Thoughts On Overhauling 200D (OM621) Engine - PeachParts Mercedes-Benz Forum

Squiggle Dog · #1 02-04-2011, 07:40 PM

I bought my 1968 200D (OM621) with what was supposed to be "2,500 miles on a freshly rebuilt engine". When I looked at the car I saw that the engine was covered in oil, so I didn't believe the seller's claim.

I could see that the valve cover gasket was torn, so I replaced it. While I had the valve cover off I adjusted the valves. Replacing the valve cover gasket helped greatly, but the engine still leaks oil enough to get me in trouble both at home and at work.

The engine smokes quite a bit on startup. Sometimes it fogs up the entire neighborhood and I leave a trail of light grey/blue smoke behind me when I drive off. One time I was in a parking lot at a hotel where there were a bunch of people standing outside. When I started the car it covered them in a smoke cloud and it was really embarrassing. Once the car warms up it tends to go away, but when the car idles the smoke returns.

The engine starts up with little hesitation even when it's in the teens Fahrenheit. It has what I think is outstanding power for a 55 horsepower engine and I don't have any problem merging onto the freeway and cruising at 75 miles per hour. It's a rather peppy car.

In trying to solve the smoking problem I have sent in the injectors and had them rebuilt with Monark nozzles by a place that specializes in Mercedes fuel injection. I also sent them the injection pump and had them replace the governor diaphragm, replace some sealing rings, and calibrate it on their test stand. The parts were reinstalled and set to factory specs. It didn't work, but now I know they are good.

I decided to check my timing chain stretch. I lined up the marks on the camshaft but noticed that someone had scribed on two extra marks. When I lined up the factory marks I noticed that the pointer on the crank pulley read 9 degrees BEFORE TDC! I thought the spec was supposed to be 9 degrees AFTER TDC for spec, or 9+ for a stretched chain. I think that the camshaft is off one tooth. I am going to disconnect the timing chain and move the camshaft over one tooth, then check the marks on the pulley. If they look right then I will run the engine and see if it helped.

I am going to have to remove the engine anyway to replace the oil seals. Before I take the engine out I am going to do a compression test. I haven't done one before because it was too hard to try to connect the fittings with the heater hose pipe in the way. While the engine is out I am going to take it apart and inspect everything. This will give me a good idea of if the engine has truly been rebuilt and what parts may need to be replaced.

I can't afford to rebuild the engine, but I am prepared to replace the valve guides and I already have a set of new stem seals. This might help some with the smoking. I've never done this before, but I hope no machining will be involved.

Also, if the piston rings are worn I can buy and install new ones. I am hoping that if necessary I can install new rings without having to machine the cylinders, though I wouldn't have a problem honing them with a ball hone. Theoretically, as long as the cylinder bores aren't damaged, I can just install the largest piston rings that will fit and call it good?

I am hoping that all of the bearings are in good condition, but I will see. I want to stop the smoking with as little money as possible. I still have to replace all weatherstripping on the car, get a carpet set, etc., all on a limited budget.

I figure that if it's not broken, don't fix it. If I pull apart the engine and find that all parts are in good condition, except the valve guides, stem seals, and piston rings are worn, just install new valve guides, stem seals, piston rings, maybe lightly hone the cylinders, replace all oil seals and head gasket, and call it good?

If I can stop the smoking and oil leaks with a small amount of money (plus my time to do it all myself), I figure, why not? Is it as easy as it seems? I know people spend around $5,000 in parts rebuilding these engines, but I think that with maybe $400 worth of parts I should be able to have a smoke and leak-free engine that will last a few hundred thousand more miles. At least I should be able to go for a few years without having to deal with smoking or oil leaks and the engine still has good power.