For a few weeks now I've been fighting with an issue with my engine speed surging at idle, and think I've finally solved it. Last night I was trying to get to the bottom of a strange noise under my hood and narrowed it down to the AC compressor. I turned the AC on to see if the noise went away, and it disappeared immediately as soon as the clutch kicked in. Much to my surprise the surging idle disappeared too and the engine ran smooth as silk. I plan on running a ZZP alternator delete pulley until I get a replacement compressor. Anyhow, can anyone explain why my engine surge straightens out when the AC is on?

Crap! If the engine speed automatically increases while the compressor is on, then that would explain it. Though the compressor does need to be replaced, it looks like it may not be the cause of the surging issue.

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My engine speed goes up by about 100RPM when the compressor engages. If your IAC was failing this may be a more comfortable position for it and it stops readjusting (resulting in surging). Cleaned yours lately? IACs are cheap and it might be worth it to replace it to eliminate it as a possible issue (if you haven't eliminated it already).

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

there is a command in the pcm to raise the rpm when the a/c is on there is a way to reset the iac also (it's supposed to at every start up, but may not always do it) I will try and find the info

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Steve
'97 GTP VS cam, ZZP PCM, and not much else yet

Suggestions noted, I'm going to go check out my IAC when I get out of work. I can't remember what my AeroForce gauges tell me, I'll post some numbers in a bit. Thanks for the heads up guys!

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what kind of noise were you hearing? if it was a metallic type rattling/scraping sound, it's likely that the bearing in the a/c clutch is going out. turning on the a/c and engaging the clutch typically will make the noise stop, until the bearing gets worn too much.

After checking it out I found that the AC compressor is definitely the cause of the noise. The idle issues I'm having are from a bad IAC. I disconnected it today and it smoothed right out. I pulled it out to see how it looked and there is a lot of side to side play in the plunger, not to mention it looks like it's binding. The fact that the idle smoothed out when I turned on the AC was only because my engine speed would increase by 100 or so RPM when the compressor kicked in. Thanks for the help guys.

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Glad you worked it out.

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

On a different note, my fuel trims have stabilized since I disabled the IAC and my exhaust smells a hell of a lot better. Now all I need to do is install an AC delete pulley and rip the AC system out. I'll put it back in once it gets disgustingly hot here in NJ, I just want to see how it will effect my 1/4 mile times.

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thats alot of work to rip it all out if youre gonna put it all back later. not to mention if you open the system up and leave it all apart, you're gonna kill the seals and such... would have to replace those before reinstall.