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  #1  
Old 07-05-2011, 03:05 PM
glenmore's Avatar
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Rough Idle 2000 C280

About 88m, car runs super but lately has developed a rough idle. Vibration is at idle in gear on a cold or warm car and you feel it easily on the steering wheel. Shifting into neutral alleviates the roughness but it is still there. Motor is not missing, drives normal, plenty of power. After a long hi-speed drive (5 hours) where the car is good and hot, the roughness is gone, idle is relatively smooth. Restart after a couple of hours and the vibration is there again.

Changed plugs but no change to vibration. I did notice that 2 of the plugs now have a good amount of oil on the threads. Three years ago at 60m, the same plugs had mostly sticky dried oil on the threads and some carbon buildup.

Any ideas? I have read that motor mounts may be the culprit but the brake/engine rev diagnostic is negative. Mileage suggests new motor mounts may be needed soon.

Thanks,

glenmore
1991 300CE
1999 C280
2000 C280
2000 C230K

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Old 07-06-2011, 04:33 AM
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replace those two spark plugs with new ones. If it makes no difference, could be that maybe plugs are oil fouling due to worn rings or oil bypassing valve stem seals and shorting out electrodes. Had similar experience with different vehicle, when put into drive engine would shake. Turned out to be valve clearances were too tight and needed to be adjusted to the right tolerances.
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  #3  
Old 07-06-2011, 08:31 AM
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To see if the mounts are the problem...

Use a jack & block of wood on the lower oil pan & slightly raise the engine (maybe 1/2") and see if the vibration goes away
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Old 11-10-2011, 07:38 AM
Dar Dar is offline
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Was this problem solved?
Hi, I'm new and have a similar problem.
Kind regards,
Dar
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  #5  
Old 11-10-2011, 11:39 AM
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Yes, it was the motor mounts. Makes a huge difference.

2000 C280 w202 motor mounts
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  #6  
Old 11-10-2011, 12:04 PM
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Quote:
Originally Posted by glenmore View Post
I have read that motor mounts may be the culprit but the brake/engine rev diagnostic is negative. Mileage suggests new motor mounts may be needed soon.
Just for the benefit of others trying to diagnose the issue, the "brake/engine rev" test isn't terribly useful for a couple of reasons. First, the ECU is unhappy when you try to rev the engine in gear with the brake on, it tries to cut power input.

Second, the mounts are hydraulically-filled, not rubber. They don't "tear away" like rubber mounts do. When the oil leaks out, they sag and transfer the engine vibration to the frame more readily; the engine isn't flopping around as with broken rubber mounts.

Hope that helps.

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