Can I Run My Boat Without a Thermostat?
Why That’s a Bad Idea

It’s a common myth among boaters: “If my engine runs hot, maybe I should remove the thermostat completely.” The idea sounds simple — get water flowing earlier, keep things cooler. But in a marine engine, removing the thermostat doesn’t solve overheating. It usually causes more problems than it prevents.

What a Thermostat Actually Does

In both outboard and inboard marine engines, the thermostat controls when water begins to circulate through the engine block. It ensures the engine warms up quickly and stays at an optimal operating temperature — usually between 140°F and 160°F for raw water systems, and up to 180°F for closed-cooled engines. When the thermostat is removed, coolant or raw water flows continuously through the block. That might sound helpful, but it creates unstable temperatures and disrupts the engine’s thermal balance.

Problems Caused by Removing the Thermostat

1. Inconsistent engine temperature Without the thermostat to regulate flow, the engine may run too cold at idle or warm unevenly. Cold cylinders can lead to poor combustion and unburned fuel.
2. Overheating at speed With the thermostat gone, water may bypass the block too quickly at high RPM. That limits heat exchange time, causing unexpected overheating despite constant flow.
3. Higher fuel consumption and carbon buildup Engines that never reach operating temperature tend to run rich. That leads to carbon buildup, fouled plugs, and reduced performance.
4. False sense of security You might think the engine is running cooler because the gauge shows lower numbers at idle. In reality, you’re losing temperature control, not gaining protection.

What We See in the Shop

Engines that run without thermostats often arrive with a list of complaints: poor idle, steam from the exhaust, fouled plugs, and rough running. In many cases, someone pulled the thermostat after seeing elevated temps, hoping to fix an overheating issue. But the root cause — like a clogged riser, worn impeller, or air leak — was never addressed. Worse, we’ve seen severe salt corrosion inside blocks due to constantly cold engine temps, which prevent full heat expansion and allow salt to settle inside passages.

EFI Engines and Thermostat Logic

Modern EFI outboards and inboards use engine temperature to control fuel mapping. Without a functioning thermostat, the ECM may think the engine is still cold and keep it in warm-up mode. That results in poor throttle response, fuel waste, and incorrect timing.

When It’s Okay to Remove a Thermostat

There is only one valid reason to run without a thermostat — during a test or emergency situation. If the thermostat is stuck closed and you’re stranded, removing it temporarily can help you limp home. But it should be replaced with a properly rated unit as soon as possible.

Conclusion

Running your marine engine without a thermostat won’t make it safer or cooler. It removes the only mechanism that regulates temperature and timing. The result is usually worse fuel economy, poor performance, and a higher risk of real overheating. If your boat runs hot, fix the problem — don’t remove the part designed to manage it.