This tutorial will help you determine if the thermostat is good or bad with an infrared thermometer gun (also known as an infrared laser).
Although a scan tool that can read OBD I live data is an awesome tool to have, you don't need one to test the thermostat. I'll explain how to step by step.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar El Termostato (1992-1997 2.2L Chevrolet Beretta, Cavalier, Corsica, Sunfire) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 2.2L Chevrolet Beretta: 1992, 1993, 1994, 1995, 1996.
- 2.2L Chevrolet Cavalier: 1992, 1993, 1994, 1995, 1996, 1997.
- 2.2L Chevrolet Corsica: 1992, 1993, 1994, 1995, 1996.
- 2.2L Pontiac Sunfire: 1995, 1996, 1997.
Important Suggestions And Tips
TIP 1: You don't need a scan tool to test the thermostat since this tutorial explains how to test it with an infrared thermometer gun (also called a laser thermometer).
If you have a scan tool that can read OBD I live data (1992-1995 vehicles) or OBD II (1996 vehicles), by all means, use it instead of the infrared thermometer to read the engine coolant temperature.
TIP 2: Don't open the radiator cap with a hot engine. If you need to add coolant (or water) to the radiator, do it with a cold engine.
TIP 3: Start your diagnostic with a completely cold engine to guarantee an accurate test result from the thermostat test described in this tutorial.
TIP 4: Take all necessary safety precautions when working around a running engine.
Where To Buy The Thermostat And Save
The following engine coolant thermostat is from a known automotive part manufacturer (not a knock-off):
NOTE: The thermostat usually comes without a gasket. Before you buy a thermostat gasket, you'll need to find out if your vehicle uses a round rubber seal (that fits over the thermostat) or a paper gasket (which goes between thermostat housing and the thermostat housing block).
You'll need to remove the thermostat from your vehicle to find out which of the two you'll need.
- The thermostat housing gasket (paper gasket) part number is FEL-PRO 35560.
- The thermostat seal (round rubber seal) part number is FEL-PRO 35614.
When ordering your parts online or at your particular auto parts store, make sure they fit your specific application.
Symptoms Of A Bad Thermostat
The engine coolant thermostat will usually fail in one of two ways:
- It will stay stuck in its open position and let coolant circulate all the time.
- It will remain stuck closed and not allow the engine coolant to circulate.
When the thermostat is stuck open, you'll see the following symptoms:
- Bad gas mileage.
- If it's wintertime, the heater will not work.
- If your vehicle has a temperature gauge (not just a temperature light), it will always show you a low reading or none at all.
- Won't pass the state emission test.
Most folks are surprised to find out that a stuck open or missing thermostat wreaks such havoc with gas mileage, but it's true.
When the thermostat is stuck closed, you'll see the following symptoms:
- The engine will overheat.
- Cooling fans come on, but the engine does not cool down.
- If your vehicle is equipped with a temperature gauge, it will max out.
- If your vehicle is equipped with a temperature light, it will shine nice and bright to let you know that an engine meltdown is occurring.
OK, let's get testing.
Thermostat Performance Test
To get this show on the road, it's important your vehicle's engine is completely cold.
If the engine has been running for any amount of time, let it cool down.
The fastest way to cool the engine down is placing a fan on top of it and running the fan for about 15 to 20 minutes until the engine is cold.
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: You'll be working around a running engine. Take all necessary safety precautions.
These are the test steps:
With the engine off, open the hood.
Check the engine coolant level. Top it off if necessary.
CAUTION: The engine must be cold before removing the radiator cap.
Check the temperature of the upper radiator hose. You can use your hand.
The hose should be at ambient temperature. If the hose is hot, let the engine cool down completely.
NOTE: The upper radiator hose is the one that connects to the thermostat housing on the engine.
Take a temperature measurement of the mounting location of the thermostat housing with your infrared thermometer gun.
IMPORTANT: Point the laser at a spot on the cylinder head where the thermostat housing bolts to the cylinder head. The arrow in photo 2 of 2 points to the spot.
You should see a temperature within 10 degrees of ambient temperature on your infrared thermometer gun's display.
Start the engine. Monitor the temperature as the engine runs.
At 150° Fahrenheit (65° C) on your infrared thermometer, check the temperature of the upper radiator hose.
The upper radiator hose should still be at the ambient temperature you noticed in step 3. If so, continue to step 8.
If the hose is hot, you can conclude the thermostat is stuck open or missing. Replace the thermostat.
At 200° Fahrenheit (93° C) on your infrared thermometer, check the temperature of the upper radiator hose.
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.
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 that is stuck in its open position will allow coolant to circulate into the radiator at all temperatures. This type of thermostat failure will prevent the engine from reaching its normal operating temperature.
Replacing the thermostat will solve the issue.