Hi, I am new to the Diesel world and just bought a 1983 300D Turbo Diesel with a W123 engine with the intention of converting to WVO. Bought it cheap as it has an engine problem. It has 305,000 miles on it, otherwise it is good shape. Everything is original and A/C works ( converted to R134 )

The engine knocks during idle and vibrates a bit. However, when I rev the engine to about 2500rpm, the knocking noise goes away and everything seems normal. It drives reasonable well and oil pressure is good. I have taken a video with sound to help diagnostics but I need a method to upload it to the forum.

Any help would be appreciated as I do not want to spend a lot on engine work.

 

Welcome to diesel world!!! I'm not a mechanic but I own 2 Mercedes 82 & 87 and both make nose when starting the car after 5 mn or soits fine.

 

Something you should understand if you are new to diesel engines, is that diesel engines tend to make more noise than gasoline engines. Diesel engines to seem to have a "knock" but it really isn't. What most people associate with a knock in the engine is simply the combustion taking place. Diesel engines do not get their ignition source from a spark plug. Rather at cold start, they have what is called a "glow plug" that heats the cylinder walls and such. A diesel engine uses the heat of compression and heat from cylinder walls and such to fire, as diesel can ignite at lower temperatures than gasoline. Doing so, it often sounds like an engine knock, which really isn't. And this sort of noise typically goes away when the engine is at a higher RPM range.

However, this may not be your issue. Heres the thing, can you tell if the knock is coming from down low in the engine, or up high near the cylinder heads? As with all engines, a low end knock typically indicates crank or main bearings might be getting bad. An engine with 300K miles, this could be the issue. Depending on the severity of wear on a low-end bearing, the knocking noise may quiet down at a slight increase in RPM. If this is the case, it can be somewhat expensive, considering if your crank is in good shape and can be reground and oversized bearings used, etc. A new crank could be upwards of 300+ dollars.

If the knock is coming from the heads, it could mean you have hydrolic lifters that are warn and needing replaced. This is pretty cheap and easy to do. It could also mean you have valve guides that are warn out, again, cheap and easy. Or it could range (with the heads) that the heads simply need rebuilt, which isn't all that expensive, about 200 or so per head.

If its not that, it could be a bad connecting rod or a bad wrist pin. A bad wrist pin, the sound usually does not go away, it just gets worse, but hey, stranger things have happened. It could also be that your cylinder walls are warn or ovaled, which means pretty much a complete overhaul of the engine is necessary, assuming your engine block is within tolerances and can be bored within reason, with oversized pistons and rings, etc. This is very expensive.

Hope I gave you some ideas to check into, and not scared you that much.

 

Hi,

I did a test on the noise and there is more information. I loosen the diesel feed to the injector one by one. You can feel the engine struggles a bitwhen the diesel feed isstarved but there is a different with the noise on one 1 cylinder.

The metallic noise remains when the injector ofcylinder 1,2,4 and 5 are loosen. However, when cylinder 3 injector is starved with diesel, the metallic noise goes away. A friend of mine thinks the piston in cyliner 3 may have a problem. I am going to swap the injectors between between 2 and 3 and see whether the noise moves to 2 or stays in 3.

If it moves then it is good new, if not then it is all mechanical problem inside cylinder 3.

Stay tune.

 

Hi;

What did you find with your test? Anything new on the noise you are concerned with?

 

I am back on line again after a while. The noise moves with the injector in cylinder 3. Good news. I replaced the injector from one in a junk yard and now the car purrs like a cat, not quite but the noise is gone.

BTW. I converted the car to run on WVO and have done 2000 miles on it, for free. I also added remote keyless control to the car. It is very cool for have keyless entry on a 24 years car. I fixed a vacuum leak andadded an actuator to the driver's door to control the valve. Total costs with control box, less then $35 + my labor.

The 300D is a great car.

 

Where did you get your remote keyless entry?

 

I got the remote control and actuator from from eBay. Any remote keyless entry would be OK. Do not buy those claiming specifically for MBZ. It is a rip-off. You can buy both for under $40 max.