Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 290Get this manual
POWER TRIM
In the up mode, as the piston rod (a) extends from the cylinder (w), the memory piston (t) clears or uncovers the pressure relief passageOil from the up cavity will enter this passage and, if requi red, causes the tilt relief piston (s) to open the tilt pressure relief valve (r)This valve lowers the amount of pressure available to lift the outboard motorWith the engine in forward gear, and at high engine rpmthe oil pressure available will not be able to overcome the propeller thrustlimiting the trim range to below the pressure relief orificeWhen the engine rpm's fall or if engine is not in forward gear, the oif pressure is available to extend the piston rod (a) up into the tilt range