Holden SUV Range

If your diagnosis is correct, the problem will be with the either sensor that detects the temperature of the coolant, the computer that creates the signal to open, the wiring that carries the message or the thermostat itself. An auto electrician should be able to sort this out by testing the components one by one until the dud one reveals itself.

But there’s another possibility in cars like this one with a thermostat that lives in a plastic housing. Over time and repeated heat cycles, the plastic can degrade and fail internally. At that point, the actual thermostat mechanism can move sideways and jam against the housing, effectively preventing it from opening., At that point the engine can overheat. Don’t rule that out until the condition of the housing has been checked.

It’s certainly strange behaviour and your theory that because it works perfectly some of the time, it’s probably not terminal is a decent one. If something internal was broken or worn out, the transmission wouldn’t work at all. Which leads any mechanic to suspect you have either a gear selection problem or an electrical glitch.

Scan the car electronically to see if any fault codes are thrown up. But also check for things like bad earths or blown fuses and relays. You can’t rule out a major internal failure until you get inside the gearbox itself, but starting with these simple things can help eliminate other possibilities. You may find the ECU (the computer than controls the driveline) is the culprit, and is having random melt-downs.

You haven’t told me which engine is fitted to your car, but if it’s the turbo-diesel engine, then the cause may be a pretty simple one. This engine has a habit of splitting the plastic trunking that takes the pressurised intake air from the turbocharger to the engine’s inlet manifold. If that happens, the air is free to escape to the atmosphere, and doesn’t make it’s way into the engine.

That’s why you’ll hear a noise like air escaping (because that’s precisely what it is) as well as a lack of power, because the engine is not receiving all the boost from the turbo. You’ll probably also notice that the car is blowing more black smoke than it should.

The fix is a new piece of plastic pipe that should be well within the abilities of any workshop to fit.