Erratic engine idle and operation

I recently had my AC repaired on 1988 mer sable. Everything is good except after car has been running a while in heavy traffic it begins to run very erratic at idle and normal engine operation is bad. I can keep car running with heel toe. When I let car sit awhile the problem goes away and car runs very nice. :confused:

A/C on, heavy traffic, stop & go, maybe it's starting to overheat? Engine cools down and it runs better until it gets hot again? Check engine temp, coolant & oil levels. I don't know if 88 had idle air control valves but they can cause rough idle.

generally the idle up and down is caused by the compressor on the air conditioner kicking on and off. you may even hear a clicking sound. this is normal... if the car is idling dangerously low you may have more of a problem... but may be able to fix it merely by adjusting the idle itself. but try this... when your car starts doing it *preferably in a driveway or parking lot etc.* somewhere safe! try putting it into nuetral or park and see if it goes up to a normal level. if it does talk to your mechanic about it. and just so you know the compressor kicks on when running anything that says A/C and when you run the defogger. vent does not cause the compressor to run which means no idle bog down. good luck!

Hi everyone who responded to my previous message. I did find the problem and it was very easy to fix. the problem was the idle by bypass motor or idle bypass valve. it was full of carbon and my was the type you could disassemble to clean. One i cleaned it i then had to reset the computer to clear the message code. These procedures totally corrected the problem, and are covered by Ford's F50 and F51 service bulletins for the 1988 Merc Sable. A machinist friend of mine who does a lot of engine rebuilding had a SnapOn troubleshooting book that covered both procedures. Very handy. Thanks again. :smile: