While trying to troubleshoot why my car died a week ago - I found that someone had turned on the AC system even though it doesn't work. When I shut it off, the idle SEEMED (can't be sure) to improve. Could that person having turned it on caused the car to run poorly?

It is my understanding that the IAC valve supplies more air to the engine to bump the idle up when the AC is running. Have you cleaned out your IAC lately? It should be done whenever you clean your throttle body and flame trap.

But in general, no, running an AC system that isn't charged up won't damage the engine and shouldn't normally make it stall out. If the condenser seizes up, it would cause some problems with the belts and possible with the power steering, but if your '90 is like most '90s with a old and tired AC, the worst that would happen in normal situations is that you wouldn't be getting the gas mileage that you should when the AC is turned off.

If the compressor is locked up, and someone turned it on, it could cause the car to die. More likely, the belt would slip, and you'd smell or hear it.
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1991 245, 61k miles, looking for a 5 speed 92-93 245 cheap.