Engine cooling problems are often the fault of the lowly thermostat, which is a simple device that resides between the radiator and the engine block to regulate when engine coolant is circulated. When something goes wrong with the thermostat, it can either become stuck open or closed, restricting or allowing full flow of the coolant whether conditions merit that or not.
Recently in our forums, a return user asked about his engine cooling problem - his water temperature gauge stopped working - and what could be done to fix it. His specific issue had two likely causes: either his gauge was broken or his thermostat was stuck open. Since gauges are expensive to replace and not easily accessed for testing, testing his thermostat was the simpler, faster option to find a diagnosis.
The mantra of the DIY mechanic should always be: test the simple stuff first.
Before checking thermostats, of course, check fluid level and condition (an empty radiator means you have no coolant in the system. That will be your problem. Also be sure that your water pump is not leaking by simply looking for coolant leaks around the pump housing or (if so positioned) the timing belt covers.
Testing the thermostat is pretty straight forward and there are several ways to do it. The first requires a fair amount of time and patience, but doesn't require thermostat removal. The second requires thermostat removal and is far more likely to tell you whether or not it is working. Which you use will depend on your level of skill and access to tools.
Testing With a Thermometer, No Disassembly
To test the thermostat without taking it out of the vehicle or draining the fluid, you'll need a reliable thermometer - preferably a mechanic's temperature gauge or touch-sensitive thermo device. Most professional mechanics will have one, but most amateurs do not. If you do not have one, you can either purchase one or use the next method instead.
Place your thermometer against the metal near the thermostat housing (which will either be below the water pump or up near the heads, depending on make and model) and record the temperature of the housing when the vehicle is "cold" (not running, not hot). Do the same on the hose leading into the thermostat. Do another at the opposite side of the radiator (top of engine or bottom of engine where the hose from the radiator connects), taking the reading on the radiator hose itself (not engine) and record that temperature. You should have three temperatures documented.
Start the engine and let it idle. Take a reading from the same three points every five minutes. Do this for about fifteen minutes (four tests, including one without engine running). Each reading should be steadily warmer than the last.
The first reading, on the engine block, should go up steadily with each check as the engine warms. This is your control reading - the reading of the engine itself. The second reading, before the thermostat housing, is getting the coolant temperature before it's sent into the engine to cool it down, so it should be cold until the engine reaches its normal operating temperature. The third reading is reading coolant that is freely flowing from the engine into the radiator without restriction - it won't be "flowing" per se, at this point, but it is being heated by the engine through induction. This reading should raise at about the same rate as the engine is heating (first reading). At the fourth (last) reading, your engine should be near or at normal operating temperature and the readings should be roughly the same. If not, allow one more test (five more minutes) and see if they are.
If the readings on the final check are out of whack - still cool in the radiator - then you probably have a thermostat that is stuck closed. If your engine has been overheating or threatening to do so, this is likely your problem.
On the other hand, if the three readings are constant - all rising at the same time, even when the engine is cold - then your thermostat is probably stuck open, allowing free flow of coolant at all times.
Testing the Thermostat After Removal
A faster test that requires no special tools beyond the wrenches required to remove the thermostat can quickly test whether or not it's stuck open or closed. Simply remove the thermostat per your vehicle's configuration (some require draining of the coolant, others do not). Refer to your shop manual for details. Never do this while the engine is hot!
Once the thermostat is out, look it over. If it's cold and you can see daylight through it (it's open), then your thermostat is stuck open. No need for further testing. If it's not open, boil some water on the stove.
Once the water is boiling, drop the thermostat into the pot and allow it to "cook" (heat up) in the water for two or three minutes. Using a pair of pliers, carefully remove it from the water and check to see if it's opened. If not, you have a stuck thermostat. If it is open, your thermostat is not likely to be your problem.
Hopefully these test methods will help you determine whether or not a thermostat is the cause of your worries.