The pump (10) has a radial impeller (11) with a radial impeller outlet (44), and a ring channel (48) surrounding the impeller outlet. An axially movable valve tube (14) is extendable for sealing the impeller outlet in the ring channel in a closed position. A mechanical temperature controlled valve tube actuator (16) is arranged in a coolant chamber enclosed by a pump housing (30), where the actuator moves the valve tube during heating of the actuator in an opening position. The actuator is designed as a wax element or a bimetal element.
