2007 Evinrude E-Tec 75, 90 HP outboards Service Manual, Page 196Get this manual
COOLING SYSTEM COMPONENTS
Adapter Housing
Adapts the cylindercrankcase assembly (powerhead) to the megaphone and exhaust housings and the water tube Provides water passages which route incoming and outgoing cooling water Provides exhaust passages which connect to the inner exhaust of the outboard's midsection
Pressure Relief Valve
A pressure relief valve is used to control water flow and operating temperature at higher speeds (above approximately 2000 RPM)Pressure relief valve activates or "blows-off the seat" as water pressure increasesSpring tension is used to set the "blow off" pressureOverheating: restricted or faulty valve typically results in HIGH SPEED overheatingOvercooling: Debris may prevent the valve from closing completely