How to Check the Bypass Valve on a Mercury 150 Outboard

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When your motor overheats, the bypass valve is rarely the culprit. Linked to the thermostat on the Mercury 150, the bypass valve diverts water away from the motor's power head--the engine part of your outboard--until the power head reaches its normal operating temperature, about 160 degrees. The bypass valve then closes and the water cools the power head. Checking the bypass valve is a process of elimination: you must first rule out other potential causes of overheating.

Connect a garden hose to the Mercury flushing attachment--the ear muffs used to flush the engine. Turn the water on full force. Ensure all persons present are aware that you are starting the engine, warning them to keep clear of the propeller.

Run the motor until it reaches normal operating temperature. Monitor the temperature of the steady stream of water coming from the bypass with a temperature probe or laser temperature gun. If the temperature is above 160 degrees--the usual factory setting for the thermostats and bypass valves--or if the "overheating" warning horn sounds before the stream of water stops, shut the motor down.

Remove and replace the impeller. If the engine isn't getting enough cooling water, it will run hotter than normal. Nearly 95 percent of the time, the problem is the impeller. After 300 hours of operation, the impeller requires changing--if you're nearing that milestone, change the impeller and check the temperature again.

Remove the two screws that hold the thermostat faceplate in place on the face of the housing, if the motor still runs hot. Replace the temperature sensors--in the outermost holes of the thermostat faceplate--and test the motor again.

Replace the two bypass valves--the two valves in the two innermost holes of the plate, if the motor still runs hot.


  • When checking the water temperature with a temperature probe, avoid getting the hot water on your hands or person.
  • Never ignore the warning buzzer or horn indicating your motor is overheating.


About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

Photo Credits

  • toy outboard motor image by pearlguy from Fotolia.com