Engine Knocking - Help
 

On my way to work this morning, as I left from a 4 way stop, I heard this horrible knocking noise. It can not really be heard at idle, it starts at about 1200 rpms. I limped it home and tried to make sure that it was not as simple as an exhuast manifold leak, I pulled the valve cover off and and ran it up for a couple of seconds. It sounds to me that it is coming from Cylinder #1 (furthest front - new to mercedes not sure what order). Is there a typical problem with the 85 300SD engine that I should look to? I am looking for a manual and figure on pulling the head off and starting with a possible valve problem.

Also has anyone downloaded any of the workshop manuals from www.workshopmanuals.co.uk ? This is the only one I have been able to find and it is downloadable.

Thanks for the help.

Search for ' skinnerbox' on this site... should be an engine manual downloadable there...

This appears to be a MAC site only??? Do you know of any that are for MS OS? Thanks

I have paper manuals and am on a slow dial up... so I have never tried to access that site... but I would be surprised if it was only Mac accessible...

Welcome to Mercedes diesels! :cool:

Assuming good oil pressure, and no bearing damage, which is not too likely since the bearings are quite long-lived and rugged unless oil flow or pressure has been lost, it's possible you are hearing 'nailing' or combustion knock.

You are corrrect, #1 cylinder is in front, #5 is in back. A few quick diagnostic tests regardless which cylinder sounds noisy...

Grab a 14mm open wrench and a paper towel. Loosen the nut on top of the suspect injector that holds the fuel injector tubing. This will create a pressure leak and that cylinder will not 'fire.' If the noise decreases, you have confirmed that #1 is the source. You can operate the throttle linkage to rev the engine and listen for the nailing... When finished, re-tighten the nut, firmly to stop the leak but don't overdo it. Wipe up the diesel drool with the paper towel. ;)

Go ahead and try other cylinders to be sure. It's like pulling the spark plug lead on a gasser to find a bad spark plug, just not nearly as dramatic since you don't get the high voltage shock from holding the spark wire! :eek:

You can also swap the suspect injector with another. If the nailing moves to the new cylinder, there is an injector problem and it might need to be cleaned and/or rebuilt.

Swapping injectors requires a 27mm deep socket. The special injector socket is deep to avoid knocking off the fuel return line arms, but other sockets will work if deep enouth. You will also need a new heat shield for each injector, shaped like a conical crush washer and about $1-$2 a piece at your friendly dealer, and a torque wrench for tightening.

Oh, while you are ordering the heat shields, get one meter of braided fuel return hose, since your hose is probably old enough to crumble when you remove it from the injectors. One meter (3+feet) will replace all of the braided hose on your engine, a quick and easy job once the old hose is off of the injectors.

Before all of this, try the the pulldown Search feature, and look for 'nailing' which will give you hours of reading. Basically, it's a result of a problem with injectors such as a bad spray pattern, bad pressure, dirty injector, etc. that sounds like, well, a hammer hitting a nail into the block...

Sorry if I rambled...

Best Regards,
Jim

Jim H - Thank you for your advice - I tried each cyllinder, in fact I disconnected the fuel line completely at each cylinder. The knocking noise did not change on any of them. Is there another typical issue with the 617 diesels that I should look at? It is loud enough that I do not want to drive it anywhere. Thanks

Also the skinnerbox site has multiple dead links, I can navigate to the point where it would pull up a pdf file and it can not find the page??? All of the menus work, but then they dead end.

Now I'll take a wild guess. Check the Vacuum Pump, in front of Injector Pump, in the area of #1... It might be making knocking noises, as in going to fail soon. If so, replace/repair SOON!

Best Regards,
Jim

Use the W123 indexing for reference.

not to be pessimistic, but that knocking noise appeared on my friend's 300CD while driving along the highway. it was VERY loud and coming from the front of the engine. I told him to drive slowly and not to over rev it. He managed to make it about 10 minutes more, but of couse the idiot he is, didn't listen to me and was going uphill doing at least 65mph.

You can see the results below, hope you had better luck than he did. :eek:

New info on same problem
 

OK - I now have the engine partially disassembled and am looking for some guidance before going too far.

I have manifolds removed, fan and shroud are out fo the way fuel lines removed, valve cover off, CC module off etc.. preparing to remove head and see if I can find the source of the knocking.

What should I look for in a vacuum pump to determine if it may be knocking?

How loose can a timing chain be, also why can I not find a master link on the chain? If I am not going to replace the chain, do I need to split it in order to remove the head.

When I cycle the motor by hand I get this wired creaking sound (like an old box spring) but only seems to be on exhaust stroke for #1 and #2??? Otherswise I do not see, feel or hear any other strange noices...

What size Torx is on the head? T60 was the largest I have.

There were two sizes, on the latter years I beileve they were 12mm 12point torx. Hazet makes a tool to fit it.

This link is the best around for rebuilding the vacuum pump:
Click Here
Most of the parts won't do harm if they fail but it the bearing goes it could be catastrophic.
Also, if you do need a rebuild make sure you build that plate they talk about. It makes the job so much easier.

This link here is for checking timing chain stretch:
Click Here

Danny

Thanks - a couple more questions
 

Thank you for your help so far. A couple more questions -


What should I look for in a vacuum pump to determine if it may be the cause of the knocking?

When I cycle the motor by hand I get this wired creaking sound (like an old box spring) but only seems to be on exhaust stroke for #1 and #2??? Otherswise I do not see, feel or hear any other strange noices...
Any ideas as to what that might be????

That vaccum pump is not too hard to pull. I would remove from engine and inspect it but also turn over engine with it off to see if bedspring noise gone if nothing visual found. It would truly be a shame to pull head if thats what it turned out to be. Not possibly a good choice but one of neccesity as you stated noise was around #1 cylinder and was not relieved by releasing injector fuel line pressure. :) Perhaps waiting till further more experienced post opinions come in prior to actual head removal would be a good thought as well. :) (never had to split the gas merceds chains to remove head and diesels probably simular.) (if you are suspecting # 1 rod bearing the front portion of oil pan can be removed to access #s 1&2 journals plus you may audibly pick up the location of that noise down there with someone tuning over engine by hand or starter. But again I would wait till more posts show.)