Help! When the cooling fans turn on the engine idles rough and bounces between 200 and 1200 rpms. When they turn off the idle returns to it's normal 600-700 rpms. The car is a 92 Legacy 2.2 n/a 5sp. Other than this the car runs great with 189,000 miles. Any ideas as what would cause this?

It's probably the idle air control valve. You can try spraying some brake cleaner in the IAC valve tube

Another possibility is the fans are causing noise in the electrical system and that is acting upon the ignition system somehow. I would check the AC component of the electrical system when the fans are 'on' and 'off' to find out if that may be the case or not. Using a good digital meter on the AC volts scale may show up something.

a third possability (which i had on an 88) is that one of the cooling fan motors are bad. I had that and it drove me nuts. I discovered it by accident whaen i changed the clogged raditor, that one of the bearings were shot. The fans were drawing alot of current, not enough to blow a fuse, but enough to make the dash go wacky.

 

nipper

thanks for the info guys. heading out to the garage now that I have an idea where to look. It's nice to know that there is help when in need!

I would suspect too much curent is being drawn by the fans for the ignition system to operate properly. Measure the voltage across the battery terminals before and after the cooling fans come on, see a difference? If so, the problem may not be the fans. It could be the altenator/voltage regulator, or a failing battery. Idling is when the output of the altenator is at it's lowest, so any weakeness in the electrical system often first becomes apparent at idle.