[brucepick]

Re. looking like mpg is up -
As long as there's enough coolant in the system, the thermostat etc. would keep the same temperature regardless of whether the system is sealed or not. So efficiency would stay the same.

But if coolant level was dropping, the ECT sensor would pick up different readings if the sensor is not covered as it should be. Would likely read hotter as it's picking up temperature from the (now drier) head it's screwed into. Might influence the computer to adjust operating parameters - more rich, more lean, who knows?, compared to regular operation when the sensor is flooded.

Losing coolant = there's a leak somewhere; you'll have to solve that or at least keep the system topped up.

Sealed system = it creates pressure, which means the water will boil at a higher temperature than without pressure. My guess is that without pressure it won't spray out as violently, but it will boil at a lower temperature. You won't see puddles etc. because it's boiling/steaming off, not spraying. I think you'll lose it more slowly with cap off due to when its under pressure, the pressure probably blows out a bigger quantity of water.