A cheap routine maintenance item to perform on the radiator is to clean the exterior cooling fins by using a water hose with a regular spray head. Remove the crowl around the engine fan to allow access with the hose and spray a jet stream of water from the back of the radiator through the cooling fins to the front grill. Don't laugh, I have a 1986 240DL that constantly overheated when the A/C was operating. After a thorough spray of water (and yes, alot of dirt & crude came out of the cooling fins) I have not had a single overheating problem. It's cheap and maybe worth a try. The worst you can end up with is a clean radiator. Good luck.

Just as a follow-up, the mechanic thought it was the temperature sending (?) unit. They put a new one in, and the guage is staying in the middle now. Hopefully it'll stay that way. Thanks to eveyone for the feedback.

How well does the heater work? Can it keep up when the fan is on high? If it doesn't stay hot there is definitely a circulation problem. Unfortunately the converse is not true. Did the mechanic position the thermostat so that the vent was at 12 o'clock?

Assuming they work properly you should be able to use the cylinder head temp sensors to verify the actual water temperature. I don't have the temp/resistance chart handy, maybe someone else has one they could post the readings you could expect.

Another test would be to supply the temperature gauge with a temperature 'standard'. By that I mean replacing the temp sending unit with a fixed resistor value for the half-way and full points and see where your gauge reads. (Sorry I don't have those values for you right now.)

The first thing you have to do is bypass the temperature compensating board. If you haven't done so already, at 12 years you're overdue for that to go bad. I'd put money on your cooling system being fine but just getting a bad reading because of the temp comp board. So do that in the first place, before you start worrying about other items that don't appear to be very likely in your case. Do a search for the instructions and helpful photo.

Until you've bypassed the temp comp board you just can't trust your gauge's readings.

If the upper rad hose is hot then the thermo is working and letting the hot water into the rad for cooling.

If the lower hose is cool, either the rad is working like it should or it is plugged. But it's not likely plugged as you've had it cleaned.

A bad clutch fan can cause overheating but that is not likely the case as you say the lower rad hose is cool.

When you turn on the heating full blast, does the temp gauge drop? If not it may be the temp compensator that's the problem. (Instructions for bypassing it are available on board.)

If the temp gauge does drop with heating on, then your car probably is overheating. Weak pump, slipping belt?

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1980 245 Canadian B21A with SU carb and M46 trans