How to Check a Thermostat in a Car for Problems

Test the thermostat in your vehicle before you get stranded on the road.

Watching the temperature gauge in your car stay low after a few miles of driving can be worrisome or just plain scary if the gauge is warning you of an overheating engine. A stuck thermostat in the open or closed position is a common problem in an automotive cooling system after miles of service, and a relatively inexpensive repair job. But before you go and buy a new replacement, check the thermostat and make sure you need to install a new unit.

Instructions

1. Open the hood of your car and locate the thermostat housing. Depending on your particular vehicle year and model, the upper or lower radiator hose connects to the housing, which mounts on the engine with two or three bolts.

2. Remove the radiator cap, if one is provided. Or make a note of the coolant level on the coolant reservoir. This is the plastic bottle on the passenger's side of the engine compartment and connected to the radiator through a rubber hose.

3. Set your transmission to Neutral, apply the parking brakes and start the engine. Let the engine idle and walk around the front of the car.

4. Watch the coolant. If you see the coolant flowing through the radiator neck, or the level drops on the reservoir, the thermostat is probably stuck open. Wait for 10 minutes, then momentarily touch the engine block next to the thermostat housing and the radiator hose connected to it with the tip of your fingers. Both surfaces should have about the same temperature, if the thermostat is stuck open. Turn off the engine and replace the thermostat.

5. Wait for a few minutes to allow the engine temperature to go up, if the coolant is not flowing as you watch through the radiator neck or the level remains the same in the reservoir. Before the temperature gauge reaches the red line, you should see the coolant beginning to flow through the radiator neck or the level drop in the reservoir. If not, momentarily touch the engine block next to the thermostat housing and the radiator hose connected to it with the tip of your fingers. If the engine surface feels hotter than the hose surface, the thermostat is stuck closed. Turn off the engine and replace the thermostat.

Tips & Warnings

• Never remove the radiator cap when the engine in your vehicle is warm. Hot coolant under pressure may spray out and burn you.

References

• "Modern Automotive Technology"; James E. Duffy; 2003
• "Auto Mechanics Fundamentals"; Martin W. Stockel and Martin T. Stockel; 1982