Well, I think that if the car is holding cooling system pressure overnight, it only proves that the cooling system passes a pressure test with flying colors. If it had a blown head gasket, it is likely that the pressure in the cooling system would have bled back into the cylinder when it sat overnight. Of course the cylinders make more pressure than the cooling system, so I suppose it is possible that it could leak in one direction only, but I doubt it.

Because you have overheated just lately, you should replace the thermostat because they can be damaged by overheating. If it were mine, I would put in a new thermostat, refill the cooling system, and drive it some more.

Is it likely that you did not have all of the air out of the cooling system? That can cause overheating.

By the way, I have noticed some of my Volvos holding pressure in the cooling system overnight. I always considered it to be a good sign. The possible bad news is that if you got it hot enough the other day, that overheating might have compromised the new head gasket.


Charley