My C240 (M112) 2.6L V6 engine recently develops a strange noise when it is cold. As long as the engine operating temperature is below normal, the engine makes a vacuum-like (woooooo) noise when the car is traveling between 40km/h and 60km/h. The noise appears to be coming from the left side of the engine. The noise is more obvious or louder when the weather is colder. But once the engine reaches its normal operating temperature at 80 celcius, the noise disappears.

Also, when the engine is cold, I can feel the vibration from steering wheel when it is idling and when the transmission is in “Drive” mode. The vibration is transmited from the shaking of the engine when it is cold. I could also hear the engine makes this “duh-duh-duh-duh” noise when it is cold while idling with the transmission is “Drive” mode.

I took it back to dealer, and the dealer could not locate the problem. I appreciate for any invaluable inputs you give me.

Thanks,
Johf

2001 C 240X
54,000km (34,000 miles)

All that I can surmise from your description is that it sounds like a valve related problem.

Good luck,

Thanks Larry,

Could you be more specific, when you said" valve related problem". I understand that it is normal for the M 112 engine to exhibits this slight diesel like clatering noise when it is idling.

Johf

Sounds like the electric smog pump to me.

This has been affectionately renamed the "Hoover" by most techs because it sounds like a vacuum.

Brian

__________________
Mercedes Benz Master Tech
ASE Master Technician
Diagnostic Technician
Owner of "Behind the Star Inc."
Raleigh NC.
www.behindthestar.com
brian@behindthestar.com

What is the electic smog pump? Does it make thos "wooooo" sound when the engline is operating below normal operating temperature while you are traveling between 40km/h and 60km/h.

Yes it does make that noise. If you lift the plastic cover on your engine it is the black pump at the very front on top.

Brian

__________________
Mercedes Benz Master Tech
ASE Master Technician
Diagnostic Technician
Owner of "Behind the Star Inc."
Raleigh NC.
www.behindthestar.com
brian@behindthestar.com

My C320 makes a pronounced intake growl now that the weather has turned colder. Sounds like a rythmic growl that increases with rpm. Sure does sound like an air pump related thing.

BTW, how would I remove the plastic cover on the C320? The front section has a hinged arrangement but pulling up doesn't seem to budge the cover. Does it require a lot of force?

Thanks,
Brian

Not too much force to lift that cover. It has metal clips that slip over rubber grommets. They hold it pretty good.

Brian

__________________
Mercedes Benz Master Tech
ASE Master Technician
Diagnostic Technician
Owner of "Behind the Star Inc."
Raleigh NC.
www.behindthestar.com
brian@behindthestar.com

Thanks Brian,

What does the electric smog pump do?

I understand that when you start the engine from stone cold, the air pump will run for the first minute. I understand that this perfectly normal.

Perhaps, I do not know if it is normal for the air pump to continue to run until the engine temperature reaches normal.


Johf

I will have to defer to the experts on the functioning of the "smog pump" on MB cars. On the American made cars I've owned, it is belt (engine) driven and is used to inject air into the exhaust stream for more complete consumption of emissions gasses. Runs all the time, but is certainly not the noisemaker that the MB unit seems to be. I'd also be interested in knowing more about the functioning of the MB unit.

Brian