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My wife's SS is idling rough when the AC is on. It's fine when driving but kinda put-puts a bit and the engine seems to shake a bit when at a stop. I haven't looked into it a lot yet but I was wondering if anyone has any ideas.

Plugs are less than 30k old. I'm also suspecting a small head gasket leak since she is losing a little bit of coolant consistently and I see a tiny puff of white smoke and some small bubbles in the coolant reservoir when the cap is off and the car revs. Other than that the car runs just fine. I've been considering throwing in a 20G/22T hybrid one of these days but don't have the time right now.

Compressor is a new old stock that I installed a couple of years ago. Doesn't make any noise and the AC is nice and cold.

I cleaned the IACV about a year ago.

Does the idle increase slightly when the AC is on vs. AC off? My car typically idled better with the AC on because it added a little load to the engine.

This can be a sticky IACV (idle air control valve); usually drenching it with carb cleaner and manually working it back and forth will free it up. This is particularly likely if RPMs drop rather than rise when the compressor kicks in.

I'll have to double check if the idle goes up or not. I think it feels like it drops a little bit. I did clean the IACV not too long ago and it didn't feel like it was sticking at all.

The car is still eating coolant so I am going to do a block test, compression/leakdown soon and do the headgaskets again if needed. I almost bought a 20G to throw in, but now I've decided to just do it right and swap over the guts from a WRX so I can tune it and have updated wiring and parts availability. So I'm looking out for a salvage or blown WRX.

I'll report back once I do the HG diagnostic. I did the HGs about 60k miles ago. My wife drove the car home after the upper radiator hose split so that may have contributed to the early failure. I decked the heads so I think everything was nice and flat.

Well, hydrocarbon block test came back negative. So maybe I have a pinhole leak somewhere in the coolant system. There is no evidence of a leak anywhere. Compression test next. So far so good though.