W123 240d Rod Bearing Replacement - PeachParts Mercedes-Benz Forum

Johnny2002 · #1 03-13-2012, 10:03 PM

Hey folks, acquired this car a few months back with a blown motor (piston was in pieces in the oil pan), got a new one off of CL with about 240,000. After about 500 miles, its got a pretty good "down low" knock at startup for a minute or so. Sounds to me like a rod bearing?

Question is, could I remove the upper oil pan in the car? Has it been done, or does the motor need to come out again?

ROLLGUY · #2 03-13-2012, 11:46 PM

If it is like a 5 cyl, I believe you can replace the front 2 rod bearings without removing the upper oil pan or the engine. However, if a rod bearing is bad, chances are that the rod itself is out of round. It may not do any good to replace bearings, as the crank journal may be bad as well. You may just have an injector nailing problem. The sound of injector nailing and rod knock is identical. If the engine idles/runs smooth, then chances are it's a rod knock. If the engine idles/runs rough, it could be injector nailing. If the oil smells burnt, it is definitely a rod knock.

Johnny2002 · #3 03-14-2012, 12:14 AM

I thought it could be nailing, but it does run smooth. And its only knocks for a minute on cold starts. Its definateley coming from the lower end

I know, the crank and or rods could be fubar but I'd like to at least give it a fighting chance

I mean this thing made 65 HP factory, how strong does a rod and piston have to be to make 15+ HP

ROLLGUY · #4 03-14-2012, 12:29 AM

Try some Lucas oil stabilizer. That stuff does wonders. It may take away the knock, and buy some time. Who knows, it may eve totally fix the problem!

Diesel911 · #5 03-14-2012, 12:53 AM

If you have a little Oil Pan on your Year and model drop that and look for Metal fragments in the bottom of the Pan and in the Oil you drained out.

If you do not find the cause of the Rod issue it will happen again.

Did Metal fragments make it as far as the Oil Filter? If so they went through the Oil Pump.

Did they say what they though caused the damaged the original Piston?

Clemson88 · #6 03-14-2012, 01:04 AM

You know if you got a rod knocking that engine is coming out of that car one way or the other.

Johnny2002 · #7 03-14-2012, 09:50 AM

Yeah, guess the motors coming out. I'll try some Lucas first. Was kinda hoping to rebuild the motor that came out over time but looks like that'll happen a little sooner!

I dont know why the piston let go originally, my guess is it wasn't maintained very well.