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Old 01-19-2002, 12:27 AM
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Question M112 engine humming noise when cold


My C240 (M112) engine recently develops a strange humming noise when it is is cold. As long as the operating temperature is below normal, the engine makes a humming noise when the car is travelling between 25mph and 40mph. The noise appears to be coming from the left side of the engine. Once the engine reaches its normal operating temperature, the noise disappears.

I took it back to dealer, and the dealer could not locate the problem. The worst thing is that they told me this is normal. I think they just could not diagnose the problem. I told them the noise was not there before, but it does not appear that they care to investigate the problem further.

I appreacite if anyone could offer any advise on this problem.

2001 C240X
38,400km (24,000 miles)

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Old 01-19-2002, 12:45 AM
Gilly's Avatar
Registered User
Join Date: Mar 2001
Location: Evansville WI
Posts: 9,616
Well, I know when you first start the engine cold, there is an electric motor with an air pump attached that runs, it's part of the pollution equipment, called the AIR pump. It runs for pretty close to 2 minutes, so try starting it cold, listen and see if it absolutely STOPS at about 2 minutes. This is a "stone cold" engine, after sitting overnight. After shutting off, after waiting about a minute, you'll also hear the motor run again for just a second, that's the computer checking the system to make sure it's functioning OK.
My bet is that it's done it all along and you just noticed it now. Sounds kind of like a vacuum cleaner noise.
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Old 01-19-2002, 12:54 AM
blackmercedes's Avatar
Just a guy
Join Date: Dec 2000
Location: St. Albert, Alberta, Canada
Posts: 3,492

My Aunt has a 2002 C240 Automatic, Classic.

Her car does the same thing. It's a "rrrrr-rrrrr" sound? She took it in, and the C240 loaner they gave her did it too!

John Shellenberg
1998 C230 "Black Betty" 240K
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