[ZL1_1LE]

Originally Posted by Patriot Motorsports USA

YES... it would superheat the coolant inside that its supposed to be holding in and prematurely push it into the recovery side of the reservoir. This is a car-show boomer part and in extreme track use would probably create more issues than it solves.

Using a sealed aluminum cap without a pressure relief valve would also cause issues due to thermal expansion, with the weakest link being coolant inside the heater core probably failing first.