acdii Report post Posted January 3, 2014 (edited) Coolant flows in one direction determined by the pump, Water circulates around the engine block and head until the thermostat, which is located at the highest point of the cooling system on the engine itself, reaches its opening point, at which time coolant will flow past it into the radiator. Up until that point the coolant in the radiator is static. The expansion tank, AKA Coolant reservoir, is at the very top of the cooling system, mainly so that air in the system can escape and not get trapped somewhere creating a hot spot. Coolant doesnt flow through this tank like in the rest of the system, but does heat up since heat rises. Since I have not seen a flow chart of the hybrid cooling system, I dont know its path, but on a conventional system, heat does not back flow into the radiator from the engine unless the thermostat is broken, and then you have more things to worry about than a warm expansion tank, like blown head gaskets for a start. When the T-stat opens coolant flows past it into the top of the radiator, depending on type of radiator, top down or side flow, the hot coolant flows through the fin tubes and gets cooled down quickly, with the cool coolant flowing back into the engine from the bottom. That is how all cooling systems work(Hybrid maybe too??). There is also a bypass that routes coolant around the thermostat, its job is to equalize pressures in the block so coolant can flow. Without it coolant would not circulate properly. It is not very big, usually a 3/8" hose is all it is and not very long either. It would take a very long time for hot coolant to get to the radiator through this line, and most of them are at the pump. The theory of coolant back flowing from the ICE doesn't hold up based on the basic design principles. One, coolant doesn't move backwards, two heat rises, and three, coolant is moving through the heat exchanger in the cabin where a good majority of the heat is being extracted from. The only thing that would heat up the coolant is the thermostat opening at some point. Based on the readings I have seen and what HB has reported, it should have not opened, however, what we don't know is the relation of the thermostat to the temperature probe, if they are in the same housing, then, it should never open, but if they are far enough apart and the t-stat is higher , then it is possible it is opening as the temp can be high enough to trigger it. If we look back to when the Fusion was first sold, there was a recall involving the cooling system. It was corrected via a "software" update. This tells me the cooling system is computer controlled, and I go back to my theory that it uses valves to control water flow, and the PCM update had made adjustments to the cooling system too. Without a complete teardown of the cooling system, it is all speculation. I do feel that there is room for improvement too, This morning it was -26, the ICE worked hard to keep up with cabin heat demands, so the radiator should have never seen any coolant flow, but it was too damned cold for me to open the hood and look(didn't even occur to me to do it either until now). The temp gauge on the dash told me the whole story, it was well below its normal halfway mark, and dropped quite often so I had very little EV. It spent a good portion about 1/4 up the gauge. This is with both grills being covered too. Weather channel said -12, they lied. It was even colder than what I took a picture of further down the road. Can't wait until Monday when it is supposed to be colder than -12, makes me wonder, will it be -35? I knew it was damned colder than -12 when I had nosicles walking from my garage to the car port 15' away. I knew it was damned cold when the condensation from the furnace was freezing at the pipe and not on the pile of ice under it. I have never seen it do that, and we have had cold weather before. Edited January 3, 2014 by acdii 2 ptjones and hybridbear reacted to this Quote Share this post Link to post Share on other sites
