The fan does come on at 100C. I don't have a coolant tank on the top of the wheel well, I just have a cap on the rad. I just replaced the rad cap with a new one last week.

The fan clutch is engaged, it has resistance when hot and when cold. It stops spinning as soon as the motor stops running, it doesn't continue to spin.

__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

Oops, my bad, was thinking of a W124.

The McParts stores do sell aftermarket coolant recovery tanks, and you could still fit one on your car.

Did you verify that the cap had a good seal and was holding the spec pressure? Lots of McPart stores have a tester for the cap.



How does the temp gage look when running down the freeway?

If temp rises when rolling at higher speeds, it's the radiator.
Low/no speed overheating is either circulation or airflow.

If high speed cooling is not an issue, and your fan clutch checks out, and you verified the aux is blowing when turned on, then that leaves circulation.

Circulation issues can be the T-stat, water pump, or clogged water passages.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles