350 sdl 150,000 miles with engine noise
 

I have gone from a backfire in the intake to valve train noise. Fifty miles into a 100 mile trip the engine got louder. A car following me stated I had smoke from the exhaust. Power seamed ok, no oil consumption but a loud value train. I bleed the injectors not being able to isolate which cyclinder so I believe something is loose under the valve cover? I had it towed back as it sounded like it was going to come apart. Our Mercedes dealer does not have much experience with diesels and I am afraid to go there. Help.

Jim Maddox

1985 300 sd @ 185,000 miles
1990 350 sdl @ 150,000 miles
1980 Unimog DoKa @ 186,000 km

uptank001,

Your problem does not sound like injector problems. It is extremly rare that more than one injector would fail it once. Therefore you can isolate a bad injector by opening the flare nuts one at a time, Usually a bad injector makes a knocking noise that will go away when you loosen the flare nut.

I hope you don't have the bent rod problem that the 350SDL is famous for.

I would take off the valve cover and take a look. Maybe it just a bad hydraulic valve lifter.

P E H

Nice loud clack clack -- could be dead lifters.

The really bad sound in a 350 is a bent rod causing the liner to wear oval and the piston to clank around, but you'd know that -- a blue trail of smoke visible for blocks (I've seen one). Not your problem, I'd bet, since you aren't using oil.

The hydraulic lifters are between the cam and the valves, and can be checked by pushing down on them on a warm engine when the cam lobes are pointing up. If you can easily push them down with a screwdiver, they are shot and need to be replaced. Very loud.

The fact that you have had noise in the intake, though, makes me think seriously bad lifters -- they also can pump up and keep the intake open, causing the engine to "chuff", or the exhaust can fail to open far enough, causing echaust gas to blow into the intake on the next compression cycle. THAT can sound terrible.

You may also have a roasted cam. Expensive (as all MB engine parts are) but not a catastrophy. Pop the valve cover a take a peek -- cam lobes should be smooth with no flat spots, and no grooves or deep scratches, there shouldn't be any loose bits in there (although I have seen the head of a bolt broken off and floating about).

Serious chain wear or a bad tensioner will allow the chain to slap on the guides or worse on the block or head, causing a terrible clank, too. Should be obvious, the chain will be loose with the tenstion all the way out. Don't remember if this is a racheting tensioner or not, though. Bad tensioner will let the chain slap, too, reguardless of wear.

An last, but not least, a bad seal in the IP at the pressure valve holder will cause a very low knock on the affected cylinder and rough running that both go away above 2000 rpm or so. Easy to fix.

Peter

The shock absorber on the serpentine belt tensioner could have worn bushings. This will make a real racket, especially with a cold engine. I recently replaced them on my '93 300SD and '95 S350. We discovered the source of the noise by placing a finger on the upper shock mount and pressing down. The noise went away! The bushings are not availble seperately. You have to replace the shock. Give it a try.