Thoughts On Overhauling 200D (OM621) Engine
 

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.

Hey Scotty. It's my understanding that one tooth on the cam sprocket is 18 degrees. So something is truly goofy. Maybe somebody installed an offset key the wrong way, although I still don't know how it could come out to 9 degrees.

Let's see. They could have advanced the chain one tooth, retarded the sprocket 4, 5 or 6 and then the chain stretched another 5, 4, or 3 degrees. That would give you your 9 degrees advanced.

Somebody has evidently screwed with the timing before you. Pull the sprocket and see what's going on with the key/keyway. Perhaps you should then do the valve lift timing method to get an accurate timing value. Then go from there. Be careful with the chain timing so you don't interfere with the valves.


Seems to me if the car starts, has good power, and is not burning oil, then you've got a good bottom end. A compression test will tell you alot, for sure.

Your IP is timed correctly, right?

I was surprised to see the extra notches on the cam, and even more surprised to see a reading of 9 degrees BTDC on the crank pulley when the factory notches on the cam are lined up. It also seems that the lobes on the camshaft are opening the valves too soon.

If a normal chain is supposed to be at 9 degrees ATDC, then having the camshaft off one tooth could put it at 9 degrees BTDC, I think. I am interested in moving it one tooth and taking new readings. I really think that the cam is off one tooth. I've checked for chain stretch on other cars before and this one is weird. I suppose I should buy a dial indicator.

The injection pump was drip timed to 26 degrees BTDC, as specified in the factory service manual.

Are you sure the crank should read 9 degrees ATDC when the marks are lined up? I'm almost certain that on my 617 it is supposed to read 0 degrees or TDC.

Of course, your engine could be different.

Your vacuum pump is not feeding oil into the engine through it's exhaust line? Any oil in the output line?

Fairly common failure with the earlier vacuum pump arrangements. Result of a breeched diaphram in the pump.

I can't remember for sure, but I thought it was odd that it gave a BTDC reading so far away from zero, when I was expecting an ATDC reading. I remember noticing that it appeared that the lobes were opening the valves too soon. I've checked timing chain stretch on other engines, but it really seems that something is wrong with this one.

Also, in the door frame is a 1980s-looking label tape that reads "ENGINE OVER HAUL 82,000". I wonder if they were trying to hot-rod the engine by advancing the cam timing. Ha. Maybe that would explain why this engine seems so fast and why it also smokes.

I was thinking that this is a possibility. The brake booster seems to be working, though. But maybe the diaphragm is cracked enough that it's letting oil leak past while still functioning. I have noticed that there is a massive amount of oil oozing out of either the intake or the exhaust manifold gaskets.

I would look into that vac pump idea very seriously. If the car is making good power and starts and runs well there is no way I would tear it apart.

Taking one of these engines down is so much work it seems a shame to try to skrimp on parts when doing so.

Though I don't advocate replaciing any parts that are within specs.

Tom

Yeah, it looks like I should buy a rebuild kit for the pump just in case I open it up and find the diaphragm is ruptured.

I have to take apart the engine anyway because the engine leaks way too much oil in the driveway and needs all new seals. It also seems that oil is leaking out from around the head gasket, but it might be oil seeping out of the manifold and creeping around the base of the head. I've had no issues with finding coolant in the oil, or oil in the coolant.

It's a pain. I definitely need to stop the smoking and oil leaks as soon as possible. I've tried unloading the car, but for what people are willing to pay for it I would be losing way too much money. I could part it out and probably even make a profit, but it would be a shame and the process can be a real headache.

If it ever stops pouring rain, I want to check out the cam timing and also rebuild the vacuum pump. If that doesn't work then maybe I can install the new valve stem seals, or at least check to see if I can tell if the stem seals or valve guides are worn. If that doesn't work then I'll see if I can figure out the chain stretch with a dial indicator. If all of that fails, then maybe I have a head gasket leak or worn piston rings.

Whatever the problem is, the engine starts up fine and has good power, and can probably be in service for years and years. But it makes an oil mess in the driveway and the smoke is really annoying, so I have to fix these problems as soon as I can. In a 2,000 mile trip it used a bit less than a quart of oil, not sure if that's too much?

You do not have to rebuild the vacuum pump to find your answer. Just disconnect the line that feeds from the vaccum pump to the engine input point and disconnect it. If the smoking stops it's the pump. Do not disconnect the actual vaccuum feed to the car itself.

I disconnected the vacuum pump line going to the intake manifold. The car still smoked as much as before and I could see nothing coming out of the line from the pump.

So, for now at least, the vacuum pump appears to be fine and the problem is likely either the cam timing, timing chain stretch, valve guide seals, or piston rings.

I want to check out the cam timing soon by lifting the timing chain and moving the cam over one tooth to see if the crank marks will give a slightly after TDC reading instead of a before TDC reading--and then I can probably do the dial test, or at least get some visual on how much timing chain stretch there is.

I have new valve guide seals and I hope I will be able to tell if the ones currently in the engine are in good condition, same with the valve guides.

If I can't solve the problem with any of the above, then it looks like I will need to check out the cylinder walls and piston rings and possibly work on those.

Today I took off the valve cover to look at the cam and timing chain. I lined up the factory notch marks on the cam and then looked down at the pointer on the crank pulley. It read about 6 degrees BTDC. I turned the crank around again and double checked. It is suprisingly easy to turn the engine with a a 27mm socket on the crank pulley. I could hear a whoosh coming from each valve as I turned the engine.

When the #1 piston is at 6 degrees BTDC the marks on the cam line up. I thought that when the cam marks were lined up the pointer on the crank pulley was supposed to point to an ATDC reading? Does this mean the timing chain is off one tooth? You can also see the extra notches marked in the cam.

I noticed that the cam lobes are scored and have flat spots. The valve cover breather elbow and tube were also full of oil and lots of oil leaks out of them and drips onto the manifolds.

When I had it back together I decided to start the engine and then take off the filler cap to see if I had blowby smoke coming out. As soon as I took off the cap it popped off and the engine started squirting out oil all over the place. The oil shot out on my face and the fenders of the car and started shooting out at high pressure from where the oil filler cap used to be so I quickly put it back in place. Is it supposed to squirt out oil from the filler cap hole in the valve cover like a geyser?

I haven't checked compression because I haven't been able to get the compression tester fittings in there with the heater pipes and injection pump in the way.

https://lh6.googleusercontent.com/_Q...200dengine.jpghttps://lh3.googleusercontent.com/_Q...00dengine2.jpghttps://lh4.googleusercontent.com/_Q...00dengine3.jpghttps://lh3.googleusercontent.com/_Q...00dengine4.jpghttps://lh3.googleusercontent.com/_Q...00dengine5.jpg

Sounds like a massive amount of engine blowby unfortunatly.

Yeah, and I'm not willing to spend more than a couple hundred dollars to rebuild the engine. The body is rusty, anyway, and even aftermarket door and windshield weatherstripping would cost nearly $1,000 (dealer price is scary). The car's not worth much as a whole. I think it's time to start listing parts on eBay to try to recoup the thousands I have into it.

I seem to recall that the timing chain is directly below the oil fill opening on that model. The chain will sling oil like crazy with the cap off.