Tracing Down Knocking Sound
 

Hello,

So I am now in possession of a W123 parts car which has a knocking sound in the engine. I do nto have much experience with knocking, so I'm looking for some advice on tracking it down and seeing how I can possibly quiet it down. My objective is probably not to save the engine, but just be able to move it around for parking reasons in my street or driveway.

I have heard talk that sometime knock can be do to an injector, a spun bearing, or a "rod".... I'm wondering where I can efficiently start to narrow it down. If it's any of these, would finding the associated cylinder and loosening up the injector quiet down the racket, or reduce further damange rate to the engine when run like this?

Any tips or tutorials on the ways of knocking sounds would be much appreciated. I'm afraid to say that I don't even know what a "rod" is...

Thanks.

How about somemore info. Is it a dull rythmeic sound or a higher pitch sharper sound?
How much is your Oil pressure when the Engine is hot and it is ideling?

To narrow down which cylinder it might be hopefully it knocks at idle- Loosen 1 of the Injector Hard/Fuel Injection Lines/Tubings up at the Injector at a time on each Cylinder. This will cut off the Fuel to that particular Cylinder.

If that knocking dissapears or the sound of the knock lessens that is the problem Cylinder.

After that to find out if it is the Injector or not switch the one from the suspected bad cylinder for another Injector (the one furtherest away). If the Knocking Sound Follows the Injector from the suspected Cylinder it is the Injector that is haveing a problem.

If the knocking sound stays in the same Cylinder after switching Injectors there is a good chance that you have a Bearing gone bad or some other problem in that cylinder.

If you model has a little steel Oil Pan you can drain the Oil and remove the pan and look and see if pieces of bearings are in the pan. The Oil Filter could also be pulled and inspected for bearing fragments.

Sometimes when a bearing is going bad the Oil Pressure Gauge pointer will fluctuate up and down.

Also be sure that the Knocking sound is not coming from something loose like a loose Air Filter housing beating itself on something.

Please put your year and model in you signature so it will appear evertime you post.
This is a Russian Parts site but most of it is in English. You can use it to learn the name of the parts and where they go.

http://www.detali.ru/cat/oem_mb0.asp?TP=1

1977 300D/ W123
 

The car is a 1977 300D/ W123.

The knock is not high pitched really, but kind of a clank-clank. It sure doesn't seem like a hose or belt slap sound, but I guess I could check.

I'll also try pulling the injectors or tubing to each cyclinder and seeing if I can narrow it down or eliminate the sound at idle at all... The guy I bought it from said he thought it was the number two cyclinder, so the cylinders are counted from facing the fan/radiator and back towards the firewall, right?

Thanks again all.

Try tightening crank pulley bolt,oil filter,intake,exhaust,turbo and check air filter mounts.Then look,one thing that can fool the best,flex plate behind flywheel.

Flexplate
 

So checking the flexplate would involve dropping the tranny or pulling the engine would it not?

What is the common failure with this component? Would I be able to do the old garden-hose stethescope trick and localize or pinpoint the sound down in the region of the tranny?

Would it go away if I put the car in Neutral?

Thanks again.

No you can see plate thru inspection cover.I do not now how to replace,wrong not oil pump,but filter housing..

"injectoring nailing" and "cracking each line"?
 

I've been reading up on possible knocking sound causes in other posts and have run across reference to "injector nailing". Can someone explain what happens to cause this and what it is?

Also, related to this, how do I "crack the hard lines" to each injector (one by one) to narrow down which injector might be nailing?

I'm still hoping for an explanation of flex-plate failure which one might miss-diagnose for rod-knocking...

Thanks.

cracking and nailing
 

Hello, cracking a line is simply loosening the nut that clamps the line to the top of the injector. Don't take it all the way off, just loosen it enough to release the fuel pressure. small amount of fuel will bubble out of the top and bottom of the nut each time the pump delivers to that injector.:)

Nailing.....I have heard of it as well but I am not sure what it is.:o

Cheers,

EddieE
87 300SDL 325K

Here are some good pictures of a cracked flex plate

www.peachparts.com/shopforum/showthread.php?t=237244

Charlie

Rods?
 

So are "rods" the piston arm which connect the piston head to the main crankshaft/propeller-shaft, or are these push-rods which control the up and down movement of all the piston-cylinder valves via a rocker-arm system?

I'm just starting to learn about the venerable five-cylinder mercedes diesels...

^^These are overhead cam engines. Open the oil cap, you're looking at the cam. So a "rod" is the piston arm, (connecting rods) but the weakest point is the wrist pin. IF that's the case, and make sure, sounds like a trashed engine.