Odd "thumping" noise at idle...

[Daesun]

I have a 98 CLK320. Starting a few months ago, I've noticed a light thumping noise coming from the engine bay on cold mornings. It's most noticeable when I'm stopped at a light, and the engine is at idle speed. It's not very loud, but it happens at regular intervals (one "tap" every 1-2 seconds). The car didn't do this at all when I first got it, even during the coldest starts. I change the oil every 3000 miles with Mobil 1 10W30, and keep all my fluids healthy. Any idea what this might be? I haven't taken it to the dealer yet, largely because they seem to have difficulty solving ANY of my problems, and the service is horrid. Thanks for any help you guys can offer.

On a side note, there are all sorts of creaks and rattles, but I've heard this is a common problem with CLKs, especially MY98-99. Again, the dealer is of little help. Any suggestions?

-Daesun

[280CEBoy]

It's possible what your'e hearing is a mis-firing cylinder....when have you had the ignition system checked? I'm willing to bet if it's put on a scope there will be a misfire shown.

[David C Klasse]

If it's misfiring, then do you notice a change in the RPMs? They might flutter a little. But if the misfire was bad enough, it would turn the Check Engine Light on.

[280CEBoy]

Funny you should mention setting a code...Last week I had a 00'
F-250 in the shop, this has the coil-on-plug setup, #2 and #8 coils were failing, but it never set a code either, it's possible if the misfire is intermittent enough that it won't. On the Benz though I'm not sure how tight the specs are for setting a code.....

[tgrand]

Check your water pump. I had a mystery thump that I noticed when I would get out of my car to get the mail. Grab the pully, with the motor off, and try and move it back and forth, it SHOULDN'T move at all. If you have any play that's a starting place.

[Daesun]

Thanks for the help, guys. I will check for both the misfiring cylinder and the water pump. Dave C Klasse, I haven't noticed any RPM fluctuations, but I'll keep an eye out the next time the thumping rears its ugly head (hasn't been doing it lately). One thing I can't stand about the dealer is their refusal to believe that there is something wrong with your car unless you can reproduce the symptoms on the spot (which, of course, never fails to be impossible). Assuming it is one of these two things, what would you estimate repair costs to be? Should I be worried about any permanent damage to the engine? Or is it something that I can have tended to at my convenience?

-Daesun