Up to 86 the 300SDL has a five cylinder engine (617). 86-87 have the six cylinder (603)
And replace your radiator cap before you take the head off! They go bad on these cars and either won't hold pressure, hence boil under heavy load (going up steep, long hills, etc) or blow under heat and won't vent, leading to collapsed heater hoses. I've seen two bad ones in the last couple months out of three 87/88 cars -- my 300E (wouldn't hold pressure) and a friends -- collapsed radiator hoses.
Mine only caused trouble if the AC was on and I stopped after a "hot" run on the highway. It would spew coolant until it cooled off.
If there is much pressure in the coolant tank (it hisses or burps hard when you open it cold) you have a bad head gasket.
Hans has been replacing a lot of them recently.
Much cheaper than a head gasket! And in the current state of affairs, some MB dealers will do a head job for coolant loss when the only real problem is a bad pressure cap, sad to say. Make lots more money that way...
If you need a new pressure cap, get a "real" MB one, not one at auto zone or PepBoys, etc. Some aftermarkets are OK, but I'd rather spend the extra five or so dollars, myself.
I definitely have a bad head gasket/head on my "new" 87 300DT sedan -- excess pressure in the coolant reservior and oil, too......! Probably severely overheated as it has a brand new rad in it.....!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!