Idle surge with A/C and defrost on

[midnightdriver]

I know this is similar to a previous post, but when I turn on my heater or a/c, every time the a/c clutch engages, the rpms jump up a couple hundred, then drop right back down to normal idle, about 800 or so. It sounds like I am reving the engine at a stop light every so often and while driving you can sometimes feel the surge as well, except when rpms are about 2,000 (ie freeway speed) or above.

Is this a dirty IAC, cus when I pulled the tube off the IAC it the car died right away, as well when I unplugged it. A small note, the A/C is not very cold, kinda just cool, could the problem be there is not enough coolant and the engine gets excessive drag?

Thanks

 

[wmburns]

AC low on Freon. Cycling on/off too fast.

The PCM maintains two tables for idle trim. One for AC on the other for AC off. When the AC turns on, the PCM immediately switches to the alternate table. Obviously, with the AC on, one would expect more air (higher IAC duty cycle) to compensate for the extra load on the engine from the AC.

I suspect that what is happening is your AC system is low on Freon. This is causing the AC to cycle on/off due to low pressure. As such, the AC is running for a very short period (not long enough for the PCM to re-learn a new trim value).

Sooooo. Follow this sequence. AC clutch kicks on. PCM switches trim tables, IAC duty cycle goes up. RPM's go up. However, the compressor is not loading the engine as much (low Freon). RPM's too high. Within short period of time, AC disengages due to low pressure. PCM switches back to non-AC idle trim. RPM's return to normal. Soap...Lather....Rinse....Repeat...

The later model Mustangs (1999-2004) are slower to re-learn new idle trim values. The PCM learns faster when the car is stopped and your foot is on the brake. In manual transmission cars, the process is slowed somewhat because many ppl do not use the brake when stopped at lights (not that it matters in your case. The AC is most likely cycling too fast).

You could try disconnecting the battery to clear the idle trim values. However, this most likely will result in the idle dropping when the AC starts. Since the AC is still cycling on/off, the PCM will never learn idle trim values that produce the desired results.

You could try jumping the AC low pressure switch thus forcing the AC to run. Let the car idle in your drive way with your foot on the brake. It may take several minutes for the PCM to re-learn but it will.

The final option is to fix your AC.

Good luck

 

[midnightdriver]

I looked under the hood at the A/C system, and this is what I found.

First pic, is this the a/c low pressure fill?



2nd pic, this is what I found when I unscrewed the cap.

 

[wmburns]

Looks like you found a leak. New valve core should fix that right up.

Unless you have an AC vacuum pump, best to use a shop for this repair.