Troubleshooting: Temperature Gauge Drops While Driving

Why the Temperature Gauge Drops While Driving?

If your temperature gauge rises when idling but drops when driving, the cause is almost always a stuck open thermostat. The reason the temperature gauge drops when driving is that there’s much greater airflow through the radiator when you’re at speed than at idle. That increased airflow drops the temperature of the coolant coming into the engine.

The job of the thermostat

The thermostat is closed when you first start your engine. As the

Engine Thermostat

engine coolant heats up to around 150°F, the thermostat starts to open, allowing coolant to circulate. As you drive, the thermostat acts as a valve, opening and closing slightly based on coolant temperature. For example, on a cold, wintry day, your thermostat may be almost fully closed because the coolant entering the engine will be much colder, and the thermostat wants to reduce coolant flow to avoid cooling the engine too much. For a more detailed explanation of how an engine thermostat works, see this post.

Engines operate best when they’re around 200°F. If the engine runs much colder than that, it will use more gas and reduce performance. If the temperature exceeds around 220° the heat can permanently damage internal engine parts.

What causes a thermostat to stay open?

A failed seal can cause a thermostat to stay open

A thermostat is a mechanical device and it can wear out? The most common failure is the rubber seal in the pellet that allows coolant to mix with the wax. When that happens, the thermostat tends to stay open full time, preventing the engine from reaching 200°F while driving. The only time the engine reaches a higher temperature is when it’s idling.

Fix for stuck open thermostat

The only fix for a stuck open thermostat is a replacement

©, 2020 Rick Muscoplat