We have replaced the thermostat and the heater is still not working. It worked for about 5 mins the other day, and when you revved up the engine. Now nothing. The car overheats quickly after about a mile of driving. There does not seem to be a leak anywhere. We have bled the system too. That did not work and there is no water coming from the bleeder valve. It is just steam. The valve cover is smoking. Checked the radiator and it is not even getting warm. We are thinking the water pump might be out. There is a ticking that is sometimes a knocking coming from the passenger side, it seems that there is no water circulating. Could this be the water pump?