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What year? There were a lot of problems with 3.5 engines - most of them have been rebuilt and/or replaced by now. Extreme oil usage and bent rods were the main problems. In general, Mercedes diesels are amazingly solid. I recently sold my '87 300TDT that was just about ready to turn 500,000 miles on it - and the engine was still rock solid - without anything other than normal maintenance on my part over the years.
