79 450 sl timing chain problems..help! - PeachParts Mercedes-Benz Forum

newslowner · #1 01-05-2006, 09:10 PM

I recently bought a 79 450 sl that I bought without hearing it run....we put in a ring gear because the ring gear has a bad section. When we got it started, it sounded really bad, with what sounded like a lower end knock and alot of valvetrain noise....sound ing like a diesel. We took it apart thinking it was a spun bearing. What we found were pieces of the timing chain guides...the plastic ones in the bottom of the pan. Upon getting the front cover off, when the chain is in a certain position the chain gets slack and allows the center gear to move back and forth producing the lower end knock noise....Help!! what do I have here....what do I need to replace, could the valves be bent? The chain is intact...what should I look for?
Any help would be appreciated...thanks John.....PS to put things in perspective...this is what I bought my wife for her 49th B-day, and I don't have it running yet!

Strife · #2 01-06-2006, 12:18 AM

Owww...definitely do not run or spin it whatsoever again right now. The upper guide pieces probably wound up in the bottom of the pan. The chain will be loose and it may have jumped a tooth. If it did or if the chain got so loose that it allowed the cams to be too far out of sync, it is likely that some valves may be bent. There are numerous posts on this subject, some recent - search on timing chain. I hate to be the bringer of bad news. I hope you get lucky and the chain was just slapping around, not enough to allow the pistons and valves to meet. In any event, you would have to replace all of the guides (you have the front cover off, so you could get at the lower ones also), the tensioner and arm, and the chain. The gears may be beat up. I hope you have the 107 manual. The front cover looks like a complex sealing job - I've never done this. How many miles are on this vehicle? Was it maintained fairly well?

86560SEL · #3 01-06-2006, 12:33 AM

Hmm. I did not know that "broken guides" would cause a lower engine knock.

Suppose a car was running fine, but had a slight engine knock @ lower rpms on acceleration, would that mean that the guides are broken, despite the smooth running?

Just wondering because someone was inquiring about that a while back.

Strife · #4 01-06-2006, 01:07 AM

Well, if the chain is intact, and the tensioner is working somewhat, and the engine is turned clockwise (only!), and the guides are in place (even with the ends broken off) there should be no position where the chain is slack.

If the guides are broken, the chain may not be under complete tension when the engine is revved up/down and the cams will not be in time with the crankshaft and pistons. The relationship has to be very exact (and the crank turns twice for every turn of the cam). I think I read somewhere that guides also prevent harmonics in the chain (no doubt you've seen long belts do this - vibrate at certain RPM's). So, you might expect anything from rough running and various bad noises (possibly the chain hitting side to side in the engine, don't know what that would sound like) to valves tapping pistons (don't want to find out what that sounds like either, really) depending on how loose it is. I can definitely say that from what I saw, the chain will be loose at some loads/rpms without the entire length of the guides in place. If the tensioner is bad (it probably is), it will be loose with or without broken guides.

I'm not sure that I have enough experience to tell the difference between a bearing knock and other equally disturbing noises. Worse, I know that aluminum engines tend to soak up noise more than case iron engines do. So, it might be hard to locate the exact source of a noise like that.

Strife · #5 01-06-2006, 01:14 AM

Disclaimer: I'm not a pro mechanic, but replacing the chain and associated parts was one of the first, and potentially the most expensive car project that I had undertaken in a long time, so I read up on it and analyzed the heck out of it before and during the "operation". I'm still of the opinion that chains don't go bad; the "chain" of events, if you will, is that the tensioner goes bad; the guides get slapped around; eventually, pieces of the guides break off and cause mistiming or worse, get stuck between the chain and timing gear. THEN the chain breaks!