How often should timing chain be changed?
 

I just bought a '79 300D with 150K on it. The Dr. I bought it from never adjusted the valves (tapping) and never replaced the timing chain & rails in 20 years. It has a new exhaust system...brakes and inspection sticker. The car is in great shape...I took it straight to my mechanic to check it over...here's the list he came up with. It handled and ran great!

1. Repack both front wheel bearings (front-end tight)
2. Replace the braided cloth injector return lines (damp with fuel)
3. Adjust the valves (tapping)
4. Change trans filter & fluid
5. Replace glow plugs
6. Check timing chain for wear
7. Replace timing chain rails
8. Do leak-down test

This car was owned by the same person for 20 years and he was clearly emotional when I bought it. He had taken "her" out for one last spin, and asked me to please take care of his "baby".
Any thing I might have missed...please tell me.

You just need to check the timing chain for wear and stretch. I have a 1977 300D and had the same concerns, 193000 miles. I took it to my tech to have the valves adjusted and he said "DON"T TOUCH THE CHAIN,,,,,IS GOOD".

They last.

JCD

My mechanic said check the chain stretch and to leave it alone if OK. I had a chain break because of bad rails on my SD...so the rails will be replaced. He will adjust the valves and replace the rails will he's in there.

When my chain broke it was in great shape (250K) . Thanks for the advice...that's what I wanted to hear!!!

Why he didn't recommend a compression test is beyond me.
If he is proficient at checking injection timing via the "drip" method he may want to do this as well. The first thing I'd recommend us the compression test though, this is the #1 indicator of a diesel engines health.

Gilly

He did recommend a leak-down test while he was checking it out...and will do one this week. The car has full records with oil changes every 3K. The car had been driven almost everyday for 20 years, and I hope the engine is tight...it started right up in 25 degree weather. I should be driving it this week and will post the results .

A leak-down test is NOT the same as a compression test.

Gilly

You are right ....and most likely hear that a lot. But this car starts on the first crank, and has power going up hills...with little blow-by. So do you think I really need to run a compression test?

Absolutely!
As a matter of fact, I see NO reason to do a leakdown test UNLESS the compression test comes up low enough to warrant repairs. You only need to do this to determine where the compression is being lost, ie worn valves (air escapes through intake manifold or out tailpipe) or rings (air escapes out of oil cap).
With 150,000 miles I would still be interested in running a compression test as this is the most critical factor in the overall "health" of the motor. With that amount of miles, assuming as you say, it starts fine, I would assume it will be somewhere between the "new" compression spec and the "ain't gonna run" compression spec. Best to know for sure before getting into a "long term relationship" with it. If the compression spec is fine to you, then I would say forget about the leak-down test. If you're not going to fix anything-who cares?
Be sure to get the valves adjusted before the compression test, and the compression test is to be done on a warm engine, not cold. The compression tester for a Diesel reads much higher than one used on a gas engine. Does your mechanic have a compression tester for a Diesel? Maybe this is why he didn't recommend the compression test as he should.

Gilly

MB Canada came out with a tech bulletin a few years back that advised changing chains every 120,000 kms (75,000 mi). I changed mine at about 300,000 kms and even then the mechanincs felt it was in great shape.

I gather this can be a DIY job but I had the mechanics do it and was glad. the job is 3.5 hours per the book but it took them 10 because the chain slipped and fell down into the engine. They said this happens to them every 10 or so jobs so I figure it was worth it as they only charged me the 3.5 hours labour.

I would be interested in finding out the bulletin number and date. I would then have a Canadian dealer or MB Canada fax me a copy of it. Very interested in finding this out. Usually directves of this nature originate from DBAG (now DCAG) and would be followed through on a world-wide scale, not just one country.
Although I did find out that MB Canada never made the removal of the Trap Oxidizer a service campaign like it was in the US, but this was emission related, and possibly what applies in one country in regards to emissions won't be regulated in another. But a timing chain replacement recommendation you would think would apply to all engines in a particular range. Besides you state that it is recommended at an interval which wouldn't make it a warranty concern.
I personally have never seen chain replacement intervals by Mercedes, only the process for checking for stretch, and even this inspection doesn't have a listed interval.

Gilly

ps Was this recommendation for all MB engines, all Diesel engines, or just a particular Diesel engine type?

You've got me on all your questions.

It was pointed out to me by Weber Motors a Mercedes dealer in Edmonton, AB (about 1,000 miles from where I live). They have done some servicing on my car when I have been unable to do it and have been familiar with the car since about 1987.

This was in 1997 and I was preparing the car for a long drive to Ontario and back...about 12,000 kms. I had done my usual work...oil, filters, tires, shocks, etc and had recently acquired a low mileage 83 SD for parts. I was swapping out a bunch of stuff into my car which had 285,000 kms....trans, prop shaft and steady bearing, exhaust, diff, calipers, etc.

I then asked the dealer if there was anything else they could recommend and they commented on the timing chain saying that MB recommended changing (they might of said checking...can't recall for sure but I generally fix things ahead of schedule as I can't afford breakdowns) every 120,000 kms.

Man, it must be a Canadian thing, eh. Normally you do not replace timing chains unless they are stretched. You measure the stretch by lining up the marks on the cam & cam tower, and read the number on the crank (would be at TDC, or "zero" degrees stretch, with a new chain.) According to published MB service docs, 4 degrees stretch is about the limit. At 3 degrees you could let it go longer, at 4 schedule replacement, and at 5 or more I wouldn't drive the car until it's replaced. If it stretches much past 6 or 7 pistons start hitting valves. Most chains end up being in the 2-4 degree range after 200kmi (300kkm). There is some anecdotal evidence showing that using synthetic oil will almost eliminate chain wear, something to consider if you install a new chain.

About compression testing, I think it wouldn't hurt. However, Dr. Booth on the MBZ.org diesel list swears they are useless, he says cold starting in sub-freezing weather is all the compression test needed! A diesel with poor compression simply won't start in cold weather (or will be VERY hard to start.) A leakdown test is more meaningful, and given the choice I'd pick that over a compression test. The compression ABSOLUTELY MUST be performed with a hot engine or the numbers are totally meaningless! Keep that in mind before you start...! :eek:



Regards,