Outboard Thermostats: The Resourceful Sailor has an Open and Closed Case

Article by Joshua Wheeler

The 2006 Yamaha T8 4-stroke outboard motor on Sampaguita, my Pacific Seacraft Flicka 20, was running poorly. Experience told me that the thermostat might be stuck open. The main telltales were the engine not getting warm (I could feel it), un-combusted fuel coming out the exhaust (I could see it), and gas accumulating in the crankcase oil (I could smell it). Moisture was also emulsifying the latter. The motor will often run under these conditions, but not optimally, and it is susceptible to more corrosion and wear. These symptoms could also mean a deeper problem, but the ease of checking the thermostat made for a logical place to start. (Note: If the thermostat were stuck closed, the motor would likely be overheating.)

Thermostat location on my Yamaha T8 - ‘Can you see the red gasket?’