Need a little advice from the pros - PeachParts Mercedes-Benz Forum

GregS · #1 05-16-2002, 08:01 PM

On this page I have seen those that say 420sel chains break because pieces of the guides break off and get lodged in the chain, breaking it, and those that say the chain breaks from stretch. So here is my problem:

I've found a beautiful '88 420sel for sale with 120k, and I think it is fair to negotiate the price of the car down based on the cost of having this issue addressed. Obviously, if all it needs is guides the repair is quite inexpensive, but if it also needs a new chain rolled in I think I am looking at between $600-$1k.

Unfortunately, the dealer (a Mercedes dealer!) is saying that the timing chain isn't an issue. I certainly don't agree, based on my two years of reading posts on this board, but I want to be more informed. In response to a similar question about a 560, stevebfl said just replace the guides. Is there some difference between the 560 chain and the 420 chain? What would you guys do?

Thanks,

Greg
'84 300D

Ashman · #2 05-16-2002, 10:01 PM

I would tellt he dealer if it isn't an issue, then why dont they check the guides and show you them.

I'm no pro though, but thats what I would look to have them do. it can't be that hard for them to pull the valve covers and check the guides..

Alon

euro 287 · #3 05-16-2002, 10:41 PM

Dealer is not going to admit to any problem with chains on this car ( they say they have no problems with harness' either). If the chain is original it is probably stretched which leads to it slapping the drivers side rail which eventually brakes it and it gets between the cam gear and the chain and there ya go jumped timing. Now replace the chain,tensioner,both rails on drivers side , upper and lower tensioning rail ,and oil rail kits.

blackmercedes · #4 05-16-2002, 11:10 PM

Right. I've yet to see any dealer admit that those plastic top rails age poorly and cause problems.

If I owned a 420 car with the M116 engine, I'd have it done. I would NOT replace the top guides without changing the chain. I'd do the chain, the guides, and the tensioner, and then sleep well at night.

MikeTangas · #5 05-17-2002, 02:08 AM

The thing with the V-8 chains is they are long - six feet long! That length and route they travel causes the chain to make many turns as the chain goes around the crank up to the head, around the cam(s) back down to the center and around another gear then up to the other cam(s) and back down to the crank. That length and route when coupled with infrequent oil changes really wears the pins.

On the other hand, the smaller 6's the chain is short and only make two turns, at the crank and the cam(s). They nearly last forever.

The plastic rails get old, discolor and become brittle. When brittle they break easily. Every 420 I have seen with a broken chain also had a broken rail in the left head and when the chain broke it wiped out the left head valve train (cyls 5,6,7,8). Usually what caused the chain to break was a broken rail lodging a chunk in the sprocket which broke the chain.

The only way to tell the condition is by pulling the valve covers if the matter hasn't been addressed and is in the records.

If you are mechanically inclined you could do the chain yourself, as well as guides and tensioner and drip tubes for about $250 in parts. If not, then try to negotiate this preventative job as part of the purchase - they do the chain etc, you'll buy it.

If you don't change the chain, guides etc, what could be a $1,000 job (if you don't DIY) will quadruple as you replace chain, guides, tensioner valves, cam(s), towers, maybe pistons and whatever else gets mangled when everything gets out of sync.

stevebfl · #6 05-17-2002, 08:58 AM

I have done hundreds of chain failures over the lats 27 years going back to 450's.

In all V8s, from 71 till the 4 valve came out in 90, there have been two timing chains. One for the 3.5, 3.8, 4.2 motors and another for the 4.5, 5.0, 5.6 motors. Wide and narrow blocks I call them - did you know that one can instantly tell a wide block from a narrow block by looking at the airfilter housing mount to the valve cover. The narrow engine has a extention bracket to get it the width of the airclleaner. The wide blocks bolt the aircleaner straight to the mount to the valve cover. The brackets take up for the small motor being about an inch narrower at the valve covers.

The only reason I can think of to replace the chain is due to stretch. After about 150k there is about 10deg retard in the right (following) cam. Twenty-five years ago i used to dial indicate and repair this timing issue with offset keys. Back when there were aluminum rails I don't think we saw the failures.

I do not think there is a chance of a chain failing due to wear in the first 250k. All the failures are due to the broken rails. In the post where I advised to just replace the rails the car only had about 100k, if I recall. I made the comment the way I did to place emphasis on where the risk lies. We almost never just due rails but when we do it someone is paying a lot of labor, it makes no sense to do part of the job.

As to the price of this car. The selling price is to be compared with reasonable facsimiles. The sum total of all such risks as you are questioning are the reason the car is being sold for a lot less than it did new. To try and use statistical projected failures as a line item negotiation misunderstands the used car concept. Why not use a list of all systems and their statistical percent of new usefullness and asign a value for the total (based upon the percent of new price). Obviously this would leave you in a worse position as I imagine there is 75% usefullness still in a nice 126 body with 120k. Should be worth somewhere over 20k...... ahhhh but the market place makes price.

sublettd · #7 05-17-2002, 11:04 AM

Steve,

If you don't mind, would appreciate your advice regarding timing chain, rail, etc., replacement on my '85 500SL. The car has 90,000 miles on it and when I asked my mechanic about the replacement he recommended it be done at 130,000 miles. What would you recommend?

Thanks,

Don

the_good_fellow · #8 05-17-2002, 12:11 PM

I heard that they have an updated version timming chain kits or something like double chains from 560 engine. If you are going to replace them may be check on that as well.

stevebfl · #9 05-17-2002, 12:52 PM

As I have said before the rails are whats important. I think they should be replaced every 75k or 5 years. I think this is way too often to do chains but every other time at 150k intervals works for me with the chain.

I have seen cars still in warrantee rattle on start up. I believe this condition is inherent in the design and capabilities of the tentioner. Since there is no way to prevent that occasional rattle, the way to security is through keeping tough rails in the beast.

Let me give it numbers. If 100 is absolutely secure then replacing the rails gets you 90. Adding the chain gets you 92. Doing just the chain gets you a 5.

IMHO maybe not so H.

GregS · #10 05-17-2002, 04:47 PM

Thank you very much for your advice Steve, its very informative. Not to belabor this issue, but has Mercedes updated the tensioner? If part of the problem lies in lack of oil pressure in the tensioner at startup, which cases the chain to slap around, I wonder if that is typical of the design of this tensioner, or if new ones are desinged better. Saab 900s have tensioner issues, and there was an updated tensioner that was supposed to be installed in all motors.

Greg
'84 300D

sublettd · #11 05-17-2002, 11:25 PM

Steve,

I strongly suspect I'm still operating on the original chain. Will suggest to my mechanic that we do the rail and tensioner replacement and see what he says. Thanks.

autozen · #12 05-18-2002, 02:15 AM

GregS,

Euro287 pretty much nailed it. If the dealer doesn't think the chain is an issue, ask them to warrantee it for 12 months as part of the deal. Now watch the salesman's knees buckle. If they won't go for that, check the private party sales. You will probably find an equally fine 420SEL for a lot less money which will leave you with extra cash to take care of any problems that may come up. AS i have mentioned in another post, these dealer million point inspections don't add up to much. A good example came from one response on the post where a guy bought a car under the STARMARK program. The wiring harness disintegrated,a known problem. The car failed and had to be towed in. Why did the dealer let the poor guy twist in the wind? As another poster asked, why wasn't it replaced during the million point inspection? A friend of mine bought a BMW from a BMW dealer where it went through a million point inspection. The rear brake pads were down to the limit. These are just my thoughts.

Good luck,
Peter

suginami · #13 05-18-2002, 01:06 PM

After reading all the posts, I think stevebfl hit the nail on the head.

The price of a used car is already discounted for wear and tear. The timing chain and / or rails are just a regular replacement item over the life of the vehicle.

Every component on the car has worn out some amount since the day that it was new. How much life is left on the struts / shocks? 50%? 75% 25%

Should you discount for that?

The point is, it is much cheaper in the long run to pay full price for a good condition, well-maintained MB, than to buy a cheaper car that needs some reconditioning to bring it back to life. Believe me and others that have done this before, you can spend several thousands resurrecting a tired MB, and not even know where the money went.