I've sorted through lots of postings regarding overheating trying to find out what to try with my 2000 Discovery. Last winter it overheated a couple times slightly, after a long highway drive, but was fine once I started up again. I wasn't sure what to make of it, but the vehicle gave me no problem during following summer.
Lately, seemingly with colder weather, I had the trouble again. I drove 150 miles, mostly highway, without a problem, then it overheated. The next day I stuck something in the thermostat to hold it open, and drove home without a problem. I had to cover up the radiator to get the thing to heat up at all.
-replaced the thermostat, and have driven around for days without a problem, then all of a sudden it will overheat. Sometimes if I just stop for a minute and start again, the temp will go right down. One night I had to prop open the NEW thermostat. Had no problem when I did that. Took out the bolt I'd wedged in there and problem returns.
-I have tried really hard to follow all the procedures regarding bleeding the system and getting air out, though I'm not really sure I've been successful. Each time it overheats a bit I seem to get more air. I've been using the right coolant.

These factors confuse me:
-problems in cold weather, summer was OK (so I think radiator is fine?)
-more trouble in city driving than highway
-fan seems to work normally
-the engine seems to cool great when thermostat (old and new) are bypassed, both highway and around town.

I have wondered if I was getting exhaust gas in the system, but I'd have thought that would be worse on the highway, and happen regardless of the thermostat situation. I also wondered if it's possible that the pump cavitates. Anyone hear of that happening? That might explain why it sometimes cools down immediately after restarting. A couple times I tried rapping on the hot thermostat and wiggling hoses. I'm not sure, but I think it might have helped. Is there any way that differing pressure within the thermostat can keep it from working? Both old and new really seem to close very tightly. I've debated drilling a small hole through to make sure no pressure differential exists, and to help allow trapped air to circulate.

I'm at my wits end, and would appreciate any other ideas. The dealer doesn't seem all that helpful, and I can't afford them right now anyway.

 

Send me your tele number and I'll call and walk you thru this.

 

Sounds like a bad water pump. The impeller is all chewed up and at slower engine speeds it cant move enough coolant to cool the engine.

 

I had a old k5 Chevy that didn't quit do that but had over heating issues and it was gunk in my system sheets of build up. I would open a hose up and check for chunks maybe your thermostat is getting clogged and you just need a good flush!

 

Did your find out what was wrong with your Disco. I'm having the same issue and awaiting your response. Thanks.