A bypass system has a source of a fluid to be cooled and a heat exchanger for selectively cooling fluid. A piston is moveable along an axis allowing movement of a valve poppet toward and away from the valve seat. The piston contacts a boot. A wax element is disposed in a chamber, such that the wax element expands as a temperature of the fluid to be cooled increases, and causes the valve poppet to move against the valve seat. The boot has wax separator structures. A bypass valve is also disclosed.