Suzuki DF25/DF30 Four Stroke Service Manual, Page 224Get this manual
POWER TRIM AND TILT
THERMAL VALVE The PTT system incorporates thermal valve for protection of the internal components, should excessive downward force be exerted on the lower unit with the motor in tilted position, or (in the case of an impact in reverse gear), the outboard clampswivel brackets and the boat transomShould the propeller strike an underwater object whilst in reverse gear, build up of pressure will be induced in the lower cylinder chamber, whereby the outboard mounting bracket andor the boat transom may sustain damageTo prevent this, the thermal valve will open to relieve the oil pressure, thereby softening the impactInternal PTT circuits are protected, as the thermal valve will open to reduce oil pressure (caused by either hot climate or abnormally heavy usage)