The OM603 engine was a straight-6 Diesel automobile engine from Mercedes-Benz used from 1984 through 1999. The 603 saw limited use in the W124, W126 and W140 model vehicles.
The 603.96 engine has a capacity of 2996cc and was an engineering marvel in pre-chamber design and a technological leap forward from the earlier 5 cylinder OM617. Versions 603.96x and 603.97x are turbocharged. Only turbocharged models of the 603 series were available to the U.S. market. The camshafts and injection pump are driven by duplex chain from the crankshaft. A separate single row chain drives the oil pump. The camshaft operated the valves via hydraulic bucket tappets; valve clearance adjustment is automatic.
Fuel is injected into a pre-combustion chamber. A Bosch M in-line injection pump is used, with a mechanical governor and vacuum-operated stop control. The pump is lubricated by a connection to the engine oil circulation and the fuel lift pump is mounted on the side of the injection pump. Preheating is by glow plugs with automatic control of preheating time.
The 603 engine was notable for several reasons: The emissions controls imposed in the US market led to the creation of a diesel particulate filter, otherwise known as a trap oxidizer. As these were mounted at the cylinder head (modern traps are mounted further away), heat from these trap oxidizers caused failure of the aluminum cylinder heads on the first generation of 603-engined vehicles; debris from the traps could also damage the turbocharger. This first version was sold in the US from 1986 to 1987. Daimler-Benz removed these traps for free, and if the turbocharger had been determined to be damaged it was also replaced. Even without the heat from the trap oxidizers, the original #14 mold cylinder heads were weak and if overheated could crack as with any overheated engine. In general, the later model #18 or #22 mold cylinder heads are considered to be the definitive cure for cracked heads.
In 1990 the 350SD/SDL debuted, using a larger-displacement 3496cc OM603.97 engine that had more torque and a lower top RPM. The engine lived on in the W140 chassis after the W126 production ended, as the 300SD or S350, with a larger yet turbocharger and thus more power and torque. By the time of the 3.5L engine, the cylinder head issues of early 3.0L engine (US 1986-1987) had been corrected. However in the 3.5L there exists a different situation that appeared on some engines; eventual head gasket erosion, and thus passage of oil into the #1 cylinder. As the 3.0L engine uses the same head oil passage design, yet does not appear to exhibit the problem - it might be that the larger bore in the 3.5 engine, the higher pressures, and resulting smaller head gasket surface area could conspire to cause a gasket erosion issue. Elevated oil consumption is an early warning of imminent problems. Some anecdotal sources suggest that the connecting rods are weak. With a bent rod comes ovaling of the bore as the 3.5L block does not have sufficient bore material to allow for sleeving. If one catches the problem before it has progressed too far (after elevated oil consumption starts, but before the bent rod occurs) the issue can be alleviated with a head gasket replacement. Generally these problems seemed to appear more in engines before they reached 75,000 miles of service than they did in engines after 75,000 miles of service.
A secondary problem with either the 3.5L (though true for any of the OM60x family diesels) is lack of maintenance of the motor mounts, which results in hard engine vibration from the engine resting on the frame members. Especially in the OM603, the vibration helps loosen one of the myriad of small screws in the crankcase which primarily hold on the windage tray. If these small bolts and screws find their way into the oil pump or passages, the resulting oil starvation can cause bearing failure, and subsequent broken or thrown rods—often cracking the block. Proper motor mount maintenance eliminates these issues.
The result of both these issues was that some 603.97x engined cars had engine problems.While never formally recalled, Daimler-Benz replaced some engines under warranty, even somewhat after the original warranty term expired (at least for the original owner).