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Rough Idle only when the A/C is on

57K views 22 replies 9 participants last post by  val3ntin 
#1 ·
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?
 
#4 ·
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?
 
#5 ·
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.
 
#6 ·
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.
 
#7 ·
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
 
#8 ·
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.
 
#9 ·
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.
 
#11 ·
okay so I found out tonight that the idler pulley and belt tensioner really need to be changed. Hopefully each of these are creating enough drag that its lugging the engine down once the A/C comes on. The parts stores were closed tonight once I realized this, but tomorrow I will replace them and find out. From what I could tell, it seemed that the A/C bearing was in good condition. Thanks again for the help guys. I will keep you informed tomorrow.
 
#13 ·
Nope. If that were true the motor would be lugging with a/c off too. The pulley and tensioner are in use all the time.

I consider 'a/c on' lugging pretty much normal on higher mileage 4 cylinder cars, older motors do not have the idle rollover torque new ones do. From what I've seen the IAC bumpup in speed really is only enough to cover pretty close to new motors. Same issue shows when fan on or power steering is hit on limit. The IAC bumpup occurs, just not enough of it. Software issue. Engine too small. Put same exact a/c system on a V-8 and motor powers right through it because more torque at idle.

Now if vibrating dash when a/c on, look to your passenger side motor mount.
 
#14 ·
So after changing the idler pulley, and the tensioner pulley, the problem is solved. They weren't horrible, but they clearly weren't operating smoothing like pulleys should. This had solved the rough idle issue for me when the A/C is on. My theory is that the car didn't have the power under idle in its old age to handle both the A/C and the crappy pulleys without getting bogged down. Also in this process I gave it a tune up (plug wires and plugs). The car runs significantly better and has no idle issue.

Lesson learned: Dont over think it and rule out the simple stuff. Hopefully this thread helps others. Thanks to everyone in the community that chimed in and offered their advice and experience.
 
#15 ·
A lot of it is fan coming on while a/c is on. That fan really jerks idle down when motor is older. May be one reason why OEM fan is engineered to come on at a later temperature than many think normal. At one time before I put in diagnostic LEDs to show when low and high fan and a/c clutch itself came on, I thought that low fan was not coming on properly. I installed a standalone adjustable fan control set to come on at 210 and back off at 190. That thing switched fan on so much during the summer it became very bothersome at traffic lights, the engine always seemed to be shuddering from the fan kick-in. I installed the LEDs and defeated the add-on control and went back to OEM fan on temp of 230 degrees, the problem quickly became much less noticeable since fan was coming on far fewer times.
 
#19 ·
Hi i have a rough idle when ac is on and an when i shift to neutral some time my engine dies when the ac is on but only when the engine is warmed up i changed the iac valve and the air pressure sensonr on the manifold and stil nothind but when i deisconnect the battery for 5 min and putt it back the engine runs like new and the next day the problem comes back why? ?
 
#20 ·
It takes a period of time driving the car for the PCM to figure out you haven't fixed the problem, only wiped its' memory to then realize after a while the problem is still there.
 
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