Uneven Mustang Idle

courtesy of Bob, The Auto Answer Man

My 1990 Ford Mustang GT 5.0 runs great without the Air Conditioner on. With the AC on it often dies when I come to a stop. The idle is usually at 700 rpm but drops to 550-600 when I turn on the AC. Also, I can notice power surges while driving at steady speeds, again, only while the AC is on.

I cleaned the idle air control valve and the problem is still there. Do I need to replace the control valve? I've recently replaced the PVC, plugs, cap, rotor, fuel and air filters, as well as the battery cables. Although these parts needed replacing they did not help the unstable idle with the AC on. Can you help me?
Bob H.

Your map sensor may also be malfunctioning. It seems, since you are also getting a problem while driving with the A/C, that the car does not recognize the increased load of the A/C. The MAP (manifold air pressure) sensor is what tells the motor how much load the engine is seeing.
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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