Thermostat swap: it’s a simple and easy DIY task.
What does it do?
As its name suggests, the thermostat in a car engine opens and closes according to temperature and regulates the flow of water around the engine. In its most basic operation it allows the engine to reach operating temperature quickly by not passing water through the engine radiator when the engine is cold. As the engine warms up, it opens gradually, allowing water to circulate through the entire system.
What goes wrong?
A traditional basic thermostat works using heat and the principle of expansion. A wax canister heats up with the coolant and pushes against a metal disc inside the unit which blocks the flow of water. A spring inside the unit allows it to fail ‘safe’ in the open position. This prevents overheating but also means that the engine will take forever to warm up. Also, in many cases the thermostat will open and close during regular runinng, especially in the British winter so if it fails the engine never reaches full operating temperature. They can also fail the other way and jam shut with the result that the engine starts running hot.
How do I know I need a new one?
If the heater doesn’t seem to be kicking out the warmth it used to, then check the temperature gauge if you have one: is it sitting slightly lower than it usually does? Also feel the radiator hoses by hand for heat.
How hard is it?
It’s one of the simpler jobs in car maintenance. In most older cars the thermostat is accessible and is located inside a housing which can be unbolted to remove the unit. You’ll lose a bit of coolant doing the job, so you’ll need something to catch it and some fresh anti-freeze to top up, but it’s a simple job in most cases.
How long will it take?
If everything goes well it really can be a five-minute fix. Even with a few problems, it’s usually a 30-minute task at most.
Any tips and tricks?
Take it easy if there’s an alloy casting – you don’t want to be snapping bolts and studs. Spray plenty of penetrating fluid on them first. And some cars like ’90s and newer BMWs, will have an electronic thermostat which interfaces with the engine management. This means you need to swap the entire plastic housing. When topping up the system, be careful not to get airlocks - the workshop manual will tell you how to bleed it.
Above left and centre: modern electronic 'stat and housing. Above right: the unit from a 4-litre Jaguar engine.
Traditional thermostat is usually to be found inside a housing with a removable cover. It's a basic DIY job.