Damn Idle!

1992 2.3LX said:

Well i just replaced the timing belt, plugs wires. And she runs great. Idles beautifully. Deadnuts on 1000 RPM. Not a hiccup. She hasn't ran that smooth in a long time.
That is until i truen on the A/C, or Defroster. Then she drops down to about 700 RPM, i know it's not a huge drop, but it gives me nasty vibrations that are annoying to boot at a light. Then it will kcik up to 1000 and be smooth for like 10 seconds. Drop back to 700 for a while. Then repeats the process. It doesn't happen when the vent is on.

What is the problem and how do i remedy this?? Not a serious problem just annoying.

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It sound like your typical A/C cycling, which puts a load on your motor. If you run it on Max A/C it will cycle faster causeing the compressor to cut on and off more often. If you run it on normal cold its slower so you probly want notice it more often. I can't tell you about the defroster since my car didn't come with that option. The only thing I can guess is that its connected some how to the A/C and they are ran together. My heater is weird like this, when I cut my heater on some how the clucth on the A/C cuts on but I get hot air only So when my compressor started locking up I had to unplug it from the clucth so it wouldn't engauge when I cut my heater/vent fan on

Yeah, the drop in idle speed is just the A/C belts and pump clicking in which puts a load on your motor. The reason why it sometimes drops to 700rpm, then will go back up to proper 1000rpm idle speed is because the belts which drive the A/A pump are turning on and off. I personally don't have A/C in my Mustang so I don't know much about it, but my best advice to you would be to maybe set your idle a little higher so it is somewhere around 1100-1150rpm when the belts are not running the pump, and that should leave it somewhere around 800 - 850 rpm when the pump is being run. This way your idle speed should be always steady and smooth. If you don't know how to properly set your idle then try this link http://projecthp.ford23.com/tech.html . The simplest way to reset it would be just to turn the little idle speed adjustment screw a couple turns to the right, but in order to do it properly you may want to follow instructions closely.

when the AC or power steering pump (there is a pressure sensor on the pressurized line) are in use, the IAC solenoid SHOULD open up (allowing air to bypass the closed throttle body butteryfly) to keep the idle up... I would suggest a poorly set idle set screw, where the IAB is partly open at idle, and then when the AC or power streering pump kick on, it opens all the way, but its not enough to make up the difference... you should be able to unplug the IAC, turn in the idle set screw, start the car, back the idle set screw off to set the desired idle, then set the TPS sensor to under 1 volt (.98 is the best) and then plug in the IAC solenoid and reset the computer... that way the throttle body butterfly is set to a specific idle, and then when accessories come on, the IAC solenoid makes up the difference for the load and keeps the idle at the correct spot...