420 sel discovery thread
 

forum i have e new project,why does the 420 have so much trouble with the timing chain?
if you have any theories please post.
i just picked up a 1987 with 217k and the previous owner had been driving making stops at shopping and such, tried to restart the engine and got a loud noise,looked under the hood and the drivers side valve cover now exposed the timing chain.
i have read a lot of the post about the same problem,and will complete a study.
chain tensioner failure would be my first suspect, would anyone know if the tensioner is the ratchet type or does it rely on oil pressure to work?
arry perkins lou ky

It is not a ratchet type, it is oil pressure based, and it wasn't the problem or atleast not the failure. It very easily caused a failed part to be the problem.

When the valve cover breaks for sure, and in almost all other cases, the problem was a brittle as glass, guide rail fracturing and pieces of it rolling up under the chain breaking the valve cover by the added height of the chain riding on the broken piece over the gear and jumping the chain out of timing.

Also, as sure as taxes it will have only bent the exhaust valves on the left head. The reason it happened was the rail, which is durable nylon type material originally got petrified over time and heat. We recommend changing the rails atleast every 5 years. I'll wager that they were original. Originally white they will probably be almost dark brown. When they turn amber its time to replace.

Meyle has new aluminum backed rails for the M116. Surprised and saddened that they're not for the M117

Typicaily, chain wear as well as guide wear cause the inner drivers side chain guide to break and the chain carries the broken piece along and between the chain and the sprocket, causing the chain to skip a tooth or two, the cam cover to break and the valves and the pistons to collide.

The head needs to come off and the valves replaced. It's a big job. You could find a replacement engine.

reply
 

well we got ideas,thats a start,just seem strange to me that the problem comes at start up,just thinking,could the motor oil be leaking from the check valve at the filter back thru the oil pump and the pump loses prime and then the chain gets loose from lack of oil presure for that instant the oil pump builds pressure back? one good test would be to let one sit, check the oil level and see if the level rises.
steve on the 5 year cycle that you recommend,do you change the tensioner also?
larry perkins lou ky

The reason the rail breaks is not due to wear. The rail never touches the chain during normal operation.

The rail gets fragile due to temp, oil immersion, and age. The rail is there to keep the chain in line when the tentioner is not doing its job. NO MATTER HOW NEW, the tentioner will not do its job occasionally. The job it doesn't do is to check the oil pressure and maintain tention without pressure when the engine is off. The result is a momentary rattle on start up that instantly goes away when the oil pressure rises. The easiest way to see this is when the event randomly occurs after an oil change. When that happens the rattle is prolonged as the pump fills the filter. The oil change is not causing the problem it is just showing the relationship to pressure when one has lost it. If one wished to verify this (and had new rails to live through it), one could remove the tentioner and squeeze it in a vice. This will drive the retained oil from the device and when reinstalled the chain will rattle till pressure rises.

I do not change the tentioner unless the rattle event is common. If I hear it during service I will infer it happens regularly and suggest a tentioner. If you are aware and notice it when it occurs and it only occrs every few months or every few hundred starts I would not worry if I had GOOD rails.

Whenever I do an oil change on an old V8, I unplug the crank sensor wire and give it a crank before starting it up for good- just in case.

and I thank you for that.

I do not ever recall a rattle sound after oil change, I once opened the right bank just to look and could not move the chain,I don't know what that would tell but I was curious about its tension,with 178000 on her and know idea if it has ever bean changed. the only trouble I have is fuel drip into the plenum chamber(intake) and the need to dry that up. JNT