Engine cooling systems – Why does my engine have a Dual poppet thermostat?

Every modern engine has 2 circuits for coolant. One is the bypass circuit that circulates coolant through the engine and the heater core. The heater core always has coolant flowing through it, even when there is no demand for heat. If you have an oil cooler and a little circuit to prevent the throttle body from icing up in cold weather, they flow in-parallel to the heater core. Also the coolant fill-tank is in the bypass circuit and is there to allow for expansion and de-aeration of the cooling system (any bubbles of air or steam that go in the top of the tank get separated in the tank and only liquid gets returned to the engine. When you start a cold engine this bypass circuit allows a good amount of coolant to flow throughout the engine to let the engine to warm up evenly and limits thermal stress in the hottest parts of the engine (like around the exhaust valve seats. The other circuit is the radiator circuit with a path that includes the whole engine, coolant pump and the radiator, The dual-poppet thermostat controls the flow between the radiator circuit and the bypass circuit. It completely closes the radiator circuit when the engine is cold (to minimize warm-up time) and completely closes the bypass circuit when things are very hot to maximize the coolant flow to the radiator. If you run with no stat, the bypass circuit acts like a partial short-circuit to the radiator circuit, because the pump can only move a certain amount of coolant. Hope this helps!

Dave