Q: engine surge at idle when the heater control is in the defrost position on 1991 Ford F-150

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When at idle, and the heater -a/c control is in the defrost position, the engine speed will increase from 500 to 700 rpm, then back to 500. If I move the heater - a/c control to the floor or off position, it will return to a smooth idle at 500 rpm. Is this a vacuum hose problem? If so, which hose is the cause?
(3) Answers
I have noticed the EXACT same issue on my 1996 escort 1.9L wagon. This morning i noticed the surging stops when not in the defrost position. Strange how this is connected to the a/c compressor when in this position, as mentioned by the previous poster. I suppose i should make sure to keep it off defrost when no longer needed as to save excess fuel consumption?
Believe it or not that is supposed to happen. With the selector in either AC or defrost , the AC compressor is turned on and the engine control module ramps up the idle speed to compensate for the load of the AC compressor. No AC compressor operation in vent or heat modes. If the idle is surging up and down very fast, the AC system maybe low on refrigerant charge. Good luck.