Date registered: Aug 2009
Vehicle: 1986 300SDL (2) 1981 300SD
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Mid 1980's 300 turbo and 350 turbo
Two things I learned after just buying a 1986 300 SDL about this engine and the 350:
300: Probably most people know that the heads can warp or crack when the engine is overheated. There is a production code of "14" on the head itself on digits 7&8 on the original heads. Replacement heads fix this problem and can be as high as "22" in the same digits. Basically, if the car overheats, pull it over as immediately as possible. Alternatives would be to put the heater on full fan and power back the engine as much as possible.
350: These sometimes have a problem of oil consumption and the cylinders "ovalling" which is a very bad situation. The dealer I bought my 300 from told me this (AFTER I bought the car). The original 350 engine had a head gasket that can compress. I don't know how often this occurs. I am told from rarely up to 50% of the 350 engines. If this happens there is a clearance problem. Then there is excessive oil consumption, rods bend and the cylinder ovalling occurs. I don't know if the order of these event is correct.
To fix this there is a replacement gasket which fixes the problem altogether. Cars that have had this done have a red dot on the head. I don't know how much this costs, but a lot cheaper than the alternative. If you have a car with the 350 engine without the red dot, either get the gasket replaced or watch the oil comsumption carefully. If it starts eating oil, change the gasket right away. Maybe it is better to just get the gasket replaced anyways.