On the throttle body you can adjust the idle speed by turning a adjustment screw. This will give you a different idle speed but over time the ECU will compensate for it and will return itself to a pre-determined idle provided the idle wasn't manually raise or lowered significantly.
I don't ever remember touching the screw since I purchased the car. Is there certain position that it is supposed to be set at, like when it came from the factory? I don't know if the previous owner would have ever messed with it.
Yes, there is a certain position. That screw is not an idle adjust screw like so many people misconceive it to be. It is the adjustment on the throttle body that allows a calibrated minimum amount of air to pass the throttle plate and it prevents the throttle plate from sticking in the bore. This setting must be precise as all of the PCM's calculations are based on this being at a certain setting. I don't know about your car for sure but to give you an idea of my car, that screw was tampered with a while back and to get it back to it's appropriate spot, I had to place a 0.002" feeler gauge between the throttle arm and the screw and adjust the screw until it just touched the feeler gauge. Then I had to turn an additional 1/2 turn to establish the proper setting. This procedure was obtained from the Ford service manual.
So from your setting, the screw was just barely touching the throttle arm when set correctly?
This is why I am asking. I purchased an underdrive crank pulley. I installed that the other day, along with a new alternator with a smaller pulley, to help with the charging. When the car is in park or drive with no lights on, the voltage is fine. In drive, with fog lights and/or headlights, the voltage drops down into the 12's. Once I start moving, the lights brighten and the volts go back up.
I thought getting the smaller alternator pulley would have helped with any problems. I guess it is not worth having an underdrive pulley on a daily driven car if it is going to cause voltage problems like this.
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