Abstract: An engine preheater of the lower radiator hose type includes a housing that is adapted to be connected to a radiator hose and which has a passage extending therethrough for passage of coolant through the heater. An electric heating element is housed by the housing and is at least partly embedded therein. It supplies heat to coolant in the passage. A thermostat is located in heat transfer relationship with the housing and is separated from the heating element by a part of the housing which is heat conductive. The thermostat is responsive to the temperature of the heating element for interrupting the supply of power thereto when the temperature sets by the thermostat exceeds a predetermined upper limit. A plug is adapted to be electrically connected to the heating element, and the thermostat is incorporated in this plug.