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Old 11-23-2012, 10:06 PM   #1
Hovs13
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idle fluctuates when rear defroster actived

When weather is less than +5C, I notice my idle fluctuates up and down between 500-1000rpm after turning on the rear defroster.

does this happen to anyone else?
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Old 11-23-2012, 11:36 PM   #2
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It's common for RPMs to drop when the alternator is under load
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Old 11-24-2012, 12:06 AM   #3
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2.5 engine is too weak
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Old 11-24-2012, 02:04 AM   #4
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It will probably do the same thing if you press the window switch to raise the window, when the window is all the way up too. It is just the load on the alternator, like they said above.
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Old 11-24-2012, 10:03 AM   #5
Hovs13
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Perhaps I didn't explain it well. the RPM drops then rises, then drops, then rises... in a continuous cycle until engine is warmed up completely or rear defroster is shut off.

This used to happen to my car even when engine was warm but recently cleaned the idle control valve and it solved this issue when engine is warm, but it continues to happens when engine is cold.

Last edited by Hovs13; 11-24-2012 at 10:05 AM.
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Old 11-24-2012, 10:29 AM   #6
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Weak battery? The alternator just maintains the charge of the battery. If the battery is weak it could be discharging quickly causing the alternator to cycle like that. Just a guess but worth a look.
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Old 11-24-2012, 08:16 PM   #7
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Quote:
Originally Posted by Hovs13 View Post
Perhaps I didn't explain it well. the RPM drops then rises, then drops, then rises... in a continuous cycle until engine is warmed up completely or rear defroster is shut off.

This used to happen to my car even when engine was warm but recently cleaned the idle control valve and it solved this issue when engine is warm, but it continues to happens when engine is cold.
That is just the rear defroster cycling on and off. It does not stay on constantly. It is most likely still the rear defroster. And it does not have anything to do with a 2.5l engine. My 330 does this as well.
Now, it may mean that the alternator is getting old, but mine has done it for 3 years with no charging problems. I think this is normal for a lot of cars. Both of my e46's have done this.
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Old 11-26-2012, 05:13 PM   #8
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Again I've had this issue for years. It used to happen when car was warm and cold but since i cleaned the ICV it only fluctuates when engine is cold.

does the defroster cycle on and off every second? because that is roughly the timing of the rev fluctuations.

I'm not worried, just curious about others experiencing this.
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Old 12-08-2012, 06:14 PM   #9
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Any other thoughts?
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Old 12-08-2012, 06:58 PM   #10
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If the ICV helped in warm weather, have you taken a look at ensuring there aren't vacum leaks? Checked out jfoj's thread on weak or irratic idle?
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