You could have a bad thermostat. These days, with parts made who knows where by anyone, quality is going to hell fast, so bad ones new in the box are not unheard of. I recommend you stick with Behr or Wahler only, get it at a good independent, the dealer, or here at FastLane. DO NOT buy one at AutoZone, etc, they usually are junk.

Yes, the coolant tank can leak, the plastic gets old. Check the hoses for pressure after it gets warm on the highway. Wrap a rag around your hand so you won't burn yourself, and squeeze the upper radiator hose. It should be quite firm. If it smashes down easily with sounds of moving coolant from the tank, you have a pressure leak. Usually have coolant loss on stopping with this problem, though.

I would also check for a blown head gasket. Does the coolant tank hold pressure cold? If so, pretty good chance the head gasket is leaking, and this will cause overheating on the highway.

So will dragging brakes, though.

I suppose it's possible to put the fan on backwards, but this would cause overheating all the time.

Peter

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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!