Have a persistent knocking sound from the engine on a 1991 300E with 137K miles. The frequency of the sound varies with engine rpm and is present at all engine speeds. The sound is coming from the right side of the engine, in-line with the front cylinder and about halfway up the block (close to were the vacuum pump bracket attaches to the block. I removed the serpetine belt and the noise is still there. I removed the valve cover and everything looks normal. I ran the engine with the valve cover removed and the timing chain was not loose or slapping. The sound is in the vicinty of the timing chain tensioner, but towards the back of the engine about 3 inches. The sound does not change when I disconnect the spark plug lead from the front two cylinders. Help, what should I do next...

Try checking the belt tensioner damper (little shock absorber). Worn or missing bushings can cause the rattle that you describe.

Good luck

~Paul

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'91 300E, 213K miles

The noise persists even with the serpentine belt removed. I think that would eliminate the belt tensioner?

And generally, the belt tensioner announces itself when idling...it's quiet during normal driving. This "rattle" is occuring at all engine speeds.

Time to borrow an engine stethoscope. It's amazing what you can hear with those things...

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Found the noise. It's a broken rocker arm carrier. A small bolt fell out that holds on the oil distributor for the cam shaft. The bolt got caught between the cam shaft lobe and the inside of the rocker arm carrier and cracked the assembly.

Anyone know were I can get the rocker arm carrier or what it is really called. The part number is 301 051 01 10. It holds two rocker arms and has two bearing saddles that hold down the cam shaft on a '91 300E.

There are three bolts that hold on the oil distributor for the cam shaft. If you have the valve cover off for any reason I would recommend removing the bolts and putting a drop of thread lock on then before re-installing.

Was your engine misfiring also? With a broken rocker arm, your valves wouldn't be opening and closing properly clearly affecting engine performance.

I agree that those 3 screws should at least have a locking washer underneath them -- they are very short and small screws that very easily rattle loose, as you found out the hard way.

You said you ran the engine with valve cover off -- did the rocker arms break after you tried this?

~Paul

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'91 300E, 214K miles

[QUOTE]Originally posted by pmizell
Was your engine misfiring also? With a broken rocker arm, your valves wouldn't be opening and closing properly clearly affecting engine performance.

The rocker arms were not broken the carrier that holds two of the rocker arms was cracked. The valves were still opening, but the cracked carrier was moving up and down slightly making the noise.

You said you ran the engine with valve cover off -- did the rocker arms break after you tried this?

I was checking for a loose timing chain when I ran the engine with the valve cover off and you can only run it for a few seconds because oil is running all over the place. I pulled and pushed on the rocker arms to check for any play and they were all tight. The crack in the rocker arm carrier was not obvious and I found it only after a close visually inspection. I did remove the carrier, which allowed the two valves for the first cylinder to close, and ran the car briefly and the noise went away.

~Paul

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i believe it is called a cam bearing housing.

You going to need to get another cam /rocker tower.
They are machined to each head to carry the cam..
To ensure the replacement tower does not bind when installed, you are going to have to remove the rest and chain sprocket
and install the new tower and see if you can turn the cam without binding.
If cam is tight, try another front cam tower.
If this fails, it's going to be a complete head.

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