Knocking - Rod or Valves - PeachParts Mercedes-Benz Forum

sdent · #1 07-12-2002, 01:29 PM

My 1987 560 SL with 234,000 miles began knocking on a short Sunday drive. The knocking started fairly quickly and did not come on gradually. I took the car to two local Mercedes specialists for preliminary diagnosis. First guy is almost sure it is a rod knock (major repair) and the second is almost sure it is a valve and a valve job should solve the problem. (less major) Both mechanics are very reputable, but each has a very strong contradicting opinion. (maybe I should settle it with a third opinion, haha)

Any one have similar experience? With this many miles, should I possibly be looking to rebuild or buy a newer engine? It runs very strong with good compression. Rest of car is in excellent shape overall. I'm still not sure which mechanic to take it to for full diagnosis, but any insight would be appreciated.

dpetryk · #2 07-12-2002, 02:18 PM

It should be easy to differeciate. A rod knock would happen at twice the frequency of vlave knock. Once per revolution of the crankshaft for rod knock, once every other revolution for valve knock.

sdent · #3 07-12-2002, 04:23 PM

Thanks for the direction, but maybe I'm not thinking too clearly. It makes sense that the Rod would knock twice for every valve knock, but if it is only one of the parts that creates the knock, how would I gage it against a part that is not making any noise to determine which is the culprit?

I'm thinking the surest way is to pull the covers and start eliminating the lesser expensive options first. Hopefully I don't have to travel too far from the top!

G-Benz · #4 07-12-2002, 05:01 PM

I've never heard valves knock...they typically click.

Check pulleys and belts. Could also be some timing chain component as well.

An engine stethoscope can pinpoint far better than just an ear near the engine...