Subject: '90 Plymouth Laser Turbo
Question: Car fluctuates at idle 900-1400rpm. Runs fine with air conditioner on. Installed used idle speed motor, did not address the problem.
Any clues?
Answer: Jay,
The idle surge you are describing typically is caused by the idle speed control motor (ISC). If you want to try some other tests of your ISC or surrounding components that might be causing your idle surge, check this web page:
http://www.vfaq.com/FAQlocator-intake.html

It may be possible you have a leak from a dried up O-ring used for your base idle setting screw (BISS). The web page I provided should give good details to check all possible causes.
Good luck!