My 1997 E300D has an intermittant mechanical knocking/slapping sound only at idle that is difficult to pin point the source. The slaping noise is loud enough to hear in the cabin and only occurrs for about 10-15 slaps/knocks and then disappears only to return again after a minute or so. With 220k on the odometer, other than the noise at idle, the car runs great with wonderful power & mileage. I'm convinced the noise is not fuel related (new injectors, new fuel lines, & new delivery valve seals all within the last 15k miles), belt related (new belt and tensioner shock), or fan related (new fan clutch and motor mounts). I'm somewhat at loss for what it could be ...... so I wonder if it could be the chain slopping around at idle against the chain rail possibly indicating the tensioner is bad. So I'm wondering if anyone with an OM606 motor has had this same problem? Is a slaping noise typical of chain tensioner failures? Is there a way to test a chain tensioner? Thanks for any help or advice you can lend.

David Roseman
Long Beach, Ca

bump. Dave dont know, was searching to see if a tensioner can affect cam timing. It *might* be the vac pump. A long length of windshield washer fluid sized hose and you should be able to pin point the problem.

If its the vacuum pump, get on it right away.

>>only occurrs for about 10-15 slaps/knocks and then disappears only to return again after a minute or so

I would suspect a hydraulic follower. These are designed to rotate a little every time the valve opens and closes - the cam isn't absolutely central. This means that the follower will slowly rotate in its bore, and when badly aligned relative to the cam, it makes noise, and when well aligned, it's quiet.

I would remove the cam cover, and inspect the upper surface of each follower - you may need to rotate the engine to relieve the load on each follower, to allow you to rotate by hand underneath the cam's base circle. This will allow you to see if there is any wear or spalling - there should be none visible.

Also, while you're there, use a non-metallic tool to check the stiffness of the follower - as described in the FSM.

There are a few while you're there jobs on the way to removing the cam cover;

- replace the rubber CCV bungs in the linlet manifold go hard and break off
- replace the injector hard line clamps which become brittle and break
- replace the injector leak off pipe and bung, which leak after a while