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Old 10-04-2002, 03:14 PM
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Loud knocking noise from the engine

Help!!! I started my '91 300E this morning and there was this knocking and tapping sound coming from the engine. It gets louder and seems like something was about to fall apart when I put the car in gear and foot on the brake. The oil pressure was fine and no warning lights came on. I popped the hood and listened and it seemed like the noise was coming from the top front of the engine by the distributor and the timing chain cover. Broke out the mechanics stethoscope and confirmed the knocking /tapping noise was coming from the forward part of the cylinder head. As I was doing this, the engine warmed up and the noise seem to decrease but was still there. Still gets louder when I put the transmission in gear.
Can anybody out there esp. MB DOC and Stevenfl give me an idea on whats causing the noise. Is this an indication of a timing chain going bad or much worse? Mileage on the car is 124K and never had major problems with it. I did not move the car at all today and probably not until I get help from the forum and Im sure I don't damage the car any more. HELP and thanks in advance.
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Old 10-04-2002, 04:31 PM
G-Benz's Avatar
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Join Date: Apr 2001
Location: Dallas/Fort-Worth
Posts: 5,711
My first guess without actually being there is that the belt tensioner shock bushing has worn out to the point that the shock is just banging around where it mounts...

...the noise (which most describe as a connecting rod about to self-destruct) usually occurs at idle, but disappears when the car is in motion.

If it is indeed the belt tensioner, then you can push against the shock mount and the noise will immediately disappear. Then you're talking about a costly $2.00 repair for replacing the worn bushing!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car
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Old 10-07-2002, 01:02 PM
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Thanks G-Benz! You were right on the money in diagnosing the problem. I didn't give the belt tensioner shocks much thought because by looking at it, it was pretty much intact. But what happened was the rubber bushing hardened up over the years and shrunk in size and started rattling. Once again, thanks.
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