An often talked about subject in the NC community, especially in Autumn/Winter.
They tend to last roughly about 40k miles, that’s very, very roughly as there is no exact point where they fail, the rubber seals distort, go hard or shrink and begin to pass a bit of water when they shouldn’t and gradually get worse over time.
I think most people realise that the ‘stat is an important part of the engine, it ensures it gets up to temperature quickly and that is stays at that ideal (minimum) temp too. Don’t ever think a cool engine is a happy engine, it is certainly not – but then neither is an overheated one either. Engines are happy at around 85 to 95’c. This where the oil viscosity is at its best and economy is good due to ideal fuel burn characteristics. Engine oil contains contaminants such as moisture, fuel and other byproducts, when the oil gets hot they evaporate off and are sucked out. This is one of the reasons why cars which are only used for short journeys need their oil changing more regularly.
Deciding whether your car’s ‘stat is faulty is relatively simple, even though the temperature gauge (as previously discussed) isn’t very accurate and in warmer weather we don’t use the heater so it often only gets diagnosed in Autumn and Winter.
Using an OBD reader which measures the real temperature will help, but also take a look at your gauge before you start the engine for the first time that day, the needle should move from there within 3 (Summer) to 5 (Winter) minutes of driving it. When the engine has been running for at least 10 mins the heater should be blowing warm, after 20 mins fully hot, if you’re driving down the motorway on a cold day with the heater on and it starts to blow a bit cooler than it was then that’s a sure sign it’s failing. Don’t perform any tests with it idling.
Changing it is fiddly, but easy enough with the correct tools. There were two types, long hose fitting (later models) and short hose fitting (early models), it doesn’t matter which you buy, they both fit as the rubber hoses move to take up the difference. We sell high quality Mahle brand for £20.