[Tcoat]

Originally Posted by paulca

My car has developed an unstable idle when the aircon cuts out. Here are the symptoms.

AirCon OFF: 650rpm stable as a rock.
AirCon ON:
- Compressor pump off: 650rpm stable as a rock
- Compressor kicks in: 800rpm stable
- Compressor knocks off: idle drops to 500rpm or below, engine labours, shakes, then stablizes again to 650rpm. Sometimes the load sensor panics and revs the car to 1000rpm in response to the idle drop, but usually it doesn't. Has caused me to stall the car twice now and it's annoying.

If I rev the car with the Air Con off to 800rpm and the release the pedal it settles down normally to 650rpm. It is only when the A/C pump stops that the idle falls too low.

Any ideas? It's really, really annoying as without the aircon the windows steam up pretty quickly here at this time of year, but with it on, the pump cycles on for 2 or 3 seconds 4 or 5 times a minute. I have taken to turning it OFF while waiting in traffic, but that is not a solution.

This is not unusual for a small displacement engine since the compressor does take a fair bit of power to kick in. The surprising part is that you just noticed it. I have had several cars that did this and it is never an issue and there is nothing that can be done about it.