M119.970 engine knocking sound

[grob115]

Hi all, I have a 1993 Mercedes 500 coupe that has developed a knocking sound. First we thought it was a broken rail so replaced with all rails, chain and tensioner with genuine parts bar the bottom rail. Lower pan was dropped and debris removed plus pick up cleaned out. There was broken side rails and the chain was only stretched by 3 degrees. New oil tubes fitted and all lifters checked when the cams were out and ok. So all back together and now the sound is still there. Timing cover was not removed but lower rail was visable and intact on inspection from underneath with the pan off. Have a video clip of the sound with engine running with belt off.
Next step we think is to remove the timing cover.
Any ideas?

 

[gsxr]

Kinda sounds like a piston/rod to me.... disconnect 1 plug or injector at a time to identify which cylinder it is.

 

[grob115]

Kinda sounds like a piston/rod to me.... disconnect 1 plug or injector at a time to identify which cylinder it is.

Done that and no difference plus a compression test was fine

 

[gsxr]

Compression test is irrelevant. But, I don't know what the possible cause might be.

 

[doolar]

Rod bearing? Since the piston and rod will move anyway when you remove a plug the sound will be the same. But I'm only guessing, last time I had a bad rod bearing was probably 20 years ago.

 

[grob115]

I changed the spark plugs the day before and ran it for a couple of minutes all was fine so shut it down, started the next day and the sound was there on start up, I thought initially I forgot something under the bonnet and something was hitting the fan.

 

[grob115]

A lot of broken bits of rails removed from the pan

 

[TerryA]

Hi all, I have a 1993 Mercedes 500 coupe that has developed a knocking sound. First we thought it was a broken rail so replaced with all rails, chain and tensioner with genuine parts bar the bottom rail. Lower pan was dropped and debris removed plus pick up cleaned out. There was broken side rails and the chain was only stretched by 3 degrees. New oil tubes fitted and all lifters checked when the cams were out and ok. So all back together and now the sound is still there. Timing cover was not removed but lower rail was visable and intact on inspection from underneath with the pan off. Have a video clip of the sound with engine running with belt off.
Next step we think is to remove the timing cover.
Any ideas?

@grob115,
From your video, to me the sound was loudest over the driver’s side front piston area. You may want to remove the valve cover and check that area again.

I once owned a 1964 Buick Rivera that I accidentally overheated the engine when a radiator hose blew and dumped the coolant while driving about 80mph in the fast lane on the freeway. By the time I got pulled over the car had overheated. When I re-started the car it sounded just like yours. It turned out that I had shrunk a piston skirt. It still ran but I eventually had to tear it down and re-build the bottom end. At that time I remember that somehow a machine shop expanded the shrunken piston skirt.

Just something to consider.

Good Luck

 

[grob115]

@grob115,
From your video, to me the sound was loudest over the driver’s side front piston area. You may want to remove the valve cover and check that area again.

I once owned a 1964 Buick Rivera that I accidentally overheated the engine when a radiator hose blew and dumped the coolant while driving about 80mph in the fast lane on the freeway. By the time I got pulled over the car had overheated. When I re-started the car it sounded just like yours. It turned out that I had shrunk a piston skirt. It still ran but I eventually had to tear it down and re-build the bottom end. At that time I remember that somehow a machine shop expanded the shrunken piston skirt.

Just something to consider.

Good Luck

Thanks I think we need some luck

 

[Alphasud40]

Noise is definitely engine speed and heavier than a top end noise. If it’s rod bearing noise it will be quiet when the engine is cold and get louder when oil heats up. Also with a rod bearing if you rev the engine you can find the sweet spot where it will really act up the. Floating the throttle at that sweet spot you can modulate the noise. Try removing the cam covers and run the engine to pinpoint where the noise is loudest. Remove spark plugs and with a boroscope look at the top of the pistons for witness marks. I have seen where an errant washer will fall into the intake and get wedged on top of the piston. In that case the noise will be continuous regardless of hot or cold. Could also be a cracked piston too. Really strong engines so unless debris blocked the oil pickup and starved the engine probably not a rod bearing. Flywheel flex plates have been known to mimic a bottom end issue so don’t rule that out.

 

[grob115]

Noise is definitely engine speed and heavier than a top end noise. If it’s rod bearing noise it will be quiet when the engine is cold and get louder when oil heats up. Also with a rod bearing if you rev the engine you can find the sweet spot where it will really act up the. Floating the throttle at that sweet spot you can modulate the noise. Try removing the cam covers and run the engine to pinpoint where the noise is loudest. Remove spark plugs and with a boroscope look at the top of the pistons for witness marks. I have seen where an errant washer will fall into the intake and get wedged on top of the piston. In that case the noise will be continuous regardless of hot or cold. Could also be a cracked piston too. Really strong engines so unless debris blocked the oil pickup and starved the engine probably not a rod bearing. Flywheel flex plates have been known to mimic a bottom end issue so don’t rule that out.

Some good suggestions, not checked the piston tops yet but makes sense. The noise seems to be the same hot or cold and the noise just speeds up with engine speed. There was some debris in the oil pick up but it was not blocked. Flywheel flex plate is another one to check

 

[grob115]

Update, 3 bits of rail jammed in the upper idler causing the noise