Abstract: A thermostat for use in the coolant passage of an internal combustion engine includes a valve seat and a valve plate with an elastomeric seal engaged with the valve seat in a closed condition and movable away from the valve seat in an open condition. The valve plate has a wavy surface disposed below the elastomeric seal that engages the valve seat, wherein the wavy surface is nonplanar with surface variations of at least 300 microns. The wavy valve plate creates a situation where the thermostat operates with two effective modes, a low flow regime for low load conditions where the valve is only traveling between 0 and approximately 1 mm while the wavy plate allows a low coolant flow and a high flow regime (valve fully open) for high engine load situations that require maximum cooling.