1990 Plymouth Laser - Dies at 1200 RPM

My 1990 Plymouth Laser Turbo idles at 2100 rpm when warm. Bought it new and it is still a very fast ride at 106,000 miles. The idle speed increases as the engine warms. Turning on the AC brings it back to normal. So I leave the AC on in town after it warms up and as long as the compressor runs, it will idle at stop lights just fine. Took it to a dealer years ago, no fix but a big bill for diagnosis. No warning lights are and mileage does not seem to be effected much. I can live with it since it is a manual, but I have wondered what the problem could be.

The first thing I would check if there’s any stored DTC’s.