An engine thermostat is designed to hold water in the engine block until it reaches its opening temperature and then allowing water to circulate the radiator to cool. No thermostat suggests to me that someone has had overheating problems in the past and removed it. Maybe you have a blockage in the radiator, or the cooling fan isn't switching on. Try bypassing the thermostat ( I believe its fitted into the radiator) ie pull the 2 connectors off it & join the 2 wires together, the fan should then run with ignition on.
Nobby
January 2007