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Jeremy5848 · #3 10-29-2012, 03:31 AM

For a 5-cylinder engine the 617 is remarkably smooth. Don't let vibration concerns keep you away from a nice 300D or 300SD with a 617 engine. I was quite happy with my '85 300D-T.

The 603 head problems have been overblown. Any engine with an alloy head can have problems if overheated. Folks would have a cooling problem and instead of immediately stopping they would try to "make it home" and crack the head. The earliest (#14) head also has some risk associated with insufficient space between oil and water passages. If you buy a car with a 603 engine, try to get one with a #17 or newer head. However I never had problems with my #14 head.

In terms of basic maintenance the only difference I remember is that the 617 engine needs regular valve adjustments while the 603 has hydraulic lifters that are self-adjusting.

In general, the 1987 300D Turbo with the OM603 engine in the W124 chassis is a far more modern car than the 1982-85 300D-T with the OM617 engine in the W123 chassis. The 124 has a driver airbag, ABS, much better suspension, and is a little quieter and smoother than the older car. By comparison, the 123 is much more the "classic" Mercedes, probably the last of which one can say that. It also drives like a tractor (said my wife).

No matter which one you buy, try for the best one you can afford. A cheap Mercedes is no bargain and you'll have many chances to regret it. The 124, a later model with more sophistication and more goodies, has more things that can break. If you can do your own work this is not such a big deal but a Mercedes is an expensive beast to have others work on for money. This is true of any Mercedes -- the money you save on the purchase price will come back to haunt you many times over. Look for a car with a complete set of service records.

Jeremy