Engine noise in 1991 Mercedes 300 SEL


There is engine noise relating to the piston connecting arm

in my Mercedes 1991 300SEL

What is the remedy?

engine rebuild.


91 300 SEL with a piston noise?

Time for a new car.


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Lots of money. :laughing:


What condition is this car in?

Some of these W126 cars . . . if they’re in good shape . . . are now considered desirable by certain folks

But I have no idea what condition your car’s in

If your clear coat’s peeling, the car’s rusted out and the transmission doesn’t shift correctly, for example, then it’s not worth an engine repair

On the other hand, if the car has been garage kept its whole life, the seats are in immaculate condition and everything except the engine is in tip top shape, it might be worth installing a good used engine, provided you get it at pick a part on 1/2-price weekend and do all the labor yourself

Any sentimental value . . . ?

Was it your dad’s car or something like that?


Not to make light of the OP’s problem, but this reminds me of an old joke:
Q: How can I become a millionaire?
A: Start with two million dollars, and then buy an old Mercedes

In any event, as was already stated, the OP’s Benz almost surely needs to have the engine torn-down and rebuilt.


By connecting arm I assume you mean connecting rod. A rod bearing knock generally means an engine rebuild.

It could be possible (assuming a rod bearing knock) to drop the oil pan and install a new set of rod and main bearings. This could quieten it down but this is also based on the assumption that the crank journals are not scored, way out of round, rods expanded too much, and so on. New bearings would need to have the oil clearance checked during the process.

Without engine in hand I cannot be definitive on this and would only consider it if the engine uses very little oil, does not smoke, the noise has not been present very long, and so on.
Whether a shop would do this is debatable.

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A 91? It’s vintage! I expect a complete rebuild of the engine is required; either that or installing another used or rebuilt engine. There’s a remote possibility the problem is only a single connecting rod crankshaft bearing. It might be possible to replace that without removing the engine working from the oil pan area. I wouldn’t count on this approach though. If I had that problem I wouldn’t even attempt that approach, likely a waste of time. I’d try to find another good used engine instead.

A used engine is $750, shipping included. $2000 to $3000 to install the engine.

Many people enjoy spending $35,000 on new vehicles, I wouldn’t consider $3750 to be a lot of money.


I tried doing this procedure on a 1993 Plymouth Sundance. It was just one rod knocking, but unfortunately, the crankshaft journal was completely ovalled out. However, I received the car in this condition, and have no idea how long it was driven like this. Someone had already tried replacing the bearing inserts from below on that cylinder, and those bearings had come apart. I ended up pulling the motor and transmission to rebuild.

I would say that if the noise just started, you might be able to get away with changing the bearing inserts from below. As a DIY project, that should not be too difficult or expensive. I doubt a professional mechanic would agree to do such a thing, however.

Excellent condition first owner only driver oil change every 3500KM oil filter every other oil change milage about 500 thousand KM all under the hood are original except some gaskets and consumables

Excellent condition first owner only user well maintained about 550 thousand KM

At around 300,000 miles, I’d say the engine is worn out. Rebuilding or replacing are your options.

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Ah, the Sundance project. Is it done?

No. The car is still sitting in my garage, with the motor and transmission pulled and wrapped in garbage bags. I was planning to do a “hope it works” rebuild this last winter–clean the parts to the best of my ability, install a used crankshaft, all new standard-size bearings, gaskets, seals, etc, but no actual measuring, etc.

I got sidetracked, because my daily driver Daewoo was running very poorly, and needed major engine repairs. By the time I got that finished, I was already in the busiest part of the year.

Now that cars have become more valuable, and these models have become more scarce, I may just have the engine rebuilt “by the book” by a professional machine shop. I want this car to last for a long time, and I want to use it for some long-distance trips which we are planning, so I need it to run well and not consume excessive oil, etc.

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That entire car should be wrapped in garbage bags…an then thrown out with the rest of the trash. They were junk when new.

A 1990 420 SEL went on the auction block at Mecums’ auctions last night.

Mint condition.

It sold for $6,500.

I wouldn’t call that collectable


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If someone is able to specifically diagnose a an engine noise to a specific part and call that part a “piston connecting arm” they kind of exclude themselves from having the skill or knowledge to make the diagnosis.

What makes you think you have “piston connecting arm” noise?