random engine knock

[Col Webb]

I'm getting a random / intermitant knock from the engine. It is coming from the front of the engine and seems to be coming from the top. It is not a rhythmic knock. No knock at idle but appears above 2000 rpm. I removed the valve cover and ran the engine (covered valve train with cardboard) and did not hear a pronounced knock. Before start, timing chain was loose. It tightened up after running.  I think the problem is a worn timing chain. Am I correct? Any input is appreciated. Thanks

[w113dude]

Have you checked your timing?
you can tell if you have a stretched chain, rotate the crank to TDC, with the two front cam lobes facing up the marker right behind the timing chain sprocket should line up, if it is right on then your chain is good. I would also check the tensioner.

[Col Webb]

Thanks for the response. I dont think its timing for it does not sound like preignition. Sounds more like metal hitting metal as if the timing chain was hitting the block or valve cover or something else such as the tensioner is gitting. I cant locate using a mechanic stethoscope touching valve cover and block. But can clearly hear that it is at the front of the engine in area of timing chain.  I'll check the timing mark. Thanks. Chuck

[Benz Dr.]

It could be a loose or worn bushing for the tach drive. Mine made a chirping noise but I suppose it could be different or every engine.

[enochbell]

If it is more a "clacking" sound you might want to check the torque on the valve cover bolts.  If it is too high you can distort the towers that support the oiler and it can interfere with the valve rockers.  Don't ask me how I know.

Greg

[garymand]

Have you inspected a rod bearing recently?      My wife barrowed my 250SL once and the accelerator stuck, motor reved high for a long time.  I pulled the small pan and checked the front bearings.  They were fine.  But, Some idiot forgot to torque one of the rod nuts.  I had a few authorities listen to the noise.  Picture a rod with only one nut.  It was strong enough to work well at low rpm and low stress.  But  It was always above 2000 and sounded high pitched clickclicking like a lifter.  It did not sound like a clack as in bad bearing.  It finally let go, giving no doubt what had gone wrong.  I now have two problems solved: a 280SEL motor and my wife will not drive the car.