When your car’s temperature gauge behaves erratically, the thermostat is a common suspect. Knowing how to check thermostat car function can save you time and money, helping you diagnose a potential problem before it leads to overheating. This guide provides clear, step-by-step methods to test your car’s thermostat, both in the vehicle and on your workbench.
You don’t need to be a master mechanic to perform these checks. With basic tools and some patience, you can determine if your thermostat is stuck open, stuck closed, or working just fine. Let’s get started with understanding what a thermostat does and the signs that it might be failing.
How To Check Thermostat Car
This section covers the most common and effective methods for testing your car’s thermostat. You can choose between checking it while it’s installed in the engine or removing it for a more definitive test. Both approaches have their merits, and we’ll walk you through each one.
Before you begin, ensure your engine is completely cool. Working on a hot cooling system is dangerous due to high pressure and temperature. Gather your safety gear, including gloves and eye protection, and have a drain pan ready.
Understanding Your Car’s Thermostat
The thermostat is a simple yet crucial valve in your engine’s cooling system. Its primary job is to regulate engine temperature by controlling the flow of coolant to the radiator. It remains closed when the engine is cold, allowing it to warm up quickly to its optimal operating temperature.
Once the engine reaches a specific temperature, usually between 195°F and 220°F (90°C and 105°C), the thermostat opens. This allows coolant to circulate through the radiator, where it is cooled before returning to the engine block. This cycle maintains a stable temperature for efficient operation and reduced emissions.
Common Symptoms of a Faulty Thermostat
Watch for these signs that may indicate a thermostat problem:
- Engine Overheating: This is the most critical sign, often caused by a thermostat stuck in the closed position, preventing coolant flow.
- Engine Running Too Cool: If your temperature gauge never reaches the normal middle range, especially in cold weather, the thermostat may be stuck open.
- Erratic Temperature Gauge: The needle fluctuating rapidly between hot and cold can point to a thermostat that is intermittently sticking.
- Poor Heater Performance: Since the heater core uses hot coolant, a stuck-open thermostat can result in weak or no heat from the vents.
- Increased Fuel Consumption: An engine running too cool operates less efficiently, which can lead to worse gas mileage.
Method 1: Checking The Thermostat In The Vehicle
This is a good initial test that requires minimal disassembly. You’ll be checking the temperature of the coolant hoses to infer the thermostat’s operation. You’ll need a infrared thermometer or a careful touch (mind the hot components).
Step-by-Step In-Vehicle Test
- Start with a completely cold engine. Locate the upper radiator hose, which runs from the top of the radiator to the engine’s thermostat housing.
- Start the engine and let it idle. Keep an eye on the dashboard temperature gauge.
- Feel the upper radiator hose (carefully) as the engine warms. It should remain relatively cool for the first several minutes.
- As the engine approaches its normal operating temperature (when the gauge needle is near the middle), the upper hose should quickly become very hot to the touch. This indicates the thermostat has opened and hot coolant is now flowing to the radiator.
- If the hose gets hot immediately or very early in the warm-up process, the thermostat is likely stuck open. If the engine overheats and the hose remains cool, the thermostat is probably stuck closed.
Using an infrared thermometer provides a more accurate reading. Point it at the thermostat housing and the upper hose. A significant temperature difference between the housing and the hose after warm-up suggests a stuck-closed thermostat.
Method 2: Removing And Testing The Thermostat
For a definitive test, you can remove the thermostat and check it directly. This method confirms its physical condition and opening temperature. You’ll need basic hand tools, a pot, water, a stove, and a cooking thermometer.
Steps for Removal and Bench Testing
- Drain the cooling system below the level of the thermostat housing. Place a drain pan underneath to catch the coolant.
- Remove the bolts securing the thermostat housing. Carefully pry off the housing to reveal the thermostat. Note its orientation.
- Take out the thermostat and the sealing gasket. Inspect it for obvious signs of damage, corrosion, or debris that might prevent it from sealing or opening properly.
- Look for the temperature rating stamped on the thermostat (e.g., 195°F). You will test it against this rating.
- Suspend the thermostat in a pot of water using a string or wire, ensuring it does not touch the bottom or sides. Place your cooking thermometer in the water.
- Slowly heat the water on a stove while stirring gently. Observe the thermostat valve.
- The thermostat should begin to open just as the water reaches its stamped temperature rating. It should be fully open about 15-20°F above that rating.
- Remove the pot from the heat and let it cool. The thermostat valve should completely close as the water cools below its opening temperature.
If the thermostat fails to open at the correct temperature, fails to close, or is stuck in one position, it needs to be replaced. Always install a new gasket when reassembling the housing to prevent leaks.
What To Do If Your Thermostat Is Faulty
If your tests confirm a bad thermostat, replacement is the only reliable option. They are inexpensive parts. When installing a new one, ensure you get the correct temperature rating for your vehicle, which is typically listed in your owner’s manual.
Replacement Tips and Considerations
- Thoroughly clean the mating surfaces on the engine and thermostat housing to ensure a good seal for the new gasket.
- Install the new thermostat in the correct direction. The spring side typically faces into the engine block.
- Use a torque wrench to tighten the housing bolts to the manufacturer’s specification. Over-tightening can crack the housing.
- Refill the cooling system with the correct type and mixture of coolant and distilled water.
- After refilling, run the engine with the radiator cap off (or the overflow tank full) to “burp” the system of air bubbles, which can cause overheating.
Finally, take the car for a short test drive while monitoring the temperature gauge. It should reach normal operating temperature and remain stable, confirming a successful repair.
Preventative Maintenance For The Cooling System
A failing thermostat is often a symptom of broader cooling system neglect. Regular maintenance can prevent many issues and extend the life of all components, including the thermostat.
Follow your vehicle’s recommended service intervals for coolant flushes. Over time, coolant becomes acidic and loses its anti-corrosion and lubricating properties, which can cause the thermostat to stick. Inspect hoses and belts for cracks, bulges, or soft spots during routine oil changes.
Always address cooling system leaks promptly, as low coolant level is a leading cause of overheating. Use the correct coolant type specified by your vehicle’s manufacturer, as mixing different types can cause gel formation and clog the system.
Frequently Asked Questions
How Can I Tell If My Car Thermostat Is Bad?
You can tell if your car thermostat is bad by observing key symptoms. The most common signs include the engine overheating, the engine taking too long to warm up or never reaching normal temperature, and erratic movement of the temperature gauge. A simple in-vehicle check of the radiator hose temperature during warm-up can also point to a problem.
What Are The Steps To Test A Car Thermostat?
The steps to test a car thermostat involve either an in-vehicle check or a bench test. For the in-vehicle test, feel the upper radiator hose as the engine warms. For a definitive answer, remove the thermostat and heat it in a pot of water with a thermometer to see if it opens at the correct temperature.
Can You Drive With A Bad Thermostat?
Driving with a bad thermostat is risky and not recommended. If it’s stuck closed, the engine will overheat, potentially causing severe and expensive damage like a warped cylinder head or a blown head gasket. If it’s stuck open, the engine will run inefficiently, increasing wear and fuel consumption, though it is less immediately dangerous.
How Much Does It Cost To Replace A Car Thermostat?
The cost to replace a car thermostat varies. The part itself is usually inexpensive, ranging from $20 to $80. If you have it done at a shop, labor costs can bring the total to between $150 and $400, depending on your vehicle’s make and model and the shop’s labor rates. Doing it yourself saves on labor.
How Long Does A Car Thermostat Usually Last?
A car thermostat usually lasts a long time, but it is not a lifetime part. Most thermostats can function reliably for 5 to 10 years or 100,000 miles. However, they can fail earlier due to corrosion from old coolant, debris in the system, or manufacturing defects. Regular coolant changes help maximize their lifespan.