Loud knocking and LOTS of Blow-By: 300SD

[cousindave76]

I have an 83' 300sd 198k. My engine has always run very smooth with all the right sounds. I just starting hearing a very loud knocking sound. I would describe the sound as being a bad valve. Because this is a 617 Diesel, I realize that may not be the case. The other new symptom that started with this sound is serious blow-by. I have checked my blow-by in the past and found that I had very little. Now that I heard this sound I decided to check it again. I found that it has a LOT of blow-by now. This scares me

I would like to know the proper steps for diagnosing this problem???

I have new glow plugs and my injectors are new as of one year ago. I have not checked IP timming. I have had a new timming chain installed and valve adjustment done by an MB shop about 6 months ago.

I would guess a cracked pre-chamber or, bad injector to be causing this knocking sound but, this new blow-by situation leads me to believe it may be something more serious.

Has anyone had a similar experience and found it to be something other than a bad valve?

Your advise please

Sounds like a cracked piston or blown out rings.

What were you doing when you first heard this sound?

[Knightrider966]

I agree with the piston and ring scenario! shut it down and pull it apart or get professional help. This is still salvageable!

[cousindave76]

I was driving as usual when I first noticed the sound. Oddly enough I have not noticed a loss of power. Still runs good!

Does that eliminate anything?

Was it after any hard acceleration/hill climbing/overheating?

Has the engine been over 5000rpm?

[kerry]

Could it be a bad vacuum pump making the noise and introducing more pressure into the crankcase? Is the vacuum system working ok?

[cousindave76]

Originally Posted by ForcedInduction

Was it after any hard acceleration/hill climbing/overheating?

Has the engine been over 5000rpm?

Normal city driving. I am actaully a very conservative driver... with exception of an itialian tuneup every two months or so.

I do not recall laying into it today at any point. I did not notice the sound until I came to a stop at home. From inside the car it is not to bad. Outside it is very loud. When I removed the oil cap from the valve cover it became VERY VERY loud. And of course, that is when I noticed the excess blow-by.

[cousindave76]

oh... and it has never even come close to 5000rmp. I don't think I have ever let it go above 4000 rpm. You know how these diesel engines are... anything above 3500 makes it sound like it's going to blow up!

Anyway, just adding that it has not reached the 5000 rpm range.

[Motorhead]

Isolate the cylinder making noise by cracking each injector, one at a time. Once the noisy cylinder is found, remove the injector and have it checked for cracking pressure AND spray pattern. A full five cylinder compression check would be advised paying attention to the noisy hole.

Is the exhaust smokey or blue?? A friend of mine had an injector fail on his diesel Rabbit and it sounded like the engine was toasted.

[t walgamuth]

i dont see how a failed injector could cause a lot of blow by.

i fear a hole in the piston. possibly a broken glow plug or something else could cause this.

good luck

tom w

[cousindave76]

My vacuum system has been working fine. I won't rule out a faulty pump though. I will double check it for proper function.

I will try to isollate the cylinder number making the noise using the above mentioned method. I will post my results. I'm helping a friend move tomorrow so I won't be able to work on it till wednesday.

Thank you so much for your fast responces! I hope this is a solvable problem without pulling the motor.

[cousindave76]

Well I have been looking it over today and have not made much progress. With it running I loosened off the fuel lines to each injector one at a time. The knocking sound never got quieter. The only effect was that with each one, when I cracked the fuel line off, the engine stumbled. I removed the valve cover to inspect cam/rocker/valves and found nothing visibly wrong. Valve clearance looked fine for all valves.

I don't have a stethoscope so I resorted to the old wood stick in my ear technique. The knocking sound seems to be most present in the very front of the block on the passenger side. I placed the stick on the alternator, vacuum pump, water pump and power steering pump while doing this and heard nothing odd from those components.

Do I need to pull my oil pan?

I am most confused by the fact that this sound is SO LOUD and yet it runs fine. I have good power and it is not smoking out the tail pipe... ...unless I were to lay into it of course, then it put's out a black smoke screen suitable to the bat-mobile as means of escaping a following car.

Is there anything inside the block that I'm not thinking of that could be causing this much knocking and yet the engine runs fine?

Can an oil pump do this?
My temp and oil pressure are normal by the way.

Can a cracked pre-chamber cause all of this noise? Or a bad fuel injector? Can either one of those thing suddenly cause more blow-by than I used to have?

Ok, I have posted a lot of questions in this one post, sorry about that... I'm just not sure what to look at next.

[rrgrassi]

On a gasser a knock like that usually means either rod or main bearings, or piston slap.

Check the vacuum pump and the timing chain tension as well.

[cousindave76]

I have found the problem!

One by one I started taking belts off and then starting it up to see if the knock was still there. I give you, the ALTERNATOR!

I had put a new alternator on the car about a month ago. I never suspected it could be the problem. It was such a loud knocking sound. I never thought an alternator could make that kind of noise! The actual problem was, the pulley nut was just a tad to loose. This caused the fan on the alternator to hit it's casing. This is why it was soooo loud. It was metal on metal hitting at constant intervals which speed up with the motor.

Thanks everyone for your help and advise! I got easy on this one!

And if you want to message me and give me all kinds of crap for not figuring this out sooner, I guess I deserve it

This is one more thing to add to the list of "So you think your diesel engine is dead when it most likely is NOT"

Seems it's always something simple when it sounds soooo bad.

One more 300SD back on the road!

[rrgrassi]

It's alot better than the alternative! Glad you found it!