Cam Timing - PeachParts Mercedes-Benz Forum

joshhol · #1 11-30-2002, 05:11 PM

Hello,
I recently had the timing chain replaced in my '76 300D. The engine has 280,000 miles on it. First of all, was it worn? When my mechanic pushed back on the tensioner, there was enough slack in the chain to skip a tooth. My other mechanic was not very concerned by that fact. So whatever, the new chain is in it now and I guess that's not a bad thing. However here is the problem. Since then, the car has run differently. It doesn't sound the same at 65-75 mph and doesn't run as smoothly. It used to find it's groove there. There was a tapping noise which turned out to be a tight valve. But the valves were adjusted 1,000 miles ago. The mechanic said the cam was worn and the valves would get tight. Also, the cam timing was 2 degrees advanced. I had my other mechanic adjust the pump timing which was at 22 degrees BTDC and now is at 24. He said the Cam timing should be right on the mark or a little bit retarded.
So, could the cam timing cause it to run just a little bit off? I'm sure it would, I don't know why my other mechanic doesn't seem too concerned about it.
Also, he recommends replacing the cam and rockers. I don't know why that valve got tight so quickly but should I pull the heads and have a valve job done or will the new cam be enough? If the bearing edges are worn, he said the valves will get tight very quickly that way as well.
What are your thoughts.
Thanks,
Josh

samiam4 · #2 11-30-2002, 08:47 PM

Josh,

I'm confused by your first mechanics@@@! I have no problems with timing chain replacement every 100-200,000 miles. Even if the timing seems close, it seems like reasonably cheap insurance to me(if they do the job correctly).
When the timing chain wears, it gets longer between the links, which means the timing should be retarded with wear and not 2 degrees advanced. The wear limit is 4-5 degrees retarded(I believe). The slope in the chain with the tensioner colapsed is meaningless salesmanship. That's why the tensioner is there!! Now, if the tensioner couldn't take all the slack out.... then it would have much more than 2 degrees of error.
Advancing a camshaft a little, in a gas engine, will boost low speed torque a little. Maybe it helps in low speed in a diesel too?). SO that being said, it should feel a little different.

My personal experience is these cars run the finest when they are setup perfectly to stock. Perfect camshaft timing and injector timing. My manual is at work, or I'd qoute you out of the factory manual specs. Maybe someone else can comment on 617.912 timing specs.

Valve job questions? Ussually, well maintained diesels don't seem to require valve jobs as much as gas cars. Does your mechanic work on lots of these 70-85 diesel mercedes????

Looking in the manual, it appears to me that the earlier cars(yours included), had a different camshaft/rocker/adjuster nut combination than later engines. The later ones got turbo camshafts for more breathing and a different piston design.

I've always heard the later setup will work in earlier cars. I do know that Mercedes used shorter valveguildes and different camshaft bearings. That being said, maybe you could rummage up a good used turbo upper end- for less than a new camshaft.

Oh, if you go new camshafts please insist on a Geniune Mercedes part. I got had some nasty run-ins with Lemforder after market camshafts... they are making them know in Turkey and have quality problems. I think wholesale on the Lemoorder was $104 and MB was $170 for mine. List was the same, hence an advantage for the shop, but no cost difference for *YOU*.

Oh, the timing marks didn't lign up with the lemorder camshaft sprocket which was the same cost as the Mercedes part... which allowed everything to lign up perfectly. It was about 6 degrees off... ran horrible until I put the factory part in and the timing was dead on. As far as timing the injector pump, I'd rather replace the chain than mess with IP timing.

If it was my labor, I'd pull the head and do a valve job. But, if it was paying someone, you are relying on their diagnostic skills. I would ask for cold or hot compression numbers on the engine to try and help determine how much $$ is justified. Hot spec is 280psi min, and 400-425 psi new. Maximum variation between cylinders is 50 psi.

How does the engine start and run dead cold?
Michael

#3 11-30-2002, 08:51 PM

I have no answer.. but struggle with what you say.. If a cam is wearing.. it will wear on the lift part of the cam.. the non lift part.. the part where you set the valve has nothing touching it.. so will not wear.. It will only have less lift.. that will affect power.. but in no way will it affect adjustment as the adjustment is done in the area where the cam is not touching the valve or lifter.... and that does not wear.. so all that is intresting.. Cam timing off.. Yes on some... There is an offset key that can be used on the cam.. However.. checking the cam timing is simple.. set the engine on TDC on # one Cyl.. There is a mark on the cam.. New chains are often just a touch ahead.. But seldom makes much difference as it will change as the chain is run some.. Timing should be 22 or 24 degrees.. depending on the engine and specs.. So did they set the cam wrong when they put the new chain in... Are you just up to tight over the whole thing.. Valves tightening because of cam wear is bull ****....

I have no clue about all this but was intresting reading....

Ken

joshhol · #4 12-01-2002, 02:13 AM

Michael, thanks for the info, it was helpful.
Yeah, the tensioner still worked but the chain had 280,000 on it so I guess it is good insurance. It was a German aftermarket though. Iwis I believe.
The timing is advanced now with the new chain. I want it to be dead on so I guess I should get an offset keyway. And I don't think I want to mix a newer cam in there. I think I'll just do it by the book, but if I could get more power. . .
And, I'll definitely get a compression test done before pulling the head. I don't think it needs a valve job, it starts great dead cold.
Thanks for the heads up on the Turkish parts. I'll stay in Germany thank you very much.
Yes my mechanic works on tons of these diesels. But they really don't seem to be that concerned by my complaints. I'm losing a bit of high esteem that I've held them in.

Ken- I hear you on the cam wear. The rocker will wear however and that is also involved in the setting of valve clearance but that should be equally as worn as the cam lobe so it should compensate in a way??? I don't really know but maybe I should replace the cam to get better breathing.
I could be uptight but I don't think so. I've driven that car for 10 years and know it ran differently before the new chain was put in. The guy didn't set the cam up wrong. I guess what has happened is the new chain advanced the timing and he didn't check that.
So I wish I could do all work myself so I don't have to rely on someone else!!!
Thanks
Josh

samiam4 · #5 12-01-2002, 03:38 PM

JOsh,

IWIS I was always told made the timing chains for Mercedes. I cannot confirm or deny this fact. But, that is the brand in my car.

If It's a little advanced now.. leave it. It will stretch out in the first 10,000 miles and be dead-on for a long time.

Offset keys are a pain, but your mechanics measurement is probably plus or minus a degree or so.

Michael

joshhol · #6 12-01-2002, 08:51 PM

Thank you very much Michael.
I think I will go with that strategy.
Josh

#7 12-02-2002, 08:37 AM

You need to double check the cam timing and ENSURE that the tensioner is working well. Once the cam timing is verified, you need to time the injection pump. When rolling in a new chain, even if cam timing is correct, it will change IP timing if the chain was worn.

Good luck,

joshhol · #8 12-02-2002, 01:47 PM

Thanks Larry.
I had the pump timing adjusted after I had the new chain installed.
The tensioner is working. I think that I should leave it alone and the chain will break in and line up right on the mark. I'm going to keep an eye on it.
I think I might need a new cam and am not sure how to tell when I need a new one BESIDES my mechanic saying so.
Josh

#9 12-02-2002, 03:44 PM

Earlier this year I had to replace a couple of 'rocker arms' , otherwise known as cam followers. They broke because of some faulty valve springs on a problem used engine replacement.
In researching the cam followers one of the shops I went to showed me that the earlier cam followers did suffer wear problems. The later cam followers are hardened on the contact areas.
I wonder if the tech on your vehicle noticed some unusual wear patterns there. I do not recall any mention of cam wear being a concern.
You should ask the tech about that. The more normal wear pattern is for the valves to wear up into the head and therefore get tighter as the miles are put on. But yes, a cam and the cam followers can wear down and shorten the distances between the parts.
Always pays to get as much information as you can.

#10 12-02-2002, 05:11 PM

When I changed my timing chain, I had excessive valvetrain noise and lack of performance, it turned out I missed by one tooth. All is well now since it was corrected, and got my injection timing adjusted.

At 275K miles my chain was stretched over 13 degrees, I am very lucky it didn't fail.