Mercedes Benz 3.5 Liter Turbodiesel Problem and Troubleshooting PDF Manual

Briefly, this engine has problems. With distressing frequency these engines show significant, grossly premature wear, requiring expensive rebuilding. Symptoms are excessive oil consumption, mechanical noise, smoke, and a throbbing idle. Diagnostic procedures usually reveal that one or more cylinders have compression that is below specification, with excessive variation in the compression among all cylinders. Mechanical teardown frequently reveals cylinders that are out of round and connecting rods that are bent.

Catastrophic engine failure has occurred quite often. These problems are clearly due to a design defect that Mercedes Benz refuses to acknowledge. These problems can not be prevented with any sort of routine maintenance, nor can they be forestalled with changes in driving behavior. In essence, the engine self-destructs during normal operation. Symptoms have appeared as early as 50,000 miles. Due to the expense of individual repairs, Mercedes honors warranty repairs only after considerable pressure is applied. Out-of-warranty claims are summarily rejected.

THE ENGINE The problem engine is a 3,449 cubic centimeter displacement in-line 6-cylinder turbo-charged diesel engine. Diesel engines were first used in the W-116 300SDL S-Class beginning in 1978, continuing through 1980. That engine was a 3 liter 5 cylinder turbodiesel, model 617. The W-126 S-Class debuted in 1981 with that same engine, which was changed to a 3.0 litre 6 cylinder (the 603.96x) in 1986-87. This same 3.0 liter engine was also used in the W-124 300D and 300TD in model year 1987. These engines are NOT the problem. That engine block and head, however, were the starting point for the 3.5 liter version. For comparison purposes it will be helpful to know how many of these 6 cylinder 3.0 liter versions were in use……

The 3.0 liter 6-cylinder 603.96x engine was an extremely reliable engine during its production
run. The problem-plagued 3.5 liter 603.97x engine is based on the 3.0 liter version and uses essentially the same head and the same block. What went wrong? For whatever reason, Mercedes decided that beginning in model year 1990 the S-Class needed a turbo-diesel engine with greater displacement than the 3.0 liter version that had been used in 1986-1987. Rather than develop a new engine, a decision was made to increase the displacement of the existing engine. Three factors determine displacement: number of cylinders, bore, and stroke. Since
the 603 block already had 6 cylinders, (the 617 had 5) that left an increase in bore and /or stroke as the only way to increase displacement.

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