Does a Faulty Thermostat Increase Fuel Consumption? What Happens When the Engine Runs Cold?

The component that regulates the engine coolant temperature and holds it steady at the ideal operating range of 80–90°C is the thermostat. Drivers usually fear a thermostat getting stuck closed and causing the engine to overheat; however, a thermostat getting stuck open also creates a huge cost at the gas pump.

Why Does Fuel Consumption Increase When the Thermostat Stays Open?

When the thermostat stays open, coolant constantly flows through the radiator, and the engine never reaches its ideal operating temperature. The Engine Control Unit (ECU) detects that the engine is continuously cold and injects a richer (larger) fuel mixture into the cylinders to try to bring it up to operating temperature. This situation can increase fuel consumption by 20% to 30%.

Most Common Symptoms of a Stuck-Open Thermostat

• Temperature Gauge Not Rising: The temperature needle dropping to the bottom or failing to rise, especially while driving on the highway.
• Insufficient Cabin Heating: The heater failing to warm up the interior and only blowing lukewarm air.
• Constant Rich Mixture Consumption: The vehicle consuming more fuel on the highway than it does in city traffic.

What Should Be Done?

A thermostat failure can be resolved with a very affordable parts replacement. Delaying this repair not only leads to wasted fuel, but also accelerates engine wear due to running continuously at low temperatures.

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