Knock knock - who's there?... Sadly it's my engine.

[280gee]

A while back I bought a pretty good E320 CDI 2002 and loved it. The car now has 108,000M on the clock.

A knock from the engine started recently and I think suddenly,
(I wasn't driving it at the time and SWMBO probably didn't hear it at first over the industrial metal music

So maybe I could call on the collected wisdom for a diagnosis, if you got a few minutes to spare?

 

[popuptoaster]

lots of things can knock, need a better description or a sound file maybe.

 

[280gee]

lots of things can knock, need a better description or a sound file maybe.

Ok, description for now, a recording as soon as my mini-disk is charged up.

The noise is there on idle, say 7-800rpm. repetition is 8 knocks per sec. as near as I can count, the knock rate rises with engine speed and disappears briefly when the throttle pedal is released.

When driven the knock is really pronounced under acceleration however slight and reduces or disappears when coasting.

 

[Dieselman]

It could be either big ends or squiffy squirting injector.

Two tests would be to listen to the engine block with an engine stethoscope for bearing slap, the other would be to disconnect the electrical connector to each injecter in turn and see if the noise stops.
The lack of noise on engine over-run suggests a fuelling problem, especially if it comes straight back on as soon as the fuel does, although it's not possible to rule out higher cylinder pressure causing bearing knock.

 

[280gee]

It could be either big ends or squiffy squirting injector.

Two tests would be to listen to the engine block with an engine stethoscope for bearing slap, the other would be to disconnect the electrical connector to each injecter in turn and see if the noise stops.
The lack of noise on engine over-run suggests a fuelling problem, especially if it comes straight back on as soon as the fuel does, although it's not possible to rule out higher cylinder pressure causing bearing knock.

Thanks Dieselman,
First two suspects were the same as yours. Injectors checked out ok as far as a Star box was concerned, so not definitive I suppose?
Ran the stethoscope over each of the injectors as well as various points on the motor. On one injector the noise was definitely up. Also the knock was really apparent with the probe on the steering pump body, = RH side.

Engine didn't want to start with injector electrics disconnected but we did disconnect it when running and the noise was much lower. On the over-run even revving it up from idling it was very apparent that the noise wasn't there as soon as the throttle was released, likewise as I mentioned, whilst driving then lifting and coasting on engine braking. Suggestive of fuelling problem but in the absence of that... big ends. Damn it.

I'd like to run this past you... an MB specialist of many years said, "a dribbling injector could have put enough liquid fuel in to have bent a con-rod through hydraulic effect. Could that have gone unnoticed I wonder? Even with the music...

 

[Dieselman]

With a bent rod you should get smoke due to low compression on one or more cylinders.

 

[280gee]

Good call, thanks!
Assuming it is a big end, I wonder why just one has decided to let go. Oh well, taking no chances now on the others a recon motor looks to be the way to go. But who to trust that job to??? )

 

[Dieselman]

Have you scoped the bottom end, if so was it banging or swishing.? Don't forget, with modern open engines noise can travel a long way and sound terrible even when it's not a bad noise at all.

 

[280gee]

No haven't scoped the bottom end yet but will try and identify a swish or a bang. I did think after a while of having the car that it was more 'diesel'y' than I'd thought a modern MB to be.

I've dealt with lots of compact light alloy motorcycle engines but mostly cast-iron car motors. I hope that I'm understanding your phrase 'modern open engines' properly, certainly for whatever reason I've a nagging doubt about this noise in mine. It does sound pretty bad though, can even feel it through the auto shift lever. Ulp!

 

[Dieselman]

can even feel it through the auto shift lever. Ulp!

That doesn't sound terribly good. As for noise modern diesels can sound like they are splitting in half just from combustion noises.

 

[280gee]

Love it! "splitting in half from combustion noises"

Yeah that's pretty much what it sounds like overall, relative to older lumps.

I hope that i haven't just "tuned in" to this particular noise, it's almost as though one cylinder is 'out' but all seem to be firing okay. Maddening as it's great car otherwise!

It's on on to phase II now....

Whether to repair or recondition the engine, and the $64,000 question. With who?
I already have an arrangement made to get engine out/in.

 

[280gee]

Phase II started... for anyone that might be interested.

The car is now with "Prestige Car Surgeons" for a heart transplant.

The noise whilst not markedly worse to my ears was easily heard by a pal who had not seen the car for a while!

 

[BlackC55]

Open CDI surgery shall commence next week.

 

[280gee]

I'll start taking the anesthetic right now!

 

[280gee]

A follow up for anyone that might be interested.

The engine was removed and sent for re-manufacture, returned in just over a week, and then refitted. All this done by Olly at P.C.S. nr. Portsmouth.

The way he managed it in such a fuss-free and satisfactory way has set a shining example in our minds of how to achieve the famed Mercedes quality!

A little more running-in has now to be done after it's mandatory flush and oil change (for the warranty conditions) but I'm expecting that to go as smoothly as the engine now does.

 

[BlackC55]

Pleased to hear you are happy Dave.

Thankyou for the post.

 

[spock500]

What do they recommend for breaking in the new engine?

This is a bit controversial but has been backed up by many posters on the US forums as being critical - not 100% sure myself, completely the opposite of my own knowledge to date

http://mototuneusa.com/break_in_secrets.htm

http://www.mercedesshop.com/shopforum/showthread.php?t=241636&page=4

 

[BlackC55]

1000 miles at low revs with an oil and filter change at 500miles. Running in an engine needs load not revs to break it in properly

 

[Smiley]

Just out of interest what had failed and why had it let go at a relatively modest mileage?

 

[280gee]

This is a bit controversial but has been backed up by many posters on the US forums as being critical - not 100% sure myself, completely the opposite of my own knowledge to date

Well the guy says amongst other things "any 4-stroke engine", and that's far too wide a definition I think. Take a 1250HP Caterpillar bulldozer for instance, the same treatment as a 250cc Honda?

There's more to it than just piston ring ring sealing as you might imagine.