Problem: Only with the A/C on and when I'm rolling the Idle on my 90 VNT Daytona hangs around 2200 rpm then slowly comes down. I reset the idle and cleaned the AIS. No vacum leaks detected.
Solution: ? I need ideas on this one.
I have had this happen on my vehicle as well and it is so intermittant I never bothered to look for it. I attributed this to a controller glitch from the factory.
You need to check that the throttle and/or cables are not binding,that the AIS is not binding or sticking in the bore and that the controller actually sees that throttle position has changed.
With a scanner AIS counts at idle will be in the 10-15 range(if base idle is set correctly and AIS has been resynced with the controller),when you come off of idle the controller sets AIS for decel and you can watch the steps increase as you are driving,when you release the throttle you should see TPS volts drop and AIS counts decrease.If the AIS counts do not decrease you have an electrical problem,if they do and the idle remains high it is mechanical.
The controller has no idea where AIS is and assumes it is correct and will only set a code if the idle is extremly high or low and cannot be corrected.
When my vehicle acts up which may be once every 2-3 months the idle could go either way.Sometimes it will stay high and other times I will leave work and stop at the first light and it may want to stall.Thinking back now this is what happened when my Baro Solenoid failed many years ago. Every so often the idle would go nuts but not often enough or long enough for me to diagnoise anything then one day about 1 year after the initial symptoms started the check engine light came on and it set a Baro code, I replaced the solenoid and the problem was gone.
I do not know if this is your problem,it could very well be a solenoid or sensor is failing but do not go throwing parts at the problem, check all the basics first, if the problem gets worse and the controller cannot bring the idle back in range it will set a code which is probably what you want so you can isolate the problem.
If you have access to a scan tool or AIS/IAC tester you can command the AIS in and out to be sure it is functioning properly.
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