Erratic Idle, Only with AC

[rs-turbo]

Didn't see anything with a search that seemed to match. I recently replaced my IAC when I suspected it was causing an occasional stall. Sometimes at start up I would have to force the engine to run by applying power, otherwise it would die. Then it would be clear the next time, or even clear up while driving. You could suddenly feel the resistance go away and power come back and all was normal. After reading here, it sounded like an IAC problem.

While swapping the IAC I put in a restrictor plate that I saw some were doing in another thread. Idle ran around 1200 with or without AC, and I figured I needed to adjust the hole in the plate. After two weeks a Service Engine Soon light kicked off, so I removed the plate, and disconnected the battery to reset everything. Idle is back to the normal 700 (and over a week now with no light). The other day I started to use the AC and I notice idle won't stay steady. Sometimes it's around 700, creeps up to 1200, and then will suddenly crash to around 500 stumble and come back to 700. Seems to run as a cycle. You can feel it while driving too if you are cruising at a lower idle. Turn the AC off and it idles normal. I would think it's the new IAC, but wouldn't this effect non-AC idle as well? And both idles were the same (although high) when the plate was in. Ideas?

 

[hotdog71]

I think someone else had a problem like this and one of the things to check was the AC clutch i think not totally sure.

But svttech is one of the main guru's in here

 

[sgarlic]

Did you replace the gasket that goes between the IAC and the TB? If that thing is ate up or in bad shape it can mess up things like you described.

 

[HoustonGT]

I have that problem as well and it could be the a/c clutch and pulley, but i'll wait for svttech to chime in

 

[rs-turbo]

Before swapping the IAC, the AC worked fine and it seemed to run fine when the restrictor plate was in, no surging, just the high idle all around. Air blew cold as normal. When I removed the plate, I put the new gasket in. Now it fluctuates.

 

[gtstangau]

Get used to it mine does the same thing. Idol is all over the place, I try to not let it piss me off!

 

[Mike97gt]

I have seen serveral cars where the idle would be erratic only when using a/c or defrost ( also uses the a/c)

on those cars I found the A/c system was low on freeon causing the a/c compressor to short cycle. ( this is when the a/c clutch only stays engaged for a few seconds then kicks off and then back on again)

what happens is the short cycle is too quick for the pcm to react, the pcm changes idle strategy when the a/c clutch is cycled on and the short cycles confuse the programming.

fto see if this might be your problem so this.

1. see if your a/c is cold. ( i mean ice cold not semi cold)

2. turn the system to max a/c and with the windows up and fan on max watch the a/c compressor, count the amount of times the a/c cycles on in 1 minute. also count how long the a/c clutch stays engaged each time.. ( you can tell the clutch is engaged when it spins with the pulley, when disengaged it will not spin with the pulley)


report this back with outside temp and humdity, also if you can a thermometer you stick in the a/c vent will help, give me the temp info coming out the dash..

 

[rs-turbo]

I haven't had a chance to dig deeper into this, however, over the weekend my wife and I took a trip out of town. Being 90+ degrees this past weekend here in Floirida, of course AC was used. On occasion I could feel a slight shudder while driving, but the stand still idle seems to have returned to the normal 700. Once in a while it seemed to stutter to 500, but would come back to normal. Now a few days later idle seems to be okay. I still feel a little stutter while driving now and then, and when I turn the AC on I can feel it drag on the engine, which I don't recall before. I was always impressed that I never noticed a power drain with AC on like I have with my other cars. I attributed this to the V8. AC still blows very cold and at highway speeds all ran cool, smooth, and no stutter.

 

[ALEX2028]

I also have the same problem when I turn on the a/c rpms drops down to about 500 the jumps to 1200 and and will then idle at 700, I notice it happens when the a/c compressor kicks in, I dont feel a diffrence while driving.

 

[steveb24]

my rpm's rise almost 200 when i turn the a/c on. idle still isnt as smooth now as it was before the t/b - plenum install, but it stays pretty constant.