What Will Cause the Temperature Gauge to Go Down While You Drive & the Heat Not to Work?

by Richard Rowe
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The cooling system isn't the most complicated system on your vehicle's engine, but it can still malfunction over time. The water pump and thermostat are the two most complex parts of the cooling system, which means that a failure will typically occur in one or the other. Consistently low coolant temperatures could only mean one thing; fortunately, the required parts are fairly cheap and easy to install.

Cooling System Basics

An engine's cooling system starts in its water jacket, which consists of a system of empty chambers placed around the cylinders and in the cylinder heads. The water pump pulls water out of the water jacket and pushes it through the radiator tubes, and thus into the radiator. A thermostat valve closes off the upper radiator hose when the engine's cold, forcing coolant to recirculate through the block until it warms up.

Heating System

The heater core is a kind of miniature radiator that sits in your car's climate control duct. The water pump's pressure-side has a small outlet that sends water through the heater core; when you turn the heat on, a valve in the heater core's water inlet line opens and sends coolant through the core. The core, and thus the heating system, gets its thermal energy from the engine's cooling system.

Thermostat Failure

Thermostat valves can fail in one of three ways: open, closed or stuck halfway. If the valve sticks in the closed position, coolant will constantly recycle through the engine without going to the radiator, causing the engine to overheat. A stuck-halfway valve will increase the time that it takes for your engine to warm up and will cause temperatures to slowly creep up under load. A stuck-open thermostat will constantly send coolant through the radiator, regardless of its temperature, causing coolant temperatures to drop and the heater to malfunction while traveling down the road.

Location and Testing

To find the thermostat, follow the upper radiator hose to the housing where it attaches to the engine. When you unbolt the housing, you'll find the saucer-shaped thermostat sandwiched between it and the block. The thermostat valve should be shut; if it's open even a little, then chuck the thermostat and buy a new one.

Replacement Thermostats

Replacement thermostats come in varying temperature ranges, and you can typically go up or down 10 degrees to fine-tune engine temperature for the application. For instance, you may opt to use a 170-degree thermostat in place of the recommended 180-degree thermostat if you do a lot of towing, or a 190-degree thermostat if you want to pick up a bit of horsepower and increase heater output temperature.

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