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Old 05-30-2012, 08:17 PM   #1
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Rough idle while a/c or heat is on?

As anybody noticed this? My car seems to idle fine around 800 rpm but if I have the a/c on the rpm will drop off a bit and rev back up to close to 1000 then settle back to around 800. It does it ever couple of minutes while at a stand still. Obviously when I'm driving I don't notice anything. Wondering if this is normal and if so, why?
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Old 05-30-2012, 08:26 PM   #2
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Does the exhaust skip? Mine kinda gets loud when you crank if then idles around 7-800. When the ac or heat is on it gets loud for a good 30 seconds then quiets down. Is this similar to you?
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Old 05-30-2012, 09:26 PM   #3
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All that is happening is your idle is readjusting to the power being used by your a/c compressor. Compressors run in cycles (not sure if it's an on/off cycle or low/high pressure). Every car does it. Compressors have been proven to use as much as 25 hp in some cars.
Many newer models will use a/c compressor for heat and cold to help dry out the air that comes into the cabin
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Old 05-31-2012, 11:11 AM   #4
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Quote:
Originally Posted by GrabberBlue5.0
All that is happening is your idle is readjusting to the power being used by your a/c compressor. Compressors run in cycles (not sure if it's an on/off cycle or low/high pressure). Every car does it. Compressors have been proven to use as much as 25 hp in some cars.
Many newer models will use a/c compressor for heat and cold to help dry out the air that comes into the cabin
Good enough for me! I knew it was something along those lines
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Old 05-31-2012, 11:42 AM   #5
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Re: Rough idle while a/c or heat is on?

I would blame the MAF, but you have a new car so it couldnt be gunked up. Your idle is probably a bit to low at idle so when the AC kicks on it take a hit and idles a bit lower and thus the shake.
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Old 05-31-2012, 11:49 AM   #6
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You could try the idle re-learn. This made it so my idle is the with or w/o the A/C on. You just hear a little different growl from the engine.

Disconnected NEG from battery for 30 minutes.

Reconnected battery and started car letting in idle while the engine gets to temperature, then wait one minute.

Turn the AC to Max a little over one minute, then turn off.

Then go drive the car for at least 10 miles.

This made a dramatic impact on idle smoothness and drivability (manual tranny) with my car.
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Old 05-31-2012, 03:36 PM   #7
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Quote:
Originally Posted by Azure
You could try the idle re-learn. This made it so my idle is the with or w/o the A/C on. You just hear a little different growl from the engine.

Disconnected NEG from battery for 30 minutes.

Reconnected battery and started car letting in idle while the engine gets to temperature, then wait one minute.

Turn the AC to Max a little over one minute, then turn off.

Then go drive the car for at least 10 miles.

This made a dramatic impact on idle smoothness and drivability (manual tranny) with my car.
Very interesting. Where did you get this idea??
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Old 05-31-2012, 04:07 PM   #8
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Quote:
Originally Posted by Mayhem48

Very interesting. Where did you get this idea??
Seen it in various Mustang forums, including this one. Apparently, it's in our owners manual, but I couldn't find it. I think this applies to 99 and newer. Somebody correct me if I'm wrong about that. This worked really well for me.
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Old 06-01-2012, 06:18 AM   #9
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I'll try it and let you know how it works. Thanks!
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Old 06-01-2012, 07:35 AM   #10
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Quote:
Originally Posted by Azure

Seen it in various Mustang forums, including this one. Apparently, it's in our owners manual, but I couldn't find it. I think this applies to 99 and newer. Somebody correct me if I'm wrong about that. This worked really well for me.
That's the idle relearn that I got from BAMA when they sent me my tunes too.
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Old 06-01-2012, 08:34 AM   #11
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Originally Posted by Mayhem48
I'll try it and let you know how it works. Thanks!
Right on!
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Old 06-19-2012, 11:17 AM   #12
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I finally tried the idle relearn and it didn't work I guess I didn't do it right. When u say let the engine get to temp do u mean when it revs down after turned on or when the temp gauge is in the middle? That's my only confusion I think
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Old 06-23-2012, 11:29 AM   #13
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Re: Rough idle while a/c or heat is on?

the car must be at normal operating temp...
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Old 06-23-2012, 12:54 PM   #14
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Re: Rough idle while a/c or heat is on?

my rx-8 used to do that, somehow my safety toggle switch plug was lose. Just had to reconnect it. Not sure if that applies to mustangs.
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