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Q AND A: Check gauges' accuracy before changing thermostat

QUESTION: With the recent warmer weather, I've noticed my pickup will sometimes run hotter than I think it should. I'm wondering if changing the thermostat to a colder one would help.

QUESTION: With the recent warmer weather, I've noticed my pickup will sometimes run hotter than I think it should. I'm wondering if changing the thermostat to a colder one would help.

--Terrence Rumsfeld

ANSWER: Let's assume that the cooling system is full of coolant -- a 50/50 mix of antifreeze and water is preferred -- that it's leak-free, and that you're seeing an occasionally high gauge reading. Start by qualifying the temperature gauge readings.

A neat way to do this is with a noncontact digital thermometer, also called a temp gun. These are now widely available, and the cheapie ones have dipped below $20. This tool has many applications -- you can check brake temperature, air conditioning or heater output, catalytic converter function, engine or transmission temperature, your refrigerator or freezer's settings -- there are endless possibilities.

You can check true engine coolant temperature by aiming the gun at the engine's thermostat housing. If you find the truck's gauge and the gun agree, when at normal and elevated temperature, we'll go from there. The truck's thermostat is designed to maintain overall engine temperature at approximately 195 degrees.


At startup, the thermostat is closed, blocking coolant flow through the radiator, encouraging the engine to warm up as quickly as possible. As coolant temperature reaches 195, the thermostat opens and continuously meters flow through the radiator to maintain that temperature. Should conditions cause elevated temperature, the thermostat opens fully, allowing maximum radiator participation. As long as the thermostat is opening as it should, it plays no role in managing temperature above 200 degrees or so.

If there's any doubt the thermostat is working properly, I'd change it in a heartbeat, with one of the specified rating. They're inexpensive, and it's typically a simple job. Changing to a colder value, such as 180 or 160 degrees, may briefly assist a weak cooling system when you encounter a single steep hill, because you're starting out cooler. But the drawbacks are many. Your engine was designed to run best, produce the fewest emissions and deliver the best fuel economy at 195 degrees. Plus the oil remains cleaner, compared to running at colder temperatures.

There are many possible causes for occasional elevated temperature: a partially restricted radiator, airflow across the radiator impeded by bugs or debris, an eroded water pump impeller, a collapsing lower radiator hose, or a slipping fan clutch. Radiator efficiency can be checked using the heat gun, comparing upper and lower radiator hose temperature when the thermostat is open and the car is at operating temperature. With adequate airflow across the radiator, you should see a difference of over 20 degrees. If desired, a heavy-duty radiator, with more air passages, can be purchased for less than $200.

Replacement could be a satisfying DIY Saturday job -- renew all hoses at this time as well. If the temperature drop across the radiator looks OK, but the inlet temperature in the top hose is notably higher than the thermostat rating, the water pump may not be providing adequate coolant circulation.

For fan clutch information and testing, go to http://www.haydenauto.com/ . Click "Training," then select a fan clutch tutorial. They did a nice job on this.

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