1991 420SE: timing chain slipped off
 

Hi,

420SE 99,000 miles. Car was running absolutely fine and parked for the night, the next morning started car and bang bang bang, switched off immediately.

Had it towed to my local Indy where he pulled both heads to look at damage. Result is the chain tensioner has collapsed and chain jumped off causing major damage; dents to top of piston with four valves mashed to not much.

Mechanic has suggested replacing with four new piston's, four valves and valves seats. Basically rebuild one side of the engine.

My question is what will be the end result by having four cylinders with a new higher compression and other four with a compression lower due to 99,000 miles on them.

Is the engine likely to shake about having varying compression's or maybe even vibrate?

Just weighing up options as to whether doing a) patch up job, b) full rebuild or c) second hand engine.

Any input will help to make a decision.

Thanks

They can check compression on the undamaged cylinders to see if they have degraded at all (unlikely with 99k on them). I am assuming with only 99k on it that this is a nice condition vehicle, I would have it rebuilt (whatevers needed) and keep on driving it. A 420SE is a pretty low-production model!

Regarding piston dings:

If the interference between piston and valve has resulted in less than about .100" of a notch in the piston, I would be of a mind to simply repair cylinder head damage (freshen the undamaged head also). The forces that were present at startup speed, while able to bend valves due to asymetric loading, are not great enough to crack the piston. Clearly you will also want to examine all of the chain guides (particularly the lower one in the timing case).

Frank, thanks for your input. Will get all guides checked.

PawoSD, yes the car is perfect in every way, transmission, interior and exterior.

Would you get just the damage rebuilt or a full rebuild?

Any ideas why this would happen on such a low Milage engine? Oil changes have always been done on time.

100k is about the norm for a chain. you bring it to me, and whether i know you and the car or not, i STRONGLY suggest at 100k, we pull the valve covers at EVERY oil change to check for chain stretch. at 6 degrees or less, look again at the next oil change. at 7 degrees or more, time for a chain, tensioner and rails. seen many a bent valve, never replaced pistons or a motor. good luck, chuck.

Benedict, recommend having your indy (or another reputable one) carefully inspect the pistons and connecting rods on the damaged bank to determine if replacement is necessary ... probably not. Have both heads reworked at a machine shop with valve stem seals replaced, as a minimum, on the good head ... whatever it takes to make both like new. Go ahead and replace the timing chain, tensioner and all guide rails ... no sense measuring the timing chain stretch at this point.

Benedict, after thinking about it some more I retract my recommendation to immediately replace the timing chain. After reassembling the engine with new timing chain guide rails, tensioner and reconditioned head(s) measure the timing chain stretch. If acceptable, don't bother replacing the timing chain. Of course, that presumes the labor to check the timing chain stretch is less than the cost to purchase/install a new timing chain. It's likely one of the old timing chain guides failed and caused the timing chain to jump.

That's terrible! 99K miles???

Does it have a single or double chain?

Many say the double chains should last the life of the engine - my M104 6 cyl is at over 250K miles and on the original chain.

What, what, what. You did say it was a Mercedes??

RayH

RayH, yes and a much loved Mercedes. It just doesn't make any sense for this to have happened at such a low milage engine. I believe it is a double chain but I'm yet to go to the workshop and have a look at the damage.

Mechanic says that the timing chain guide is what failed.

Freedman, thanks for your input. Just to be on the safe side, I will probably put in a new chain. At least I know it is done and hopefully have trouble free motoring from there.