I am going through the same thing and in my case there were cracks in #5 and #6 and there had been moisture in all of the cylinders. The car was overheated for the first time 18 years ago and my guess is that it has not run well since(see my post from last week about my 300 SDL autopsy). I think that the cracking head was a dormant condition that went way back but just happened. The oil was clean but the radiator hose was hard as a rock upon starting. It didn't miss at higher RPMs because the cooling system was pressurized to the point that the cracked cylinders couldnt leak enough to screw up combustion. The header tank was black from soot.

I opted for a new "22" head from Metric because the car is worth fixing. In my case I needed 2 exhaust valves, 6 intake valves and all new prechambers in addition to the head for a total of just over $2,500. Add in 6 new glow plugs, a head gasket set, new tappets and a new style (non exploding) vacuum pump and that gets you up to the $3,000 that Hatterasguy is talking about. The springs were all OK and the injectors pop tested out fine.

What you should do is remove the head and look for cracks between the prechamber and exhaust valve. The cracks on mine were almost invisible but they were there. any pitting in the valves and in the ridge area indicates fluid leaks into the combustion chamber. With the head off, you can evaluate the cylinders and camshaft for wear. In my case the cylinders still had the factory crosshatch on the thrust side, the cam was beautiful and the chain had no stretch which goes along with what most mechanics say about the durability of the bottom end of Mercedes diesel engines.

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL