I bought a 97 Discovery SE7 last year. It ran well. When winter rolled around, I realized that the heater pumped out warm air at best. I replaced the thermostat, which was stuck in the open position, after which the truck began over heating. I did some research, which I should have done ahead of time, and then bled the cooling system to rid it of air. The truck starts immediately every time and idles well, but eventually over heats. Have I destroyed this wonderful vehicle?

 

I'd want to get a test to see if you have exhaust in the coolant and make extra sure that the system is properly and completely free of air

 

If you've changed the t-stat and your sure the t-stat is opening you have a couple more things to check.

Viscous coupling (fan clutch), it lives between the water pump and cooling fan, probably shot as most are that haven't had attention. Motor will overheat at idle once stat is open. To check the viscous leave motor off and cold and spin the fan by hand, if it free wheels easily it's toast. The fan should resist, not even a quarter turn when cold.

The expansion tank. If you still have the old black expansion tank they're known weakness is the seams, they split. It's very common. The new improved are white and I highly recommend you change yours if in fact yours is black. Check the expansion tank cap too while your there. If it looks old just replace it, it's cheap.

Hoses, check that you don't have a coolant hose leak. Give the hose clamps a double check, give em a twist if you would.

If you don't rectify your overheating it always leads to head gasket failure, always.