if you have a small rachet and socket set. What you will see once it is removed is a bellows-like valve. The bellows should push the valve (shaped more like a small hat) into the opening when it is in the closed position. If debris is wedging it open , then you will need to clear the debris. Be careful as this thing is just plastic and if you ruin or alter the function of the bellows, the ISC is like $400.00 to replace. You can try applying a 12 volt source to make it function, however, I suspect it is more complicated than that because the opening is varied during operation by the ECU. You may need to use a reostat (dimmer) to vary the voltage. You may also want to consult your shop manual.

 

Manual says if idle goes up and down to check pcv valve, spark plug, throttle sencor, solinoid valve, intake air leak.

 

Meh, check the tps

 

I checked the pcv valve, and it only allow air flow in one direction. I also checked the tps and the voltages are correct. I'll try the ISC next and post an update...