My 2005 GT manual normally idles at a little under 1000RPM . . . . . .just recently when I turn on the air conditioning the idle alternates between 500 and 1000 rpm, then stalls! AT highway speeds the the AC works like a champ . . . . any ideas? I am just coming upto my 60000 service (which includes a fuel and air filter).

 

Only thing I can think of is a week battery and or alternator? At idle your alternator is not producing enough voltage to handle the draw from the A/C, but at freeway speeds with the engine turning a couple thousand RPMs it manages?

We recently had similar problem with a corvette at work, except it was the radiator fan, not the A/C. At idle when the fan kicked on the rpms would start to stumble and then it would die. Turned out to be a bad alternator..

 

Interesting! My AC blower fan is also very weak on level 1 (maybe non-functional) . . . I changed the resistor, and still had the same issue . . . .could that be related to the battery/alternator?

 

I'm not sure that the weak fan would be a symptom of a bad battery/alternator, but it could definitely be part of the problem. If for example you've got some debris (leaves/dust/etc) build up in the fan and adding additional resistance to turning, that might be harder on an already weak electrical system. Id definitely go down to autozone or whatever and have them test your charging system. That should be a quick and painless way to confirm or rule out that cause.

 

Throttle body's dirty. Clean it.

That's my guess anyway if the alternator/battery isn't the problem. Although, the weaker fan motor could be pointing you straight at the charging system or the fan itself.

Awesome did something similar, and cleaning the TB solved it. Of course, you retrain the computer too, it won't like the 'new' results of the cleaning.

 

OK, I no longer have the near stall during idle with Air Conditioner ON! I changed the fuel filter and air filter (per the recommended 60,000 mile service) . . . immediately after the filters changed I still had the issue with the air conditioner almost stalling the car during idle . . . by "accident" I filled my car up with 91 octane gas on my last two fill ups (my 05 has NEVER seen 91 octane gas, in 7 years and 60,000miles) . . . . . but no the idle issue is GONE! is it the filters? or is it the 91 octane gas?

 

I was just wondering if your problem has gone away completely? The funny thing is that I recently picked up an immaculate 05 with only 20k miles on it and I'm having the exact same issue at idle with the AC on...

 

The Mustang GT was made to run Regular Gas. And NOT e85. Unless you are running a Tune, You can.... I would not do it. Also a can of Seafoam may help. CLEAN THAT THROTTLE BODY FIRST!!!!!!

 

Who said anything about e85? Are you talking about the 10% blends of ethanol (90% real gas) (common in the midwest gas stations, but not very common here in California).

 

I just removed my Throttle Body and cleaned it with CRC spray . . . . runs perfectly. Houtex, you are smart guy!

 

Naw, I ain't sarmt. I jes had the same thing happen to me.

Glad it's fixed!