Timing Chain takes out another 420SEL - PeachParts Mercedes-Benz Forum

#1 01-04-2003, 12:07 PM

Timing Chain takes out another 420SEL

Last week the Timing Chain on my 1986 420SEL with 212K miles skipped when my wife tried to start the car. She did not notice any loud or unusual sounds while trying to start our 420 just that it would not start. When I arrived home from work I tried to start our 420 but it would not start and would just turn over as if there where no spark.

I had the 420 towed to the MB dealer. After inspecting the timing chain, pulling the left valve cover, and a compression test they reported that the timing chain skipped and that there is evidence that some of the vales bent on the left side. (The result of the compression test (cyl:psi): 5:0, 6:0, 7:0, 8:0, 1:75, 2:125, 3:100, 4:125.) They stated they could rebuild the top end but that could cause problems with the bottom end.

The 420 was my father’s car and I would really like to keep the car but I’m not sure about the condition of the bottom end. Additionally before the timing chain problem the car was leaking oil and the AC had gone out. If I have the top end rebuilt I still have the additional costs to take care of the oil leak and AC.

Questions:
1. Does the result of the compression test give any indication of the condition of the bottom end?
2. Does the leaking oil give any indication of the condition of the bottom end? The oil leak appears to be coming from the main seal but the leaking could be from other places as well.
3. What should the psi rating be for an engine in good condition?
4. Any general advice?

Thanks for your help,

Robert

#2 01-04-2003, 12:20 PM

If you are considering any engine work on this vehicle I would STRONGLY recommend you find an experienced, reliable, independent Mercedes shop rather than using the dealer. It will save you a ton of money!

#3 01-04-2003, 12:40 PM

I'd love to know how repairing the top end would have a negative effect on the bottom end. My BS detector is making a few faint ticks.

stevebfl · #4 01-04-2003, 12:51 PM

My shop has done hundreds of these repairs. There is never a problem with the bottom end. I have one customer with 650k miles on one (91 420SEL) and we have done two of such repairs to it. I felt bad when the first came back after only 120k broken again. It turned out that when we did the first one with what we thought was 150k it was really something like 300k (speedo had been replaced). When it got somewhere over 400k true it wore through the lower left side rail. When we did the first one we didn't go for that rail because it doesn't wear out till after 350k.

He has over 200k on it since the second repair and he repainted it a while ago so I guess he is going to use it another couple years (he drives 90k miles a year).

ctaylor738 · #5 01-04-2003, 01:04 PM

The important things for the compression are to be adequate and even. I don't have 420 information is front of me, but you would certainly want to see 125 +/- 10%.

If oil consumption is reasonable and oil pressure is good, the bottom end is probably OK.

The right side is clearly hosed with bent valves. The left side has a couple of low readings. You have to fix the right side to get the car to run. I would think you are looking at another 3-4 hours of labor plus a few hundred dollars at the machine shop to do the left side.

I would say that this (both sides) is the minimum investment if you are going to keep the car. If you decide to sell it just do the right side and have some XP33 run through the engine to see if you can improve the compression.

There used to be a saying "If you valve, you've got to ring." The theory was that good compression would put pressure on worn rings and cause additional oil consumption. But I haven't heard anyone say that about the later Mercedes V8's.

My $.02

joe p · #6 01-04-2003, 02:35 PM

As long as a 116/117 has not been run hot, the bottem ends are damn near indestructable.

Fix it, it'll be fine.

Just get a good indy to do it and go buy a widescreen with the money you'll save.

Joe

ILUVMILS · #7 01-04-2003, 07:58 PM

Am I missing something ? Who does a compression test on a motor with bent valves and cam timing off? I hope you're talking about a leakdown test, but the numbers posted don't sound like it. In any case, pull the heads and see what's broken. I've never seen a jumped chain hurt anything in the bottom end of a 116.

Gilly · #8 01-04-2003, 08:24 PM

I have to agree with ILUVMILS. If the timing chain is toast and the one bank jumped time, THAT'S probably a good indication to say, uh, STOP cranking the engine over.

Gilly

suginami · #9 01-04-2003, 08:43 PM

Robert, you are one lucky guy!

You make your very first post, and got four good replies from four different Mercedes technicians, one after the other! I've never seen anything like it.

gregga · #10 01-06-2003, 10:39 AM

The problem is famous enough to be in a magazine: http://www.import-car.com/ic/ic40024.htm
Mine, two years ago, broke one valve and took out a piston. I'm just now putting in a short block and correcting all the screw-ups my "low-cost independent" incorporated in repairing it. I've drilled out eight socket heads, found the intake gasket was leaking and discovered broken spring shims in one head. Make sure you get references from someone who has had major work done, not just tune-ups. Good luck. By the way, Auto Sports Unlimited in Zeeland, MI, offers this engine remanufactured at a reasonable cost as well as remanned heads. Compare it against repair of yours. I found out I could have replaced mine cheaper.

#11 01-06-2003, 05:12 PM

There is a big difference between a low cost indy and a knowledgeble indy. Make sure you find the latter.

#12 01-07-2003, 12:04 AM

I agree with what Ned said. Cheaper does not always mean better. The MB dealer in my area gets around $83 USD per hour and my independent mechanic who has specialized in Mercedes/Porsche for over 25 years charges around $62 USD per hour. The indy always tries to get parts from junkyards for older MB's to save me money where the dealer always installs new. Unless the car is still under warranty I will go to my independent shop every time. In my case, my independent guy is cheaper and better.

stevebfl · #13 01-07-2003, 08:16 AM

There are lots of reasons to choose a competent independent. The nomenclature describes the reason. He is independent. This means all techniques are accessible including factory approved methods.

This independence allows all sorts of repair techniques not suggested by the manufacturer, including used parts, non OE parts, improved parts (parts redesigned for handling, horsepower, longevity, etc). It includes repair techniques including A/C hose repair (here again the independent can improve on the part by rebuilding hoses with barrier hose where new would be the same old R12 junk), TIG welding, machining, electronics soldering and repairs.

Generally rebuilding of any nature has been discouraged at the dealer level. If one chooses the dealer level of repair the point of obsolecence is reached at a much younger age. Obsolecence being the point where repair costs make new purchases economical. Sort of in their best interest not to lengthen that time span.

ctaylor738 · #14 01-07-2003, 09:48 AM

I agree. From practical standpoint, you would have to be Bill Gates to afford a repair like this (chain and heads) on a 420 at a dealer.

But you do need due diligence. Look at the recent post from chrisgoodman with his valve train problems.

gregga · #15 01-07-2003, 07:08 PM

On a related note, how do you time the cams? My Mitchell manual mentions doing it but doesn't describe it. I'm putting in a short block and need to start from scratch. The only timing mark referenced is on the distributor drive gear.