How to Tell if Your Car's Thermostat Is Stuck Closed

Your car's thermostat allows coolant to circulate when the engine is warmed up and should be closed when the engine is cold so that the car can warm up faster. It's a fairly inexpensive part that's replaced easily, but when it fails, your car can overheat, causing costly engine damage.

Difficulty: Easy

Instructions

1. Warm up the car but don't let it overheat--don't let the temperature gauge go into the red.

2. Turn off the engine.

3. Open the hood.

4. Find the upper radiator hose. It's black, is made of rubber and is about 2 inches in diameter, with metal clamps on either end. The upper hose goes into the top of the radiator.

5. Locate the lower radiator hose. It looks similar to the upper hose except that it attaches to the bottom part of the radiator.

6. Touch each hose very carefully (they can be extremely hot). If the temperature gauge is indicating that the engine is warmed up but one hose is hot and the other is cold, the thermostat is probably stuck closed, and the coolant isn't circulating through the radiator.

7. Have the thermostat replaced if this is the case.

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[kenl123]

on 6/16/2009 Step 6 makes no sense. If the thermostat is NOT stuck closed, hot water will flow to the top of the radiator. The purpose of the radiator is to cool the water, so the bottom hose will be cool.

[bgd73]

on 1/2/2008 A thermostat sticks open and closed randomly for several reasons. headgasket sucking air, water pump sucking air, frigid cold can fail even a good thermostat into bizarre actions. This after acknowledging the system is not using coolant. Also a reservoir will fill up with cold fluid if thermostat is stuck closed in the cold. An engine can do many different things and make a thermostat problem elusive. You could also pack up and move towards 70 degrees where none of this will be of concern for decades at a time and hope the crazy north gets manly parts that function after 100+ years of the internal combustion engine resides in paradise with the other engineers of no concern.

on 11/22/2005 If you're driving down the freeway and your car is about to overheat because you have a stuck thermostat, turn off the air conditioner, and put the heat on full blast. Engine coolant will run through the heater core and be cooled by the heater blower. This should get your coolant temperature down a little bit to keep you going. Yes, it will be very hot, but open the windows and think of how you'll no longer be stuck on the highway.

on 2/12/2007 This should be done only as a last resort and should confirm any doubt after following any of the previous tips.

Remove the thermostat and place it in a coffee mug of boiling water (roughly 180-200 degrees). If the thermostat fails to open, it is faulty. If it does open, your problem lies elsewhere.

on 6/11/2007 With the engine cold, remove the radiator cap and warm the engine up. Normally, at startup, the coolant should be still. But as the engine warms up to normal operating temperature, the thermostat should open, and the coolant will stir with the water pump. The churning action of the water pump would be more apparent if you rev the engine up. Based on this normal thermostat operating scenario, you should be able to determine if your thermostat is stuck open or closed.