Your car’s thermostat is there to keep a portion of the coolant in the engine block and cylinder head until it fully warms up. Despite the water pump turning, the coolant in your engine doesn’t actually move at first, thus helping your engine get to normal operating temperature quicker.
Once warm, the thermostat activates and opens up, and when it does, the coolant can flow freely around the rest of the cooling system, including the radiator and heater, which then brings the coolant back to a safe operating temperature.
If the thermostat fails, you’re going to be in trouble. The coolant trapped in the engine will get boiling hot, while the coolant in the rest of the system remains stone cold. Or, if it is stuck opened on a cold day, it will take miles for the motor and oil to come up to temperature, causing more wear and a chilly drive with no heat.
This inexpensive part has a simple but important job, and like any hard-working part on your car, it needs replacement from time to time. If you suspect the thermostat may be bad, by the time you get the housing off to check, it is easiest to just replace it. But how do you know if your thermostat is failing?
01 Always Running Cold
If you notice that the temperature gauge in your car seems to always read low, this isn’t a good thing. Admittedly, it’s better than the engine being too hot, but too cold means lower fuel economy, more wear, less effective heating, and higher emissions.
Typically, this is a sign that the thermostat is stuck in the open position, and as such, coolant isn’t being kept in the engine until it is warm enough. Your engine needs heat to run properly. If it’s too cold, other fluids like the oil will be thicker and won’t flow properly.
Furthermore, if the thermostat is stuck open, the heater won’t get warm for quite a while and be less effective.
What should you do?
- Remove the thermostat housing and look at the thermostat to see if it is indeed stuck open.
Solution: If the thermostat is stuck open, the only thing for it is a replacement with a new part. But once you have the housing off, replacing the part is only another minute of work.
02 Always Running Hot
Too much heat is a very bad thing for your car’s engine. It will cause the engine and the oil within it to exceed operating temperatures, making the oil thin and increasing wear. If the coolant boils and the thermostat isn’t open (depending on the engine design) it may not be able to release the steam normally and may blow out a core plug or the head gasket.
What should you do?
- Once the engine is cool, remove the thermostat housing and inspect the thermostat. To see if it operates, boil the kettle and submerge the thermostat in boiling water. Most should open at somewhere between 80 and 100C. It not, it’s junk.
Solution: The only option is to replace the thermostat.
03 Sporadic Rises and Spikes in Temperature
Is your temperature gauge all over the place? Does the gauge stay cold, then suddenly shoot up to the red zone? Or rise gradually to hot, only to suddenly drop to normal? This could be down to the thermostat sticking or being clogged with corrosion.
Proper, bona fide coolant won’t let this happen. However, if the engine is, heaven forbid, filled with just water, rust and corrosion will become an issue.
What should you do?
- Inspect the thermostat first. If it is indeed gummed up or full of corrosion, it won’t be operating correctly.
Solution: You could clean it up, but honestly, for the sake of what it will cost, you should just put a new thermostat in. Drain and flush the system to get rid of any contaminates/corrosion. Drain the existing coolant by the tap on the bottom of the radiator or by disconnecting a bottom radiator hose (see your Haynes manual). If the coolant coming out is really dirty and rusty, refill the system with water and drain it several times until it is clean. Fill the cooling system back up with the correct rated coolant for your engine, diluted properly, and follow the procedure in your Haynes manual to be sure there are no air pockets.