If you're wondering how to test the engine coolant thermostat, this tutorial provides a comprehensive guide to help you do so using an infrared thermometer gun.
All test steps are explained in detail, allowing you to determine if the thermostat is functioning correctly or has failed.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 2.8L Chevrolet S10 Pickup: 1988, 1989, 1990, 1991, 1992, 1993.
- 2.8L GMC S15 Pickup: 1988, 1989, 1990.
- 2.8L GMC Sonoma: 1991, 1992, 1993.
This tutorial also applies to the 2.8L Chevy S10 Blazer, 2.8L GMC S15 Jimmy. See the 'Applies To' box on the left column (desktop) or at the bottom of the page (mobile device) for more info.
Important Suggestions And Tips
TIP 1: Never attempt to open the radiator while it's still hot. A hot radiator poses a significant risk of burns and other injuries!
TIP 2: To accurately test the engine thermostat, the engine must be completely cold. Testing the thermostat with a warmed-up engine can give you a false-positive test result, leading to an inaccurate diagnosis and repair.
TIP 3: In order to follow the instructions provided in this tutorial, you'll need an infrared thermometer gun. This tool will be used to measure the temperature of the upper radiator hose and check the thermostat's performance. If you don't have one, check out my recommendation here: KIZEN Infrared Thermometer Gun (LaserPro LP300) (at: amazon.com).
TIP 4: You'll be working around a running engine, so to take all necessary safety precautions. This includes wearing appropriate protective gear, such as gloves and safety glasses, and being aware of the dangers of moving parts and hot surfaces.
Where To Buy The Thermostat And Save
The following engine coolant thermostat is from a known automotive part manufacturer (not a knock-off):
Symptoms Of A Bad Thermostat
The engine thermostat is the component responsible for regulating the temperature of the engine by controlling the flow of coolant. Unfortunately, it'll eventually fail, which can lead to a range of issues. Generally, an engine thermostat fails in one of two ways.
- The thermostat stays stuck in its open position and allows coolant circulate all the time.
- The thermostat remains stuck closed and does not allow the engine coolant to circulate.
When the thermostat stays stuck open, the engine coolant flows continuously through the engine, resulting in one or more of the following symptoms:
- Richer air/fuel mixture: A stuck open thermostat can cause the engine to run cooler than intended, which leads the ECM to compensate by increasing the amount of fuel injected into the engine.
- Longer engine warm-up time: A thermostat regulates the engine's operating temperature by controlling the flow of coolant through the engine's cooling system. A stuck open thermostat can cause the engine to take longer to warm up, which leads to reduced fuel efficiency and increased wear on engine components.
- Increased emissions: A richer air/fuel mixture caused by a stuck open thermostat can lead to increased emissions of harmful pollutants such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx).
- Bad gas mileage: A stuck open thermostat can cause the engine to run cooler than intended, which leads the ECM to compensate by increasing the amount of fuel injected into the engine.
- Heater will not work: A thermostat also controls the flow of coolant through the vehicle's heater core, which is responsible for providing heat to the vehicle's cabin. A stuck open thermostat can prevent the heater from working properly, which can be particularly problematic during the winter months.
- Increased piston ring wear: A richer air/fuel mixture caused by a stuck open thermostat can dilute the engine oil with gasoline, which can lead to increased wear on engine components such as piston rings. This condition can cause increased oil consumption and reduced engine performance.
- Black exhaust smoke: A stuck open thermostat can cause the engine to run excessively rich, which can lead to black exhaust smoke being emitted from the vehicle's tailpipe. This condition can also cause damage to the vehicle's catalytic converter and oxygen sensor.
When the thermostat stays stuck closed, it blocks the flow of coolant, leading to one or more of the following symptoms:
- High engine operating temperature: When a thermostat is stuck closed, it can prevent coolant from flowing from the engine into the radiator. This can cause the engine to overheat and potentially lead to engine damage.
- Engine overheating: When a thermostat is stuck closed, it can prevent coolant from flowing from the engine into the radiator. This can cause the engine to overheat and potentially lead to engine damage.
- Temperature light illuminated: If the engine begins to overheat due to a stuck closed thermostat, the temperature warning light will illuminate to alert you of the issue.
- Temperature gauge (if equipped) will max out: If the thermostat is stuck closed and the engine begins to overheat, the temperature gauge may show the maximum temperature reading, indicating that the engine is running too hot.
If a thermostat is stuck closed and the engine overheats, it can cause damage to engine components such as the cylinder heads, head gaskets, and pistons. In severe cases, a stuck closed thermostat can cause the engine to seize or result in catastrophic engine failure. It's important to address this issue promptly to prevent serious engine damage.
OK, let's get testing.
Thermostat Performance Test
I know I'm starting to sound like a broken record, but it's important to start with a cold engine to ensure an accurate test result.
You can verify that the engine is cold if the upper radiator hose is cold to the touch (which also indicates that coolant is not flowing into the radiator).
If the engine is warm or the upper radiator hose is warm or hot to the touch, the thermostat may not be closed and can lead to an inaccurate diagnosis.
To cool down the engine quickly, place a fan on top of the engine and run it for about 20 minutes.
The test instructions assume that you have an infrared thermometer gun, if you don't have one, check out my recommendation: KIZEN Infrared Thermometer Gun (LaserPro LP300).
NOTE: The OEM thermostat temperature is 195° F.
CAUTION: Take all safety precautions when performing this test, as it requires the engine to be running. This includes ensuring that the vehicle is in a well-ventilated area, wearing protective gloves and eye protection, and keeping all loose clothing and hair away from moving engine components.
IMPORTANT: Ensure the radiator is full before starting the thermostat test.
These are the test steps:
PART 1:
- 1
Check the temperature of the upper radiator hose with the engine off. You can use your hand.
The hose should be at ambient temperature. If the hose is hot, the engine is still hot and needs to be cooled down further.
NOTE: The upper radiator hose is the one that connects to the thermostat housing on the engine. - 2
Take a temperature measurement near the thermostat housing with your infrared thermometer gun.
IMPORTANT: Point the laser at a spot on the intake manifold near the thermostat housing, rather than pointing it directly at the thermostat housing. - 3
You should see a temperature within 10 degrees of ambient temperature on your infrared thermometer gun's display.
PART 2:
- 4
Start the engine. Going forward, monitor the temperature at regular intervals with your infrared thermometer gun.
- 5
At 150° Fahrenheit (65° C) on your infrared thermometer, check the temperature of the upper radiator hose.
- 6
The upper radiator hose should still be at the ambient temperature you noticed in step 3. If so, continue to the next step.
If the hose is hot, you can conclude the thermostat is stuck open or missing. Replace the thermostat. - 7
At 200° Fahrenheit (93° C) on your infrared thermometer, check the temperature of the upper radiator hose.
- 8
At this temperature point, hot coolant should be circulating from the engine into the radiator, and the upper radiator hose should now be hot to the touch.
- 9
Turn the engine off.
Let's take a look at what your test result means:
CASE 1: The upper radiator hose got hot at 200° F. This is the correct and expected test result and confirms the thermostat is functioning correctly.
CASE 2: The upper radiator hose WAS NOT hot at 200° F. If the radiator hose was still at ambient temperature (cool to the touch), the thermostat has failed and is stuck in its closed position.
A thermostat that is stuck in its closed position will NOT allow coolant to circulate between the engine and the radiator, causing the engine to overheat.
Replacing the thermostat will solve the issue.
CASE 3: The upper radiator hose was hot at 150° F (or less). This test result lets you know that the thermostat has failed and is stuck in its open position.
A thermostat stuck in the open position will allow coolant to circulate through the engine's cooling system and into the radiator at all times, regardless of the engine's temperature. This type of thermostat failure will prevent the engine from reaching its normal operating temperature.
Replacing the thermostat will solve the issue.
