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.

The Territory has a yellow check-engine light just on the outside of the tachometer’s face, around the 2000rpm mark. Perhaps that’s what you saw flickering on. This is probably nothing to worry about as the light immediately switched itself off, suggesting that whatever the problem was, it was a fleeting one.

This light is linked to the car’s onboard diagnostic system which monitors the entire car for faults and then logs them for future problem-shooting. If a problem occurs twice within a certain time-frame, then the check-engine light must illuminate to warn the driver to have the car checked and the problem fixed.

If the light reappears and stay lit up, then you need to act to have the problem found and fixed before it has the chance to do any permanent damage to the car’s driveline. As a side-note, a car with an illuminated check-engine light is technically unroadworthy.

Any time you have oil mixing with coolant, you have a pretty major problem on your hands. In the case of your Territory, it could be that the head gasket has failed, allowing the two fluids to mix, creating the milky murk known in the trade as a 'milkshake'. It’s a bit unlikely but it could be even more serious and your engine may be suffering from a cracked cylinder head or a split cylinder liner. A blown head gasket is a lot more likely, though.

But just as probable is a problem that occurs pretty frequently with this make and model. The Territory’s transmission is cooled via a heat-exchanger that is cooled by the engine’s coolant. If the plumbing on this cooler fails (usually because of friction due to junk inside the tubing) then the engine coolant and transmission fluid can mix and also form a milkshake which you’ll see in the radiator or coolant catch-tank.

Unfortunately, if this is the case, the transmission’s computer and its hardware may also be damaged beyond repair by the coolant diluting the transmission fluid and generally causing havoc inside the transmission.

The best advice is to not drive the vehicle any farther and have it diagnosed by a workshop to see exactly what has happened.