engine runs smooth when A/C is on but as soon as you turn it off the idle starts becoming rugh?

Hello there, I have an 1989 325is manual transmission it has been one of the best cars I have ever owned, engine runs smooth when A/C is on but as soon as you turn it off the idle starts becoming rugh to the point that the engine light comes on and off at times happens whe you are in a complete stop, can you give me an idea what can be happening here I will appreciated it.

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[Kerry at Pelican Parts]

I must admit I am a little baffled! Usually the additional load of turning an AC compressor makes the idle drop or unstable but in your case it is the opposite. When you remove the load of the AC compressor the engine runs poor. I would need to know is the car running rich or lean? Is it surging or loping? Take off the oil cap and see if the engine runs better with the AC off. This would be creating a vacuum leak. If the car runs better then you are running rich. While it's idling spray carb spray down the air intake. If the engine speeds up and runs smooth then you are lean. I hope this helps

- Kerry