Troubleshooting my engine knock (long-ish) 1982 300D - Page 2 - PeachParts Mercedes-Benz Forum

yblocker · #16 01-22-2010, 12:32 PM

I think you can eliminate a loose object banging around in the combustion chamber. You would hear that all the time and I think it's too regular in its period.

I'm more of a gas engine mechanic, but if it WERE a gasser, I'd disable that # 1 cylinder. This takes the load off the piston, piston pin and rod bearing and noises like this that are piston/rod related will be reduced. Can you disable the cylinder (the injector)?

Can you run these motors with the valve cover off? Might be messy, but instructive....

Steve
(soon to take possession of my first diesel car - 1982 300D Turbo)

konstan · #17 01-22-2010, 12:39 PM

Quote:

Originally Posted by yblocker

I'm more of a gas engine mechanic, but if it WERE a gasser, I'd disable that # 1 cylinder. This takes the load off the piston, piston pin and rod bearing and noises like this that are piston/rod related will be reduced. Can you disable the cylinder (the injector)?

Disabling for one of those diesels is equivalent to loosening the fuel line nut, that prevents the fuel pressure from activating the injector. The noise does not go away.

Yea I ran it with the cover off too. Messy but fun...

Good luck with your first 300D Steve!!!

barry123400 · #18 01-22-2010, 07:09 PM

The cam and vacuum pump turn 1/2 crank speed. Because of the proximity to the number one cylinder I would check out the vacuum pump myself.

There is a bushing of sorts in there that may make a noise if too much end clearance is present. I also wondered about less noise coming off the freeway. The vacuum pump would not require as much cam pressure at that moment perhaps as the vacuum is fully charged so to speak. I might start by opening up the principal vacuum line or output fitting on the pump to see if things change at all.

konstan · #19 01-24-2010, 10:54 PM

Opened up the main vacuum line while it was running, the knock didn't really change. The pump is definitely making vacuum, my finger almost got sucked into the line

Poked around with the stethoscope some more... the knock is so darn loud, its real hard to pinpoint where it is coming from. It is fairly loud at the pump... for example, much louder at the pump than at #2 injector...

layback40 · #20 01-24-2010, 11:21 PM

I would be removing & inspecting the Vac pump first. I dont know if you can run the motor with the vac pump removed, maybe some one can comment.

Did you check for timing chain stretch?

Have you run the motor with no belts (fan etc)?

I am just trying to eliminate easy to fix things before you need to take the head off.

konstan · #21 01-26-2010, 12:32 AM

Its not the vacuum pump. Today I removed the pump and ran it for a few seconds without it. Still has the knock...

layback40 · #22 01-26-2010, 01:59 AM

It is starting to sound serious. Have you run the motor with no belts?
After that, my next line of inquiry would be a couple of Mercedes repair shops.
Maybe a diesel pump specialist as well.
If that gives no joy, head may come off, I am far from convinced that you will find much though. If its not something simple, I would be going back to an early suggestion of it being a cracked piston, possibly the skirt. It would be good to get some other opinions though.

jt20 · #23 01-26-2010, 02:30 AM

if you've gone this far, the sound must be serious.

I would take off the lower, black oil pan and search for metallic debris / shavings... whatever. it will only cost you 2 gal of oil and some time.

it will also give you an inside view of the lower end.

layback40 · #24 01-26-2010, 02:44 AM

jt20,
Probably a good idea. Will be able to check No1 bearing as well.

jt20 · #25 01-26-2010, 02:46 AM

only #2, maybe #3.... can't get to the #1 on the turbo (main or rod)

konstan · #26 01-26-2010, 10:06 AM

Quote:

Originally Posted by layback40

After that, my next line of inquiry would be a couple of Mercedes repair shops.

Well, the one fella I took it to said the next step is the head... he said the camshaft and the valves all looked good.

Quote:

Originally Posted by layback40

Maybe a diesel pump specialist as well.

Using the HF stethoscope, I can determine that the knock is much weaker at the pump than at the head.

Quote:

Originally Posted by layback40

If its not something simple, I would be going back to an early suggestion of it being a cracked piston, possibly the skirt.

I was working under assumption that it is not the lower engine problem (bearing, cracked piston, etc) because I had the car on the interstate for test drives a few times, at 50-60mph, and nothing came flying out of its side... or maybe this is a faulty assumption?

I will take the pan off, see whats in there.

konstan · #27 01-26-2010, 10:19 AM

I looked on youtube for similar sounding videos... found one with a very similar sound that the owner said turned out to be a broken piston skirt.

I will try to make a cleaner sound clip of mine and post it, too.

kerry · #28 01-26-2010, 10:43 AM

If my memory serves me correctly, ForcedInducation had/has a knock in his engine that has been there for a long time. He knows what is causing it. He's not here anymore but you could find him on one of the other forums. It might be worth exchanging some info with him since he might be able to help diagnose yours.

layback40 · #29 01-27-2010, 08:27 AM

With th3e pan off you should be able to see the insides of the pistons, the motor will run with a big chunk broken off a skirt. If it is you may just need 1 new piston & rings for it. i have known of people using second hand pistons successfully. You may decide its easier to find a second hand motor.

konstan · #30 01-27-2010, 10:58 AM

Ok tonight is the time for pulling the pan off. Yay!

Why does a piston with a broken skirt make noise? Because it is not balanced?