1999 Volvo Penta "WT" Models Workshop Manual, Page 337Get this manual
Insufficient water supply: aProcedureDisconnect water supply hose from transom bracket at thermostat housingOperate engine at specified idle RPMHold end of hose level with the top of the flame arrestorbResults and Conclusionsinch (2,5 cm) head of water discharge, water supply is goodIf less than V2 inch (1.2 cm), look for source of water losscCheck forBlocked intake screens; damaged impeller housing O-ring or impeller plate gasket; broken or worn impeller; defective pivot housing water passage O-ring or water drain screw seal; loose pivot housing-to-gimbal housing water hose clampsThermostat malfunctionoverheating or slow warm-up: aProcedureOperate engine until indicated temperature exceeds 160 (71" C)Touch hoses between thermostat and exhaust manifoldsbResults and ConclusionsHoses cold, thermostat is stuck closedHoses warm or hot, thermostat is goodcCheck forThermostat stuck open or closed, defective thermostat O-ring, correct thermostat style for engine type, clear thermostat housing bypass passage ,:tmIinj'Iit''Wr'mFdoverheating or hard starting