OM603

Launched in 1985 as the six of the OM601/602 family and developed out of the OM617, it kept prechamber injection, a Bosch M inline pump and hydraulic tappets, and only turbocharged versions were sold in the United States. Tightening American emissions rules brought a trap oxidizer — an early particulate filter, mounted at the cylinder head rather than further down the exhaust. Heat radiating off it cracked the aluminum heads of the 1986 and 1987 cars, and debris shed by the trap could destroy the turbocharger. Daimler-Benz removed the traps free of charge and replaced ruined turbochargers, then withdrew the six-cylinder 300 D from the market after 1987.

The heads were suspect anyway: the original number 14 castings would crack if the engine was ever allowed to overheat, and the later 17 and 22 molds are regarded as the cure. Then the 3.5-liter 603.97 of 1990, fitted to the 350 SD and SDL, produced trouble of its own. Owners reported head-gasket erosion feeding oil into the first cylinder, and connecting rods that bent and wore bores oval in a block with too little material to sleeve. There was never a formal recall; Mercedes-Benz replaced engines under warranty, sometimes after the term had expired. The three-liter carried on more quietly in the W124 300 D and the W140 S 350.