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Old 11-17-2012, 08:42 PM   #1
blakeclevenger
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Rough Idle only when the A/C is on

I have a 2002 ZX5 (DOHC Ztech, Automatic) that runs great usually. Just recently, I started getting a rough idle, but only when the A/C is turned on. I have already changed the plugs and wires, but this didnt help. Any suggestions on where to go next? I am thinking its the ICV or TPS. Am I on the right track?


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Old 11-17-2012, 08:45 PM   #2
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sounds like it's not idling up when the AC is on.
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Old 11-17-2012, 08:51 PM   #3
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Mike you are right on. It just seems to drag down when the A/C hits. This is especially true once it is hot and the fan is running as well. What is causing it to not adjust the idle? Any ideas?
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Old 11-17-2012, 09:17 PM   #4
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I'm not aware how the focus controls the idle up function when the compressor is engaged... I'm guessing probably just the pressure switch on the AC which would mean the Idle Air Control (IAC) valve is not not operating correctly.

It runs worse warmed up because when the engine is cold it has a higher idle anyway. How the idle is controlled at start up I don't know... maybe it doesn't use the IAC valve, but it should. If it does, then the IAC is operating correctly but perhaps the computer is not receiving the signal to adjust the idle... There is only one pressure switch according to the diagram I have, and it's a dual pressure switch. The AC engages (correct?) so there shouldn't be a problem with the wiring to it. There appears to be only two wires to the IAC as well, power and the ground which is controlled by the computer.

The engine runs fine when it's cold and when the AC is off, correct?
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Old 11-17-2012, 09:22 PM   #5
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Yes the A/C runs just fine. I just try to leave it off since it causes the rough idle. So do I needs to change the IAC or is there more diagnostic that needs to be done? From what you are saying, may be it is a computer thing? I haven't received a check engine light yet, so I guess it isn't idling "that bad," but its definitely off and needs to be addressed.
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Old 11-17-2012, 09:27 PM   #6
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I would suggest cleaning the IAC valve first. I don't know where it is on the zetec but it will be right around the throttle body. Hopefully it's easy to get to.

edit: couldn't find a how-to with working pictures but that certainly looks like a pain to get to. Not the worst though I'm sure. You don't have to remove the manifold, but You'll be getting at it from the bottom. It's on the plastic manifold on the back of the engine. Here's a good picture I found of it at least.
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Old 11-17-2012, 09:32 PM   #7
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Is the idle bouncing? I could be a bad bearing or ac clutch in the compressor. Turn the car on and turn the ac on. Pop the hood and look at the serp belt. The pulley at the bottom is the ac pulley. If its a bouncing idle you should see the pulley freeze momentarily and then kick back on I have this in my car. Good luck, I suggest exhausting every possibility for diag before throwing the parts dart and buying tons of stuff that you don't need to replace
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Old 11-17-2012, 09:37 PM   #8
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Thanks for all the help guys. I am going to give the AC bearing a check. May be it will be something fairly simple. Im new to the forum and not good at diagnosing stuff. I guess I fire before aiming because I dont know enough than just to guess. Hopefully the shared expertise around here will allow me to grow in knowledge and learn how to diagnose things. Thanks again.
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Old 11-17-2012, 09:40 PM   #9
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yes definitely verify before replacing anything. Check the simplest stuff first. If the belt drive seems to be OK I'd say the IAC probably needs a cleaning.
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Old 11-17-2012, 11:09 PM   #10
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My money is on the IAC needs cleaned or replaced..
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