- what the heck has this to do with the question at hand? Or to my posts?
- no, it will not. The radiator cap may operate properly, but the engine won’t build enough heat for the coolant to exceed the radiator cap pressure and there’ll be no movement of the coolant to the reservoir. And the coolant will boil at too low a temperature and the engine will overheat.
What will happen with the T-stat stuck open, assuming the radiator cap is operating properly, is that there’ll be no movement of coolant from the engine & radiator (the cooling system) to the reservoir and no drawing of coolant back into the engine. The coolant in the system will expand as the engine warms up to whatever temperature it reaches, but the pressure created will be maintained by the radiator cap’s ability to hold pressure in the engine and radiator. As the coolant cools and contracts after the engine is shut down, the pressure will drop, but not to a level lower than that of the coolant in the reservoir (at ambient). Since the pressure in the engine & radiator won’t drop below the pressure in the reservoir, there’ll be no drawing of coolant back into the engine & radiator.
In summary, the engine and radiator will always be at the same pressure. If it gets greater than 15-16psi above the pressure of the reservoir, the radiator cap allows it to release some pressure (coolant) into the reservoir which will always be at ambient. The engine will operate normally with the pressure higher than the reservoir. As the coolant cools and contracts, the radiator cap allows it to freely draw coolant back into the engine to prevent a pressure lower than that of the reservoir from developing.
Truth is, it is only if the pressure in the engine/radiator (the system) drops below that of the reservoir (ambient) that coolant is drawn back into the engine. If that doesn’t happen, everything remains stable. If the night gets cold, the coolant in the system will contract and will draw coolant back into the engine.