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I recently changed a leaking thermostat housing and thermostat on my 2l petrol 2000 mk2 mondeo. Now I find that at idle it is fine, I can leave it idling and the fan cuts in an out, and the temp stays about the quarter mark where is usually is... all is well. Driving in backstreets at about 50k's it is fine. However if I'm driving at 80k's or above for any more than a few minutes, it will overheat when I come to a stop. I have bled the air out of the system, and performed an engine leak down test, and a pressure test on the radiator, which showed no leaks to or from the cooling system. Going to flush the radiator tonight, any words of wisdom on other things to check while trying to nut this one out as I've never had a car that only overheats upon stopping after travelling at a high speed.

  • Welcome to the site. When the car overheats at idle, does the fan come on? – Zaid Mar 6 '16 at 11:10
  • And how's the water pump? – Zaid Mar 6 '16 at 11:21
  • The fan will cut in and out every few minutes at idle, but I don't think it cut in straight away after travelling at higher speeds then stopping. – user15528 Mar 6 '16 at 11:24
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If you're confident that the coolant lines are properly bled and the fan is running as it should, overheating under high loads is usually indicative of insufficient coolant flow.

I would have the water pump checked out to make sure that it is still capable of delivering the required flow rate at higher loads.

  • It doesn't overheat while travelling at high speeds, only when I come to a stop. I did notice that the hose from the thermostat housing to the radiator seemed to have more pressure than the hose from the radiator to the water pump & expansion tank had. Is there a way to test my water pump in a car that doesn't have a cap on the radiator? – user15528 Mar 6 '16 at 11:30
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So I gave it a flush and changed the water pump and thermostat. The old water pump looked fine, but the thermostat (only a few months old) failed at test. Couldn't get it to overheat last night, so it would seem at this point that the problem is solved. Cheers for the advices :)

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