C280 idle speed - Mercedes-Benz Forum
 
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post #1 of 8 (permalink) Old 09-20-2018, 06:14 PM Thread Starter
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C280 idle speed

My 1998 C280 idles at about 700 rpm in neutral but drops to about 500 rpm when put in gear. This results in some vibration. Should the idle speed be dropping this low?
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post #2 of 8 (permalink) Old 09-21-2018, 10:51 PM
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Our 99 c230K drops similarly, but no vibration. Very quiet and smooth.
Have you check where the vibration is coming from?
Perhaps engine and tranny mounts?
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post #3 of 8 (permalink) Old 09-22-2018, 05:44 AM
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These RPMs under load are normal, vibration is certainly not...
Quote:
Originally Posted by khomer2 View Post
...Perhaps engine and tranny mounts?
I have to agree, motor mounts would be the first thing I would inspect... This video, about 0:40 sec into it would give you a good visual of what the failing motor mounts "look like" https://www.youtube.com/watch?time_c...&v=6HrRLjoPark

Regards,
D
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post #4 of 8 (permalink) Old 09-22-2018, 08:48 AM Thread Starter
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The motor mounts are less than two years old and have about 25,000 miles on them. Its just a slight vibration due to low engine speed. The thing is, I don't remember the idle dropping this low when the car was newer, and I wondered if the computer was supposed to bump up the idle to keep it in the 700-800 range to compensate for load.
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post #5 of 8 (permalink) Old 09-22-2018, 09:11 AM
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Our w202 kompressor rpms dropped and ran very smoothly after:
1) maf cleaning with CRC maf cleaner
2) bottle of berryman (into both the vacuum intake and throttle intake) - to cleanup what the previous owner's minimal maintenance routine
(got some misfiring and an engine light after this which is normal...)

3) air filter change (did not do an oil change yet as the color was good)
4) oil separator and line checks
5) spark plug checks
6) egr vacuum check (diaphram working ok) - did not open up to check the lines to see how clogged up they may be - too lazy.

Have you scanned for any codes?
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post #6 of 8 (permalink) Old 09-22-2018, 09:18 AM
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Quote:
Originally Posted by duh_vinci View Post
These RPMs under load are normal, vibration is certainly not...

I have to agree, motor mounts would be the first thing I would inspect... This video, about 0:40 sec into it would give you a good visual of what the failing motor mounts "look like" https://www.youtube.com/watch?time_c...&v=6HrRLjoPark

Regards,
D
I agree, but, honestly, Iv'e dealt with other cars where the motors mounts had failed and the engine ran fine with minimal vibration/shaking. My neighbor, a bodyman, was the one that picked up one of our minivans ONLY after taking a closer look and noticed a slight "clunk" when the tranny engaged into drive or reverse. Every car is different and I see your point.
Doesnt hurt to still check them visual, that's why I asked.
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post #7 of 8 (permalink) Old 09-22-2018, 03:56 PM
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some more nfo that ay assist you.
https://www.benzworld.org/forums/w20...-rpm-drop.html
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post #8 of 8 (permalink) Old 09-23-2018, 06:40 AM
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Quote:
Originally Posted by fatherfixit View Post
The motor mounts are less than two years old and have about 25,000 miles on them...
Then I would stick with the list of khomer2 suggestions/steps. As at 500-600 rpm range with D or R engaged, one should have no vibrations.

Regards,
D
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