Have a 2011 Chevrolet Aveo LT, actually have 19 of these cars we use as delivery. So very much acquainted with the craziness of some normal issues. This one has me stumped though, 2011 with 41K on the clock, the car will once a week, peg the temperature gauge, and push all the coolant out of the resevoir. So far I have replaced the radiator, radiator cap, thermostat... Cooling fans are operational during overheat issue, and coolant is flowing. I have performed a block test with the test fluid all of which show negative. Was able to catch the car while doing its overheating spell and performed block test at that point. Fluid never changed colors... The only thing it really leads me to believe is combustion gases are still getting into the coolant either thru bad head gasket or an issue with head or block. Anyone had this issue before?