What are the methods for checking the thermostat?

[MacIsabella]

Here are several methods for checking the car thermostat: 1. Inspection after engine start: Open the radiator filler cap. If the coolant in the radiator is calm, it indicates the thermostat is working properly; otherwise, it indicates malfunction. When the coolant temperature gauge shows below 70°C, if there is coolant flow at the radiator inlet pipe and the coolant is warm, it means the thermostat's main valve is not tightly closed, causing premature major coolant circulation. 2. Inspection after coolant temperature rises: In the early stage of engine operation, the coolant temperature rises rapidly. When the temperature gauge indicates 80°C, if the temperature rise slows down, it indicates the thermostat is functioning normally. When the coolant temperature gauge shows 70-80°C, open the radiator cap and drain valve, and feel the coolant temperature by hand. If both feel hot, the thermostat is working properly. If the coolant temperature at the radiator filler is low, and there is little or no coolant flow at the radiator upper tank inlet pipe, it means the thermostat's main valve cannot open.

[VonThomas]

I did thorough research on thermostat inspection right after replacing mine. After starting the engine, simply feel the temperature of upper and lower radiator hoses. If the upper hose gets scalding hot after 10 minutes of cold start while the lower hose remains cold, the thermostat is likely stuck closed. Be alert if the temperature gauge struggles to reach midpoint - I encountered this last winter. Another crude method is boiling the removed thermostat in water; it should gradually open around 80°C, but remember to measure if the opening gap reaches 8mm. Electronic thermostats are trickiest, requiring an OBD scanner to read coolant temperature data stream - I specifically bought a diagnostic tool for this. Actually, heater performance helps too - poor cabin heat often indicates thermostat issues, a trick that's saved me three towing fees.

[OCourtney]

Checking the thermostat actually requires a few steps. When the engine is cold, first confirm if the thermostat is installed in the correct orientation—the valve should face toward the engine. I once had a repair shop install it backward, causing the temperature gauge to skyrocket. The hot engine test is even more critical: once the temperature gauge stabilizes, suddenly rev the engine to 3000 RPM. Under normal conditions, the temperature difference between the upper and lower radiator hoses should quickly decrease. Remember to use an infrared thermometer to scan the thermostat housing—the valve body should noticeably heat up in the 80-90°C range. If you suspect a fault, it’s best to perform a pressure test by connecting a cooling system pressure gauge. Abnormal pressure fluctuations during warm-up indicate faulty valve operation. Don’t forget to check the thermostat’s sealing ring—last time, a leak was caused by this aging rubber ring. For complete peace of mind, also test the antifreeze’s freezing point while you’re at it.

[Daedalus]

We northern drivers know best how to judge a thermostat. After starting the engine in the morning, keep an eye on the temperature gauge—the needle should move within five minutes normally; if it doesn't move after ten minutes, the thermostat is definitely stuck. While driving, pay attention to abnormal noises from the engine compartment—a hissing sound means the valve isn't sealing properly. A simple method is to feel the temperature difference between the radiator's inlet and outlet pipes—if the difference exceeds 20 degrees, there's definitely an issue. It's most noticeable in winter when using the heater—if cold air blows from the vents, stop and check immediately. I once discovered a thermostat failure on the highway this way. For older cars, also check the coolant color—if it's rusty or murky, the thermostat is likely clogged.

[DiHolden]

Checking the thermostat depends on vehicle characteristics. For mechanical types, rely mainly on physical tests - like removing an 88°C valve and boiling it in water; if it doesn't open after 3 minutes of boiling, replacement is needed. Electronic types require measuring heating resistance with a multimeter, with standard values typically being 20-30 ohms. For newer vehicles with electric water pumps, combine coolant temperature sensor data and use a diagnostic tool to read factory-set temperature values for greater accuracy. Always bleed the cooling system before repairs - one customer's misdiagnosis was caused by airlock. Pay special attention to thermostat specifications for modified vehicles; installing low-opening thermostats in regular cars can cause frequent full coolant circulation leading to low temperature issues.