Fluctuating RPM's. Need help diagnosing.

MZWIE · 01-09-2014, 06:57 AM

I need help tryng to diagnose this problem.

Under normal cruising (ie not under acceleration) the engine RPM fluctuate. It is not a surge, but rather a momentary loss of power. The actual speed or RPM per se does not seem to make any difference. I tested using the cruise control just to make sure I wasn't somehow moving the throttle inadvertently, and sure enough it did it again.

I thought it may be only under load. So, with the car in neutral I checked it at idle and it did not do it.

Then I pressed the accelerator to 2000 rpm's and maintained it there. Then the "surging" starts again. Drops maybe 500 RPM and then back again.

So my thoughts, in order if what I would check are as follows:

1 - Spark plugs. I am going to replace these right away.
2 - Plug cables
3 - Injectors
4 - VANOS
5 - Vaccuum leak

No CEL's at this time. Not sure of any codes since I can't read them. Any ideas from all you guys will be greatly appreciated.

MZWIE · 01-09-2014, 09:58 AM

Wow...93 views...no comments...

Chris. · 01-09-2014, 06:17 PM

Dct or manual trans?

If manual, possible clutch slip

1descending · 01-09-2014, 06:24 PM

Quote:

Originally Posted by Chris.

Dct or manual trans?

If manual, possible clutch slip

Wouldn't that make the RPMs shoot up rather than down? Also, he said it happened in neutral as well.

Chris. · 01-09-2014, 08:18 PM

Yes, but one could argue that the "bogging" is just the clutch finally holding.

Edit: after rereading it sounds like an ignition issue. I suspect plugs/coils

MZWIE · 01-10-2014, 07:12 AM

Sorry. AT. I'm replacing plugs tomorrow morning. Will report back.

01-09-2014, 06:57 AM	#1
MZWIE Lieutenant Colonel 389 Rep 1,660 Posts Drives: M2 Join Date: Jun 2007 Location: GEORGIA iTrader: (0)	Fluctuating RPM's. Need help diagnosing. I need help tryng to diagnose this problem. Under normal cruising (ie not under acceleration) the engine RPM fluctuate. It is not a surge, but rather a momentary loss of power. The actual speed or RPM per se does not seem to make any difference. I tested using the cruise control just to make sure I wasn't somehow moving the throttle inadvertently, and sure enough it did it again. I thought it may be only under load. So, with the car in neutral I checked it at idle and it did not do it. Then I pressed the accelerator to 2000 rpm's and maintained it there. Then the "surging" starts again. Drops maybe 500 RPM and then back again. So my thoughts, in order if what I would check are as follows: 1 - Spark plugs. I am going to replace these right away. 2 - Plug cables 3 - Injectors 4 - VANOS 5 - Vaccuum leak No CEL's at this time. Not sure of any codes since I can't read them. Any ideas from all you guys will be greatly appreciated.
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01-09-2014, 09:58 AM	#2
MZWIE Lieutenant Colonel 389 Rep 1,660 Posts Drives: M2 Join Date: Jun 2007 Location: GEORGIA iTrader: (0)	Wow...93 views...no comments...
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01-09-2014, 06:17 PM	#3
Chris. Private First Class 2 Rep 103 Posts Drives: 2008 Alpine White 135i M-Sport Join Date: Jul 2013 Location: Southern Illinois iTrader: (0)	Dct or manual trans? If manual, possible clutch slip
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01-09-2014, 08:18 PM	#5
Chris. Private First Class 2 Rep 103 Posts Drives: 2008 Alpine White 135i M-Sport Join Date: Jul 2013 Location: Southern Illinois iTrader: (0)	Yes, but one could argue that the "bogging" is just the clutch finally holding. Edit: after rereading it sounds like an ignition issue. I suspect plugs/coils
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01-10-2014, 07:12 AM	#6
MZWIE Lieutenant Colonel 389 Rep 1,660 Posts Drives: M2 Join Date: Jun 2007 Location: GEORGIA iTrader: (0)	Sorry. AT. I'm replacing plugs tomorrow morning. Will report back.
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