My '91 350SDL w/ 126Kmiles is running hot--radiator, water pump, and thermostat replaced in 6/04. Temp gage reads as follows after warming up with ambient temperature in the mid-90's:
20 -50 mph = 105C/200F
60 mph = 110C/208F
75 mph = 115C/216F
Otherwise, engine is strong and smooth running with no unusual oil usage.

 

Get infra-red thermometer and check for the cold spots on the cooling system.
My bet is radiator.

 

yep!

 

While the radiator may have blockage, I would also be concerned about the thermostat, the auxiliary fans, auxiliary fan thermo switch, and fan clutch. All of these cooling system components are subject to fail at some point in your engine's history.

Restricted flow through the radiator, or a failed thermostat, sound to be more likely based on your symptoms.

 

This problem is still a mystery in my '91 350SDL as :
The thermostat has been changed, the year-old Behr radiator checked for flow rate (in the car), the fan clutch replaced, the injection timing is perfect, the auxiliary fans are both working and coming on when they are supposed to, no water in the oil and no oil in the water and not losing any water, soooooo.....what else might we check. The year-old water pump will be removed and checked this week and the radiator will be completely removed and sent out for a pressure check/flow rate. Should we consider a cracked head?

 

I'm sorry to say, but I'd expect a bent rod before I'd expect a cracked head in that 350 engine!

But I don't think a bent rod would cause hi temps. The heads used on this engine had been redesigned and is either a #17 head, or higher number. You would have to seriously overheat this head, unlike earlier 603 heads in order to crack it! You should try and confirm the instrument gauge some how, although I haven't heard of one that was faulty by way of giving 5 and 10c greater readings. They usually either work, or they don't work!

 

The mystery continues--radiator, water pump, t-stat all check OK. Next the compression will be checked.
To reiterate--no water in oil, no oil in water, no blue smoke, no white smoke, cooling system pressure OK,
fuel injection timing perfect, but car continues to run up to 110-115C. Hopefully the compression will be
checked tomorrow 9/30. Until then, any other suggestions?

 

What did the infra-red thermometer indicated?

 

Have you checked the temp guage? A fellow at a bmw e12 forum took a while to track down his hot running motor to a high reading guage.

 

My mechanic feels that the temp guage is accurate but if the compression is OK,
then we'll check that and do more infrared temp testing.