Rough idle with a/c & Defrost - Mustang Evolution

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Old 04-03-2013, 09:24 PM   #1
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Rough idle with a/c & Defrost

Hello everyone, whenever I turn on my a/c or defrost the car idles very odd, you can hear the compressor switch on and the engine revs to 1200 then dips to 450 then goes to the normal idle of about 650. Whenever the a/c is on all it does is repeat this cycle, no matter what the operating temp is.

I have already replaced the IAC and cleaned the entire upper engine including t/b and MAF.

The car will also surge slightly when driving around. The surge happens for about the 5 seconds that the compressor is switched on, its awefully annoying.



This problem has me stumped, I also can here a slight "whistle" or humm sound when I'm driving down the highway and my exhaust isn't drowning everything out.

I'm stumped and I'm about just give up and take it to the ford dealer so they can empty my pockets. Any help is very much appreciated, thank y'all.
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Old 04-03-2013, 09:45 PM   #2
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Im not sure on that maybe losing volts some place..
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Old 04-03-2013, 09:53 PM   #3
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Im not sure on that maybe losing volts some place..
My volt gauge is normal and all my accessory's work fine, the lights do dim though when the idle dips
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Old 04-03-2013, 09:57 PM   #4
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Sound like the A/C clutch is going bad.
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Old 04-03-2013, 11:36 PM   #5
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My mustang does the same
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Old 04-04-2013, 01:39 AM   #6
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If u have underdriven pulleys I think it can cause it. I can't remember what the part is but turn on ur heater or ac pop the hood and look at the bottom of the pulley on the left side. U can see the part click on n off. I'm pretty sure it's normal. Annoying tho I have the same issue. Only happens when heater is on. Revs a bit higher like its on a cycle I have no clue why lol should look into it

---------- Post added at 10:39 PM ---------- Previous post was at 10:36 PM ----------

A/c clutch pulley*
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Old 04-04-2013, 01:45 AM   #7
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I'm pretty sure the click and increased idle is normal, squealing or grinding would indicate failure
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Old 04-04-2013, 04:37 AM   #8
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The whistle or hum sound may be the ac clutch going bad or the compressor itself. You could have the system pulled down and have the shop put fresh ac oil in the compressor when it is refilled. Could have water in lines causing too much pressure. On the other hand the whistle sound could be a cracked leaking intake.
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Old 04-04-2013, 09:03 AM   #9
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Thanks For All the replys guys, im going to check the A/C compressor out, the pulley still spins but the cycle is only for 5 seconds and this is when the RPM rises. When the cycle stops and the pulley stops spinning is when the RPMs dip. Ill post back with what i find out so maybe i can help all you other guys out. I know the RPMs are supposed to kick up a bit to deal with the extra load on the engine but dipping to 450 isnt normal. I also took a video today and ill post that up for you guys to take a look at when i get the chance.
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Old 04-04-2013, 10:36 AM   #10
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Here's a video of the idle
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Old 04-04-2013, 10:42 AM   #11
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Same problem..subscribed
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Old 04-04-2013, 12:57 PM   #12
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video is private?
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Old 04-04-2013, 01:02 PM   #13
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It's normal for your motor to do his when the compressor kicks in, it puts more load on the engine and your PCM does not adjust for it immediately, it takes a second, over compensates them resettles, hence the dip, surge, then normal
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Old 04-04-2013, 01:41 PM   #14
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Video is public now, I know the idle will adjust but it almost stalls sometimes which isn't something I prefer happing a lot T

The car will also surge slightly when driving around. The surge happens for about the 5seconds that the compressor is switched on, its awefully annoying.
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Old 04-04-2013, 07:47 PM   #15
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Does the air work? Compressor could be going out and when the clutch engages the compressor is so stiff it almost stops the motor
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Old 04-04-2013, 08:31 PM   #16
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Does the air work? Compressor could be going out and when the clutch engages the compressor is so stiff it almost stops the motor
Air Blows Cold
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Old 04-04-2013, 11:25 PM   #17
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One other idea. My alternator was going out. I had it checked at autozone and the meter said the diode was bad causing irregular voltage. You can get yours checked that would be just one more thing easily done.
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Old 04-05-2013, 08:28 AM   #18
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One other idea. My alternator was going out. I had it checked at autozone and the meter said the diode was bad causing irregular voltage. You can get yours checked that would be just one more thing easily done.
Yeah i plan to get that done, Really thanks so much for all the suggestions guys, i have a friend whos a mechanic who is going to look at it to diagnose the problem.
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Old 04-05-2013, 08:02 PM   #19
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Mine is doing the same only i dont have the noises but idk my car is loud as hell. Ive come to the conclusion its something to do with the vacuum system ive replaced most of them except the ones inside the dash and its not as bad as it was. Check there as well
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Old 06-19-2017, 10:46 PM   #20
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mine does exact same thing only when ac or defrost is on. surging idle. did you ever find out the problem
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Old 06-22-2017, 10:16 PM   #21
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Compressor could be seizing up. If it's harder to turn than normal when engaged that could account for it.

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