head gasket but no oil in water or other way around?

Usually if its a blown headgasket you can see bubbles in the overflow ....heater won`t work either ..possiable bad therm or a bad radiator not leting coolant flow

bg
June 2006

It sounds like you have a faulty thermostat. Fans usually only kick in when the engine is up to working temperature and the vehicle is stationary. It looks like your never gets up to working temp and so your fan will never kick in. A faulty stat may also explain a sudden rush of water.

BTW as you don't say what the car is, no one can answer your question about the ECU even if it was the problem.

Terry
June 2006