Let the car idle and the coolant bottle was hardly pressurized at all. It gave me a tiny hiss. Say 1-2 psi. The engine did not get warm enough to cycle the thermostat so i dont know how much steam could have been produced. But it had been idling for about 5 minutes. No cloudiness in the bottle. The water still has a bit of black crap and oily residue floating on the surface.