Thermostat Troubles? Here’s How to Replace Your Car’s Engine Thermostat

Cooling System

By Rodney (Mechanics)

Overheating the engine is one of the most common problems that can happen if you have a defective thermostat. The thermostat itself is a component that regulates how coolant flows from the radiator to other parts of the engine. Normally, you can locate the thermostat in a housing where the engine is connected to the radiator hose.

There will be a buildup of heat in the engine if the thermostat is experiencing failure and has difficulty opening. This condition will lead the engine to overheat. In turn, you might have a problem with the coolant seeping out if the radiator boils over. This can also result in a damaged condition on the head gasket and other components inside the engine.

Engine Thermostat

Meanwhile, suppose the thermostat has difficulty closing. In that case, you will be able to notice that your engine will also have difficulty maintaining a proper warm-up, especially the first time you turn on the engine in the morning. This condition will lead to the inability of your engine to operate at normal temperature. One of the results is reduced fuel economy and worn-out conditions on some engine components.

How the Thermostat Works

The thermostat consists of a non-corrosive brass tube filled with wax that changes its properties when heat is applied. The hot temperature will expand the wax inside the tube, opening the thermostat valve and allowing the coolant to reach the thermostat. The thermostat unit will have its operating temperature stamped on the body. Normally, recent thermostats are calibrated to perform the opening mechanism at 195 to 200 degrees Fahrenheit. When the unit reaches a temperature above 200 degrees Fahrenheit, the thermostat will be fully open to allow maximum flow. The unit should then maintain a temperature point between 200 to 230 degrees Fahrenheit.

When the engine is running, the thermostat will perform in a normally open condition, maintaining the normal operating temperature by measuring the coolant temperature that circulates from the engine to the radiator. This mechanism ensures the vehicle has optimal fuel economy and lower emissions. Meanwhile, the thermostat will temporarily close the unit when the engine is idling. This is because the engine temperature will be lower, and the coolant flow needs to be restricted so the engine temperature can rise to the optimal temperature range.

Also, after the engine is turned off and the temperature has gone down, the wax inside the thermostat will contract, allowing the mechanism to close the thermostat valve. This will ensure the engine and the thermostat are ready for the next drive cycle.

How The Thermostat Works

Electronically Controlled Thermostats

On some newer vehicles, the thermostat also has a feature that can control the unit electronically by using the engine computer or control module for the thermostat. Aside from the non-corrosive tube filled with wax, the thermostat will also have a small electric heater element on the electronically-controlled unit. You might also notice a small electrical connector to the housing of the thermostat unit. This mechanism will also use a coolant sensor to monitor the engine temperature.

The thermostat with an electronic controller inside the unit will stay closed to ensure the engine can warm up more quickly. Normally, the unit will try to maintain an optimum engine temperature between 220 to 230°F. When the engine temperature reaches a higher number than the optimum range, the heater element will be energized, forcing the thermostat to perform the opening mechanism. This will lead to the coolant flowing again between the engine and the radiator. This method is more beneficial to the engine temperature since it can control the better temperature in various driving conditions. Besides, the engine will get faster to warm up, and the fuel economy will rise while the emissions will be lower.

Some new vehicles do not use a thermostat to manage the engine temperature. For instance, the 2020 Silverado and Sierra from General Motors lack a thermostat. To regulate the coolant temperature in the engine, the PCM is used by those vehicles to split the coolant flow between the oil cooler, heater, and radiator. The mechanism also employs a second rotary valve that will redirect the coolant to the heater when the engine needs warming up during cold weather.

The PCM will then activate the coolant sensors to monitor the temperature inside the engine. It will also use an electric water pump to control the coolant flow. This system is a complicated and sophisticated mechanism to optimize the coolant flow that matches various vehicles’ operating conditions. Other automakers are starting to use the same setup similar to this mechanism, and more applications are likely to use it.

Diagnosing a Bad Thermostat

Older thermostats have a bigger chance of being defective or damaged since thermostats have a limited range of service life.

If you feel that your engine has frequently been overheating, you can check the condition of the thermostat by manually feeling the temperature of the upper radiator hose. You can do this after turning the engine on and letting it run for 5 to 10 minutes. If you cannot feel the hot temperature on the upper radiator hose, you can be sure the thermostat is blocking the coolant flow.

The possible cause for this condition is that the thermostat is experiencing failure and will need to be replaced. But, it would help if you also suspected that the condition occurs because of a low level of your coolant. A bad water pump or trapped air bubble under the thermostat can also be the culprit for this condition. You can start troubleshooting this problem by checking the coolant level and other connected components.

You can test the condition of your thermostat by removing it from the engine and carefully dropping it into a bucket of hot water. You should notice that the thermostat unit will open the machine when submerged in hot water. The unit should also be fully closed after removing it from the hot water. If the mechanism fails this test, you must replace the thermostat unit.

You can also use the help of an ohmmeter to check the integrity of the heater element if your vehicle is equipped with an electric thermostat. The first thing you need to do is disconnect the heater connector and connect the pins of the ohmmeter to the thermostat. If the heater element is in good condition, you will be able to notice some resistance on the ohmmeter. Refer to your vehicle’s manual to know the exact specifications for your vehicle. You must replace the thermostat unit if the resistance number is outside the normal specification. You should also know that the heater element is shorted if the ohmmeter shows no resistance. Meanwhile, an infinite resistance number will indicate that the heater element has been opened or burned out.

NOTE: Excessive heat can also damage the non-corrosive tube components filled with wax elements. It is recommended to replace the thermostat unit if your engine has been experiencing frequent overheating recently.

If you notice that the heater is not giving out enough warmth during cold weather conditions, you should suspect that the thermostat is stuck open. Another symptom of this condition is that the temperature gauge indicates that your engine is very slow to reach a normal operating temperature. This condition might be caused by a defective wax element or a bad condition on the thermostat return spring. The only fix for this condition is usually a replacement.

Meanwhile, if you frequently use your vehicle with high mileage, you should also replace the thermostat for preventive maintenance. A good thermostat condition will reduce the risk of engine overheating from thermostat failure. While replacing other cooling systems components, such as the water pump or the radiator hose, you should also perform a thermostat replacement. Other reasons to replace the thermostat include engine rebuilding, head gasket replacement, or when the engine is older than ten.

Choosing a Replacement Thermostat

The main thing to notice when you want to replace the thermostat with a new one is to ensure it has a similar temperature rating. Suppose your vehicle is used, and somebody else has replaced the original thermostat with a new one with a different temperature rating. In that case, you should ask a certified mechanic to recommend a thermostat with the same temperature rating as the original for the vehicle.

Name-brand thermostats are also better to be used as replacements compared to cheap ones. Cheap thermostats may not perform the opening and closing mechanism at optimal temperatures. You might also need to replace your thermostats more quicker than the name-brand ones.

A thermostat with a colder temperature rating may be required on some vehicles requiring high performance, such as muscle cars with highly modified engines. This is because the thermostat will be able to prevent the engine from reaching a higher temperature. But, using a thermostat with a colder temperature rating is not recommended on most general-use cars and other vehicles equipped with computer-controlled engines. This is because this type of thermostat will not contribute to colder engine temperatures and might prevent the optimal operation of the engine control and emission systems.

Meanwhile, a thermostat with a slightly hotter temperature rating might be optimal to use in very cold climates. This will be able to increase the heater output for better comfort. But, the user should only be done during very cold weather conditions, or the thermostat will contribute to a hotter engine temperature. An improper application can also cause the engine to burn out.

WARNING! Never run your engine if the thermostat has not been installed properly. The engine will need a coolant flow to manage the proper temperature for different components. Without a thermostat, the system cannot regulate the temperature. You will also risk your cylinder head getting cracked and damaged if you have an uneven cooling system inside the engine. Other conditions that might come from this include damage to your head gasket and moisture buildup in the crankcase. The engine will also have difficulty getting significant heat if you don’t have a thermostat during cold weather.

Install The Thermostat Correctly

You should also not remove a good thermostat if you notice that your vehicle is experiencing frequent overheating during hot weather conditions. This is because the condition might not be caused by the thermostat performance but by other conditions related to the cooling system. For instance, the overheating might result from an obstructed or clogged radiator or an improper cooling fan operation. Weak water pumps can also be the reason for this condition. Meanwhile, suppose your vehicle has a modified engine that gives you a high horsepower. In that case, the overheating might be caused by other components, and you might need to get a thicker radiator to reduce the effect of extra heat coming from your modified engine.

You will also notice that different heat ratings will apply to various replacement thermostats. For instance, thermostats for older vehicles that manage hot weather driving conditions should use replacement units with heat ratings between 160 to 165 degrees Fahrenheit. But, you cannot use this type of thermostat on newer vehicles since you must calibrate the cooling system for heat ratings between 195 to 200°F. The engine cannot get into a “closed-loop” operation if the cooling system cannot manage a proper temperature. As a result, the engine will experience worse emissions, lower fuel economy, and bad performance.

How to Replace the Thermostat

Replacing the thermostat is relatively easy, and you will only need a wrench, pliers, screwdrivers, sandpaper, a bucket, and a funnel. A new thermostat and gasket are required to replace the parts. You will use the gasket on the housing of the thermostat. You can also get a new antifreeze product to replace the old antifreeze simultaneously. If your vehicle has high mileage, you might need to replace the thermostat housing if it has been corroded.

Thermostat Housing

  1. Place your bucket under the radiator drain plug and then open the plug to drain some amount of coolant from the engine cooling system. You will not need to get all the coolant liquid out of the engine completely. If you want to change the antifreeze, you must do the drainage completely. If you only need to replace the cooling, ensure the coolant level is drained until the liquid is below the thermostat.
  2. Find the thermostat’s housing inside the engine, which will normally be near the upper radiator hose. You can then remove the bolts and get the housing freed from its place. You might need to tap the component lightly to help you unstuck it. You will not need to disconnect the radiator hose for the installation if you can remove the component easily.
  3. Make sure that you remember the thermostat location inside the housing. Proceed by removing the thermostat unit. You should also replace the housing if the component has been badly corrupted or damaged.

Older vehicles with old thermostat housing will need to replace the component since there is a big chance that the housing has been corroded.

Replace Thermostat Housing

  1. Close the engine opening to prevent debris from falling inside the cooling system. You can then use sandpaper to scrape the old gasket material from the engine mating surfaces and the housing flange. Make sure that the surfaces are clean and flat before you proceed to install the new gasket.
  2. Put the new thermostat inside the housing with the correct positioning. Ensure the thermostat is installed correctly because an improper installation will cause your engine to overheat since the unit will not open correctly.

Thermosta Vent Hole

Some thermostats might also have a bleed hole preventing air bubbles from forming under the thermostat unit. The existence of the bleed hole will also make it easier for you to refill the engine cooling system.

  1. Get a gasket sealer and apply a coat of that material after you install the new gasket. Some gasket units have backings with an adhesive material that can hold their correct position.
  2. Double-check the positioning of the thermostat before tightening the unit down using the housing bolts. Ensure that the bolts are not tightened too much because breaking the aluminum housing from improper installations is possible.
  3. The next part is to check the condition of the radiator hose, clamps, and other components related to the cooling system. You should also check if there are any physical damages to these components. Replacements are required for components that are damaged or have been used for more than ten years. This is to reduce the risk of a hose failure in the future.
  4. Fill the engine with a new coolant liquid to refill the cooling system. You can also use a 50-50 mixture of water and antifreeze for this step. Ensure you refer to the vehicle’s manual for information about the recommended coolant.
  5. You can turn your engine on and let it run in idle condition for a few minutes. This will open the new thermostat and push the coolant to flow in the engine. Check the coolant level and add some more to maintain a full level. Put the radiator cap back on and check for leaks in the cooling system.
  6. Drive your vehicle for a while, and then turn the engine off to let it cool down. Recheck the level of the coolant liquid and add some more if required. You do not need to overfill the coolant. Instead, maintain a certain level between the FULL and LOW marks.