Odd 300D Engine Noise

[patterson]

I have posted about this before and thought that it was related to the timing chain, but after a new tensioner, chain, and tensioner rail the noise is unchanged. It seems that I have an odd noise because I have searched through the archives and can find nothing similar. It is a loud rattle/knock that seems to come from the top end. The noise increases frequency with RPM increases but the intensity never seems to change. The engine runs great good oil pressure (about 2 bar at hot idle) does not smoke, and doesn't use oil.

I have had this to various people that know these engines and they say they have never heard anything like it. But their theory seems to be that it is something related to a wrist pin or the prechamber. The noise changes but doesn't go away when the #2 injection line is loosened, and none other cylinders effect the noise. I pulled the oil sump earlier and no metal was in the pan and rods looked good and tight, so at this point I am stumped. I will probably pull the head off at the least or pull the engine to find out what it is before it ruins my engine.

Thanks in advance for any help you can give.

[SW]

Have you run a can of diesel purge through it to see if the noise goes away?

[patterson]

Wow that is a quick reply. I have not tried Diesel Purge. I don't think that it is an injector knock because usually the injector knock it more pronounced at idle and diminishes with increased engine speed. I may be wrong, but I am willing to put in new injectors if that is a possible source of the noise. Thanks for the new idea to think about.

[tcane]

A wrist pin/piston pin knock is heard more clearly in the upper area of the engine in the water jacket area of the cylinder at issue. A big end rod knock is heard more clearly in the oil pan area. To test bring the engine up to about 2,500 RPM for a very brief moment, snap the throttle full open and release it fully, a wrist pin knock will sound about the same, and a big end knock will usually get louder. Loosening the injector line will usually reduce a wrist pin knock and a big end knock will get worse. Good oil pressure at idle usually indicates the crank/rod bearing are OK. Listen for sound using an engine stethoscope, a screwdriver with a long shank, or a piece of wood.

Your info sounds like a wrist pin knock. Use the above test to narrow it down. M-B diesel wrist pins are very tight: non-turbo engine .0005 to .0009 inches or .012 to .023 mm pin to rod clearance, and .0000 to .0039 inches/.1mm for the pin to piston clearance; turbo engine .0007 to.0011 inches or .018 to .029mm pin to rod, and the pin to piston clearance is the same.

Good Luck!
Tom

[patterson]

I though that I would follow up with this post and tell all that had helped me out what was wrong with my engine. I pulled the head to check everything out and it became evident what was wrong with my engine. You could rock the #2 piston in the bore easily and there were some scratches on the bore. I was lucky because it was the #2 piston, becasue I could drop the sump and remove that piston. I will try to post some pictures of that piston so that someone can tell me what would cause it to do what it did. The piston scirts are severely scored and the piston was worn to the point that you could slide a .020" feeler gauge beside the piston in the bore.

This was not the obvious diagnosis because the engine ran smoothly without blow-by. And this cylinder was holding 400 psi of compression! But it had a wicked knock that seemed to be on the #2 cylinder.

Well I got a new piston, honed the cylinder and put the engine back together. The skirt to bore clearance was within specifications, so that is a testament to how hard the cylinder walls are. The engine runs great, and I am now a happy member of the Mercedes Diesel community.

Glad you got it solved, I was going to tell you to check your injection timing, if that is off the engine can make all types of wierd knocks and snaps.

[Nate Stanley]

Patterson and other fellow gearheads,

Congrats on the relatively painless fix!

One quick question about that noise--
I have a tapping sound not related to valve clearances (they are set to within + - .001" of specs).

I don't want to tear into my engine without due cause, but I wanted to ask about the noise you had.

Was it present at all throttle settings, or did it go away while coasting? Mine is almost silent while coasting, but even the slightest pressure on the accelerator makes the tapping noise reappear.

My engine's oil consumption is better than 2K miles/quart , 239k miles, 23-25 mpg no matter what speeds.

I REALLY want to rule out a wrist pin, but am not sure if it isn't a bad injector or an exhaust leak as well.

Any thoughts?

Much appreciated,
Nate

[patterson]

My noise was there at idle, load, and most every other situation, but it would slightly change at coast. You could tell that it got louder when you got back into the fuel after coasting.

I bought the car making this noise so I can not personally attest to the oil consumption but I know the fellow I bought it from well and he swore that he never had to add oil between changes to it.

My noise sounded very valve related, but I adjusted the valves ad that did not help, I swapped injectors and that did not help so then I know that this was a mechanical problem. As bad as it was I was really lucky that it was such an easy fix. But in a diesel the first thing I would expect to cause a knock is something fuel related especially if it goes away when coasting. Check your injectors before you think the worst.

Has anyone else encountered a problem like this with the 617 turbo engines? The engine compression is good with around 400-410 psi across all cylinder and the oil pressure is good but this is a high mileage engine. The car has 352,000 and according to the original owner this is the original engine. Well this was my first experience with a Mercedes Diesel and I must say that I have grown a respect for them. These engines are really built well.

[psfred]

Nate:

If the knock goes away off throttle coasting and comes back with fuel, it is most likely injector related. Check for diminished or vanishing knock with the cap nut loose -- this is diagnostic for injector knock.

you could also have bad IP pressure valve seals, but this usually causes rough running at low speed, too.

Peter

[Nate Stanley]

Patterson & Psfred,
Thanks for the clarification. I'm hoping it's an injector issue as well- I do suspect #3 is the culprit, but have been so busy I haven't had time to get another injector and swap it out.

I'm assuming all 5 injectors are original and would like to replace them all (239K on engine), but with the smoothness and relatively good fuel economy (24-25), maybe I can get away with swapping just 1 for now.


Low speed at idle is usually ok, except after a hard freeway run. I have a new rack damper pin ready to install this weekend. I've already done the diesel purge as well.

I'll keep you posted--
Nate

[wheelguru]

I had the same noise on my old engine, ('78 no turbo) it was bad when I got the car and knew that it would eventually cause trouble but the car was relatively cheap to start with. the noise was a rapping sound, always present, and it would change pitch when you let off the gas. eventually the car just quit one day and would not restart. I had already been searching for another motor, and found a car with a decent one for $600. That motor is now in the car. The oil pan of the old motor was filled with the remains of a piston.
Soooooo, the newer motor also has this sound, but it is much quieter, you have to listen for it. seems to be in the front of the engine. Always there, a can of diesel purge didn't help. My question is: how hard was it to change the one piston? I like to think I have OK mechanic skills, but the only motor I have ever torn down was a VW aircooled, I doubt this will be that easy. Will I be able to remove the head and oil pan and then unbolt the rod on the offending cylinder and remove just that one to replace it? Does not seem like it would be so hard. will it be?
thanks for all opinions, Andy

[patterson]

Wheelguru,

I would suggest that you check compression on the engine before you consider tearing down to replace one piston because if the compression is low on all cylinders and one cylinder gets a new piston and good compression it would throw off the power balance of the engine. But, if the engine has good compression I would not hesitate to tear down the engine for one piston. You can access the front two rods through the sump. So if it is one of the front two it is as easy as pulling the head and the sump and fixing the problem. But you know if it is another piston you are going to be pulling the engine and may as well rebuild the whole engine. Well good luck.

Matthew

[tcane]

The sound is probably louder toward the front of the engine because the 240 and 300 engine's #1 piston to cylinder wall clearance is greater than the other cylinders (.0011-.0019 in. for #1 and .0007-.0015 in for the other cylinders). Add in the noise from the timing chain and components connected to the chain and you should hear more sound toward the front. The turbo 300D's engine, however, does not have the difference in piston to cylinder wall clearance as the 240 and 300D non-turbo.

A worn piston like patterson had would not make as much noise when the engine is at operating temp. versus cold because the aluminum piston expands more than the steel cylinder liner does taking up some of the excess clearance and when running the noise would be greater when a load is put on the engine versus coasting when little load is put on the piston from combustion.

Patterson's problem shows what I found out a long time ago - you can do all the diagnosing you want, but in most cases the only way to know what the problem really is is to do a TDI (tear down and inspection). Just like what medical doctors often do - ever hear of the medical procedure called exploratory surgery to find the problem? For example, my old engine had low compression in three cylinders because of a blown head gasket, the other two cylinders had 325 compression (#1) and #2 had 410 compression. After a TDI , I found that #2 had no cross hatch left on the cylinder wall and the cylinder wall was as smooth as glass, but had excess piston to cylinder wall clearance despite 410 psi compression.

If you search the old posts you will find some by Wm. Lewallen who repaired M-B diesels for over 20 years as a professional. He makes reference to repairing just the problem if everything else was OK - in other words, if one cylinder was bad he would fix that cylinder and go (like a new piston and/or a new liner bored to correct clearance, etc.). My own vast experience is similar, fix what's broke and leave what's not broke alone.

Good Luck!
Tom

[wheelguru]

What a wealth of information! exactly what I wanted to know, and some that I didn't know I needed. thanks for the replies. I will check the compression first and then consider the TDI approach. -Andy