My '91 350 SDL with 145K miles runs great and doesn't seem to burn very much oil. But recently I noticed a sharp knocking or cracking sound (almost like a snapping sound) coming from around the timing chain area in the front of the motor. The sound is intermittent and seems to be more prevalent with the airco on. Additionally, the sound disappears when the RPM's are increased from idle.

Is this likely the timing chain? Is this motor getting ready to let go?

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'91 350SDL 165K miles
Lovin' her until she blows...
'01 Dodge Ram Cummins 4X4 DWR
Stump pullin' beast...

Get you timing chain checked for stretch. It could be getting kinda long. Also, the tensioner could be stuck, allowing the chain to slap.

You could also have a sticking injector -- they make a loud snap or crack when they stick open or open very late. Usually makes the engine idle rough, too. Sounds almost like someone is tapping on the block with a steel hammer.

The bad noises on this engine are clanks, like a piston is loose and banging (which is it). Oil consumption goes sky high first, though, as the cylinder wears from the bend rod(s).

Later engines appear to be better than earlier ones.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

One other place to check would be the belt tensioner. They seem to have a service life of 75Kmi. The idler must be exactly parrallel to the belt, if there is misallignment relplace the unit and the shock. If it goes out completely it usually strips out the mounting hole in the timing cover $$$. I have heard some odd clicking and clacking from these as they start to go.

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Bob D.
Parrish, FL
1 SDL, 1 D, 1 TD, 1 Mog

Hey Mav.

Always wanted to say that ...

My 91 SDL has 125K on it ... runs great, although with all the bad statistics on this engine, it seems like it is only a matter of time. Love the combination of the S class and the low end diesel torque from this enginer.

The first owner had a history of this car of replacing the accessory drive belt tensioner about once every two years. It makes an aweful rattling noise and allows the belt to vibrate (it is loose) at idle. if yours makes the noise at idle, but not above about 1000 rpm it could be this.

The part number for the tensioner is reportly different than the original part number for the same item - many believe the original part was a failure mode item and MB finally figured it out. Part was about $25 and took about a hour to replace (first 45 minutes spent figuing out how to do it, then 15 minutes to do it).

Please let me know if it is determined to be internal as it would likely relate to the information one of the responders noted on the ovaling out of the liner in the cylinder by the piston. Big time failure mode.

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

Whoops ... forgot to mention a couple of items.

I recently used some injector cleaner (Redline 85+) ... seems to have smoothed the idle out somewhat - hopefully, I am not simply imagining the change.

Valve noise at idle has always been more "clacky" than it was on my previous solid liftered 123 diesels. Likely worn hydraulic lifters and or "just the way it is." Hopefully, this is not indicative of internal failure.

Finally, the engine runs much smoother, starts easier, etc when I changed over to synthetic oil (Mobil 1 15-50) immediately upon buying the car. Perhaps this will help extend life of what is an unargueably poorly re-designed engine.

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg

Pull the oil filter housing cover and replace the two o-rings on the bottom of the long shaft. These go bad and cause low oil flow/pressure to the lifter gallery.

I replaced mine and eliminated the lifter noise on the 87 300D. Didn't fix the rough idle, but did fix the lifter rattle.

You can make a "stethoscope" out of a piece of wooden dowel rod -- large enough not to fit into your ear, please -- and hold one end up against your ear and touch the other end to placed you think the noise may be coming from. Carefully avoiding rotating parts, eh? Should tell you very quickly where the noise is.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks ALL for the advice on 350SDL engine noise. I'll take an old stethoscope to the motor tomorrow to isolate the noise. I’m confident that it’s not the bottom end and not related to the belt tensioner.

Incidentally, when I changed the belt a few months back, I ended up with, at first, air conditioning problems, then an overheating issue and finally power steering problems. After double and triple checking the Gate’s belt from NAPA, I finally purchased one from MB and it solved all the problems at once. It seems that the Gate’s belt was not “tacky” enough to grip all the accessories without slippage. To be safe, the tensioner was inspected and the shock and spring replaced. To this day the tensioner is parallel to the belt with no wobble.

The cracking sounds like it could be coming from a sticky injector but the engine runs very smooth at idle and runs hard in gear.

Since I purchased the car in May with 118K mikes, I have put over 25K miles on it mostly at 80 mph (Florida highways). Top speed so far: 125 with lots of pedal left.

Lovin’ her until she blows…

Mav

Thanks ALL for the advice on 350SDL engine noise. I'll take an old stethoscope to the motor tomorrow to isolate the noise. I’m confident that it’s not the bottom end and not related to the belt tensioner.

Incidentally, when I changed the belt a few months back, I ended up with, at first, air conditioning problems, then an overheating issue and finally power steering problems. After double and triple checking the Gate’s belt from NAPA, I finally purchased one from MB and it solved all the problems at once. It seems that the Gate’s belt was not “tacky” enough to grip all the accessories without slippage. To be safe, the tensioner was inspected and the shock and spring replaced. To this day the tensioner is parallel to the belt with no wobble.

The cracking sounds like it could be coming from a sticky injector but the engine runs very smooth at idle and runs hard in gear.

Since I purchased the car in May with 118K mikes, I have put over 25K miles on it mostly at 80 mph (Florida highways). Top speed so far: 125 with lots of pedal left.

Lovin’ her until she blows…

Mav

125? I dont know about that! My 87 300SDL will only go about 107, pedal to the floor.

I would think an 300SDL would do more than 107mph....I have had my 92 140 up to about 100 and it seemed like it had more...and I am carrying several hundred more pounds than a 126 car....and I think I have less horsepower...148 versus 155(?)



Warren
1992 300SD 127K
Columbus Ohio

My purpose here was not to get cocky, just to get an idea how fast these cars are supposed to go without coming apart. In fact, I was told that my car could hit 130+ if someone had the brass to get there. Currently, I don't. My 350SDL seems to love 100-110mph speeds and the roads here in Florida are perfect for long range/high speed runs. The FHP's are less forgiving though.

Mav

Did some preventive maintenance today on the ’91 350SDL. Permanently removed the vacuum line to the EGR so the intake will not suck in crud. Then removed the crossover pipe and was completely amazed at how much crud was in there as well as the intake manifold. Cleaned up the crossover really well but didn’t want to mess with the intake without new gaskets. I know that some crud fell into one of the plenums but I’m sure the motor has already digested a bunch of this stuff.

Next re-routed the valve cover breather so it will vent under the car instead of back into the intake pipe right before the turbo and plugged up that hole. Since I broke the brittle breather elbow in the project, it gave me a chance to remove the valve cover and inspect the timing chain from the top. It looked pretty tight with no noticeable slop. Is this an effective way to check the timing chain?

Finished with an oil change of Rotella. Do you guys like this oil? I get less smoke and less consumption when using it. I’d switch to synthetic but have encountered leaks with older motors such as mine.

The noise is gone for now and the idle is real smooth!

Thanks Jim (oldsouth) for the tips below.

Mav

Jim wrote: I could not get this to go through by reply so I am sending it this way. (email)

Maverick, The first thing I would do is disconnect the egr valve. Try it and see if the computer cuts the boost. It does on a ‘95 model but I am not familiar with how the ‘91 is set up. If the computer cuts the power, the only way to get around it is to change the wastegate actuator on the turbo. Mercedes uses a vacuum unit that has to have vacuum to close the wastegate. Call Garrett (air research) and get them to send you a pressure model. You will have to adjust your boost after installing. Shorten the rod by turning the clevis to increase boost. Anyway plumb this into the intake manifold where the vacuum line runs out to the overboost sensor and aneroid valve. All you need is a ¼” hose going from the turbo to the intake and tee of of this to the aneroid valve. You can tie in a gauge here to adjust boost. Specs are 14 psi at 4000rpm in third gear. Another trick is to take the vent that comes out of the valve cover and run it down to the belly pan. Don’t stop it in the belly pan but run all the way where it will be nothing between the bottom of the hose and the ground as some oil will come out of it. Plug up the hole in the intake pipe with a bolt or something. The purpose of all of this is to have the engine intake only clean filtered air, not exhaust or oil clouded air out of the crankcase. Now you can take the crossover pipe off and clean it up real good. You also may have to take the intake manifold off and clean it also. Mine got so stopped up before the mods I described here that it started whistling. Totally stopped up. Does it have the exhaust catalyst or oxidizer? The big thing on the exhaust.
Mine does not. If it does I would take that off also. Good luck
Jim
One other thing - I would not turn up the fuel on this engine. I think it is high enough like it is.

Maverick - When you disconnected the EGR did the computer cut boost or did the car feel weaker? If it did you would know it for sure. My S-350 will go 118 before it tachs out and computer cuts fuel. Pulls right up to 118 and stays there.

edit - that speed is GPS speed not speedometer speed as the speedo is about 3.5 mph off.

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K

Jim,

I didn't notice any loss in power after disconnecting the vacuum line to the EGR. Also didn't test 0-60 because I didn't want any loose chunks of crud to dislodge and enter the intake before they get a chance to get cooked and stuck to the side of the intake. 0-60 last week was roughly 13 sec.

I'll take a gps unit on the highway tomorrow morning and open her up and get back in a few days. In regards to a speed cutoff, I haven't noticed anything on my MB thats like my gas cars that start sputtering at 105, 110mph, etc. Are you coming up on a governor or is it electronic?

I'll likely remove the exhaust catalyst or oxidizer this weekend and weld a pipe in its place.

Hey...thanks again for the tips!

Mav

I don't know what I am bumping at 118 but the RPM's are in the red zone, about 4500 and the cutoff is very smooth. Just pulls to that speed and holds. Not like a GM product that just aburptly cuts you off. My 0-60 is 11.5-11.75.

__________________
1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K