Thermostat and temperature gauge problems in my 2011 Ford Territory

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I have recently replaced the frozen thermostat in my 2011 Ford Territory 2.7 diesel seven-seater. After 24 hours of appearing to have fixed the problem, now I have the temperature needle flying from half to the-engine-is-going-to-blow-up within 15 seconds. As soon as I spotted the gauge rising, I hit the hazard lights turned the motor off and pulled the car to the left. I left it parked for 15 minutes, topped up the water and, 15 mins later, tried again. The same thing happened. When the radiator cap is opened, the system is under pressure like its blocked somewhere. Can someone point me in the right direction?

It might be a case of super-fast overheating, but one would expect the temperature needle to take more than 15 seconds to go from normal to dangerously hot. You could have a broken or faulty gauge, a short-circuiting wire somewhere in the gauge’s wiring or even a temperature sender unit that is giving false readings to the gauge once it gets to a certain temperature.

The first thing to do is work out just how hot the engine really is getting. You can do this with an external temperature gauge or an infra-red thermometer that can be bought for a few dollars. Aim the infra-red beam at the top tank of the radiator, or the top radiator hose and see what reading you get. You should see a value of between 85 and 95 degrees Celsius. At which point, the engine wouldn’t appear to be overheating (provided your new thermostat is working properly).

By the way, the coolant being under pressure when the engine is at operating temperature is perfectly normal. It’s why you shouldn’t remove the cap on the expansion tank when the engine is hot as you can easily get burnt by the escaping coolant. Modern engines pressurise their cooling systems to actually raise the boiling point of the coolant and make their cooling systems more efficient.

Disclaimer: You acknowledge and agree that all answers are provided as a general guide only and should not be relied upon as bespoke advice. Carsguide is not liable for the accuracy of any information provided in the answers.

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