V8 Timing Chain: What's up with it?

[KCM]

I'm a newbie to the whole Mercedes V8 experience, and have read a lot on this site about the need to replace the timing chain, apparently after 100,000 miles. I am familiar with Mercedes diesels, and these engine run much longer without the need to replace the timing chain, plus they run an injection pump which can pull pretty hard. But it appears that the V8 can self-destruct sometime after 100,000 if it is not changed. I know the early 380 engines used a single roller chain, hence their failures.

My question is what exactly goes wrong with the timing chain? Does it completely break, or just stretch enough to cause the valves to hit the pistons? Does it need changing only after it has reached a certain stretch (and if so, what amount is that) and is okay until then, or after so many miles automatically (such as 150,000 miles) regardless of the stretch, or lack thereof? Or does proper maintanence play a large factor as to longevity past 100,000? Design flaw? Guide breakage?

What's the scoop? Any layman or technical explanations welcome.

[Walrus]

Originally Posted by KCM

My question is what exactly goes wrong with the timing chain? Does it completely break, or just stretch enough to cause the valves to hit the pistons? Does it need changing only after it has reached a certain stretch (and if so, what amount is that) and is okay until then, or after so many miles automatically (such as 150,000 miles) regardless of the stretch, or lack thereof? Or does proper maintanence play a large factor as to longevity past 100,000? Design flaw? Guide breakage?

What's the scoop? Any layman or technical explanations welcome.

The V8's chain is nearly 8' in length... With some 190+links which do stretch more than a shorter chain would under similar circumstances. This said, the chain does not necessarily NEED to be changed at 100,000, if measured stretch is within acceptable Tolerance. Now for the real concern: The Chain Guides (especially the upper). The guides are plastic, and prone to brittle failure after so much TIME! exposed to heat, oils, etc, and can break loose and scotch or hang a chain, which can cause premature breakage (with the resultant cam/piston/valve damage you have heard about). A low mileage 1980ish SL will need attention to the guides before the chain is stretched enough to matter (a degree of cam rotation or two). It is time, as well as mileage that takes its toll on the guides! Facts are, as long as you are in the vicinity of the chain, changing out the guides, why not go ahead and change the chain? That is the logic most often used when addressing the timing gear of the V8SL. Another point of concern is the tensioner... If it fails to hold the chain taught, the chain can slap around a bit while the oil pressure finally builds up enough (if it ever does) to tighten the chain. This can accelerate the chain guide wear, causing premature guide failure, etc.

Numerous posts on every SL newsgroup concerning this topic. I encourage you to search the fora and read the posts for even more info on the subject.

[JBR]

I have a 73 450sl with 91,000 miles, the chain and guides were replaced prior to my ownership. I removed the valve covers for sanding and repaint.
I was very supprized to see deep grooves cut into the side walls of the engine, that chain must have been very loose to slap the sides that hard.
I would recomend to you, remove one vale cover and see how tight the chain really is. On mine the valve cover only had 4 bolts, do not over tighten when putting back on.

[Benzmeister]

end of the engine rebuilt and claims and is on the the service order, for the rebuild said the it was converted to dual timing chains, I asked the owner about that and he said if one failed the other would keep the engine from seizing. I guess I have to take his work and that of the shop, the rebuild was in 95 but since then only 5K miles were clocked on the odo. Sincd I bought it, I have clocked over 2K. Can anyone vertify this is a legimate modification to the engine?

thanks!

[Walrus]

Originally Posted by Benzmeister

for the rebuild said the it was converted to dual timing chains, I asked the owner about that and he said if one failed the other would keep the engine from seizing...
thanks!

Obvious BS, or serious misunderstanding... The 450 had originally a double-row chain. I have never heard of two, separate timing chains, and seriously doubt the voracity of the claim (after all, the timing cover on the front of the engine is only so deep, and where would a broken chain go that would not foul the other one). The US version of the 380Sl, for the years of 1981-1983 were the only V8 107's to use the single-row timing chain and gears. The European versions of the 380 for all years had double-row chains.

[Benzmeister]

The guides and chain were replaced I read the serviced record wrong and for some reason misinterpreted wha the previous ownver had told me, after only 6 weeks of owning the car. Sorry!

[Walrus]

Originally Posted by Benzmeister

The guides and chain were replaced I read the serviced record wrong and for some reason misinterpreted wha the previous ownver had told me, after only 6 weeks of owning the car. Sorry!

Thats cool! You have around 15 years or 100,000 miles not to worry with the chain and guides, unless the tensioner gives out (cheap, easy fix if it does). Keep the oil changed on regular intervals, and run her to full operating temp often and you'll be fine.

[1991 560 SEC]

After sitting for a week or longer, and after an oil change, disconnect the EZL wire and crank the engine. This will prime the chain tensioner so that the chain dosen't flop around.

John

[derryb]

good info on measuring chain stretch and replacing chain.

[embe560sl]

Originally Posted by 1991 560 SEC

After sitting for a week or longer, and after an oil change, disconnect the EZL wire and crank the engine. This will prime the chain tensioner so that the chain dosen't flop around.

John

Disconnect the EZL wire ? Where is this?

[Benzmeister]

Originally Posted by KCM

I'm a newbie to the whole Mercedes V8 experience, and have read a lot on this site about the need to replace the timing chain, apparently after 100,000 miles. I am familiar with Mercedes diesels, and these engine run much longer without the need to replace the timing chain, plus they run an injection pump which can pull pretty hard. But it appears that the V8 can self-destruct sometime after 100,000 if it is not changed. I know the early 380 engines used a single roller chain, hence their failures.

My question is what exactly goes wrong with the timing chain? Does it completely break, or just stretch enough to cause the valves to hit the pistons? Does it need changing only after it has reached a certain stretch (and if so, what amount is that) and is okay until then, or after so many miles automatically (such as 150,000 miles) regardless of the stretch, or lack thereof? Or does proper maintanence play a large factor as to longevity past 100,000? Design flaw? Guide breakage?

What's the scoop? Any layman or technical explanations welcome.

self destruct at 100K (usually), I seen cars with 125K plus miles and the original chain. But as Walrus said they ar chains and the stretch, a link could break. I have also seen timing chains, 100K miles type break at 50K miles. By the way do Mercedes V8 seize when the timing chains breaks or do they stop running. I forget what they call these different designs? Walrus help me out on this one.

I posted a definition of DOHC and it is vague at best, it described a DOHC as a engine with 2 cams, but didn't say anything per bank etc?

[Walrus]

Benzmeister,

Your definition was spot-on... for one bank. Reference the statement, one cam drives the exhaust valves, the other the intake valves, ie 2 cams on one bank=DOHC. Usually, this geometry results in a hemispherical head, with the sparking plug dead center atop the combustion chamber, between the two cams (but not necessarily).

Our engines are called interference fit, or simply interference engines. This means the piston can (and will) contact the valve in its open position if the valves get out of time with the crankshaft. In one example, for our engines, a slipped or broken chain will stop the cam(s) from turning, leaving a valve or valves protruding into the combustion chamber. The piston comes along and smacks it quite violently, driving the valve upwards, breaking the cam, the cam brg mounts, etc, as well as bending the valve, damaging the piston, scoring the cyl walls from the debris, etc. Usually, the valve cover(s) are damaged as well, as everything tries to get out of the way. The engine doesn't immediately sieze; it tries to continue to run for a bit with all that momentum built up in the flywheel/torque convertor and crankshaft. By the time everything gets stopped, you may have damage on only one bank, the left one if I recall correctly, or both, depending on the rpms at the time of failure.

While I am not experienced with this failure personally, I have read a few horror stories. In each that I have read, I have not been satisfied that initially the chain broke, but that the guides broke and wedged between the chain and cam gear. This lifts the chain and contacts the valve cover, usually punching a hole thru it. Then the timing gets off due to slipped cog, broken chain and the damage ensues...

Incidently, there has been a reported case of piston/valve damage with just replacing the chain. The mech let the chain slip on one of the gears as he was turning the engine by hand. There is significant leverage on the crank and piston rod, which was imparted onto an open valve... Care MUST be exercised, as it only takes approx 12 degrees of cam timing to cause contact (someone may correct me on teh degrees as I cannot say I remember for sure). Count the teeth, and divide the number into 360, and you'll see it does not take too many teeth to get out of time enough to matter.

If you are lucky, you may hear a bit of chain slapping before impending disaster. This is especially true if the tensioner has failed or otherwise not keeping the chain taught. If this should happen (the chain slapping), one should discontinue operations immediately until it can be determined what the cause actually is. It could be as simple as a malfunctioning tensioner, or worse... I would go as far as to recommend the vehicle be towed, not driven, to service in the case of a slapping chain (which can take its toll on the guides fairly quickly, I surmise).

[1991 560 SEC]

The EZL wire is on the left inner fender, closest to the fender. It's the coil wire. Just trace it back.

John

[embe560sl]

Originally Posted by 1991 560 SEC

The EZL wire is on the left inner fender, closest to the fender. It's the coil wire. Just trace it back.

John

By left inner fender...do you mean the drivers side? and do you mean attached to the fender? lastly,( PLEASE BEAR WITH ME) trace it back towards what, the firewall?

[1991 560 SEC]

Yes the drivers side. Look behind the headlight as the inner fender starts to rise, you'll see two round electrical plugs it's the one on the right closest to the fender. It goes to the coil that is located on the frame near the exhaust manifold.

John