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Old 12-07-2012, 10:37 AM   #1
twlott
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Engine starts to idle rough when brake pedal is pressed

noticed the other day that when you are at a stop light or in drive thru that the engine starts to idle rough but once you let go of the brake it runs like it should.

I was either thinking it could be a vacuum leak from the master cylinder or it maybe a IAC. Anyone have any ideas?


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Old 12-07-2012, 12:58 PM   #2
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Brake light wiring shorting to ground, putting a increased electrical load on the alternator, which drags down the motor.

The IAC should compensate with idle speed increase to prevent dying, if it runs rough at 800 rpm, the IAC can't fix that.

(just a wild arse guess)
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Old 12-07-2012, 04:13 PM   #3
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Small enough leak in PCV hose will do it with same result.

Your vacuum leak would be power booster not master cylinder. Quick way to check. Go somewhere like big parking lot at 3 in the morning and disconnect booster vacuum supply hose and plug it. Your brakes will have no power assist then so much harder to push pedal and car will not stop as quick, the reason for being in big space by yourself. If the motor at that time idles correctly then problem is the booster.

Passenger side engine mount going bad can seem to idle rough if motor beginning to shake, it will show every time you put on brake and disappear as soon as you let off. The difference is true engine roughness versus vibration felt through chassis, the two often perceived by many as the same thing................
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Old 12-08-2012, 05:19 PM   #4
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Can this vibration be felt when you disengage the clutch? I have a really rough vibration that comes and goes when I let up on clutch. When it's pushed in, vibes go away. Would a bad dogbone mount cause this?

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Old 12-30-2012, 12:07 PM   #5
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I have the same problem with a 2011, which is barely 2 years old. I can engage the emergency brake and release the pedal and the RPM's increase to around 850 and the vibration goes away.
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