Thermostats are very simple mechanical control devices that work until they don’t. There is usually not much in-between partial function. While they are not expensive to replace, testing one that you suspect isn’t working is a good way to narrow down a problem. If you don’t know for sure, then you are just guessing and throwing money at a problem. Thermostat testing is the best way to know for sure if this little part is doing its very big job.
A Little Background Information First
One of the most basic principles of physics is that energy is neither created nor destroyed; rather all energy is merely transferred from one type of energy to another. The most common transference of energy comes in the form of heat. Nearly every single transference of energy creates heat as a byproduct; this heat energy must be dissipated. In a vehicle, this heat is conducted to the atmosphere through the radiator cooling system.
You don’t want this to occur in an uncontrolled manner, the engine needs to be at a certain temperature to operate as efficiently as possible, for most modern engines, this is around 200 degrees. The method of controlling this temperature is the thermostat. A simple mechanical device, the thermostat is a small spring-loaded device that is about as pure as a mechanical device can be. Inside the unit is a wax pellet. As the temperature of the coolant in the engine increases, the wax melts, expanding inside the device, which pushes the plunger open, allowing the hot coolant to flow from the engine into the radiator, where the heat is exchanged with the atmosphere. As the coolant cools below the melting point of the wax, the spring on the outside of the thermostat pushes the plunger closed, reducing or stopping the flow of coolant.
Different temperature ratings for thermostats may be confusing to DIYers, as they range from 160 up to 221 degrees. What your vehicle needs depends on the original OEM specs, but can be changed due to geographic location, engine swaps, and performance upgrades. The rating listed on the thermostat is the point at which the device begins to open. This is not the full open temperature, which is typically 15-20 degrees above the listed temp. For example, a 180 degree thermostat begins to open within three degrees of 180, so 177 to 183. By 200 degrees, the thermostat is fully opened. The operation of the thermostat, open to close, is a range of about 20 degrees, so with different temp ratings, you can fine-tune the run temp of your vehicle. The colder the thermostat temp, the quicker it will open. The flip-side of that coin is that your engine wants to run in a specific range, you don’t want to just change temp settings without good reason.
So now that we know how they work, how do we test a thermostat to see if it is working? Unlike many other parts, you can’t just look at a thermostat and know if it is good or not. Thermostat testing is easy and it is kind of cool to watch, at least it is if you are a gearhead or science nerd. All you need is glass jar, some tap water, a microwave (or stove/hot plate), thermometer (not from your medicine cabinet, a diagnostic thermometer or a cooking thermometer), and a thermostat.
Start by inspecting the thermostat. If the unit is open at room temperature, then it is bad, and you need a new one. Sometimes they get stuck open, this is usually due to the wax on the inside leaking and when it hardens, it pins the unit open. Once this happens, it is junk.
Fill a jar, pan, or bowl with tap water and heat it until it is at least twenty degrees above the open temperature of the thermostat. You can do this on a hot plate or stove with the thermostat in the water, but it is best if you heat the water by itself, and NEVER put the thermostat in the microwave.
Carefully lower the thermostat into the water. Use the thermometer to monitor the temperature of the water. Watch the unit slowly open. The water needs to be at least 20 degrees hotter than the unit’s rated temp. If the unit does not open at all or does not fully open, it is bad.
Continue to monitor the thermostat as the water cools. Once the water temp is exactly 20 degrees above the rated temp, the unit will begin to close as the temp falls. Note the temp when it starts to move and when it is fully closed. If it is closed with three degrees of the rated temp, the unit is good.
Check out all the heating & cooling systems parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more thermostat testing tips and tricks, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.