Exhaust valves operate in a very harsh enviroment. The temperature of an exhaust valve in a petrol engine operating under favorable conditions is around 750 to 850 degrees centigrade. As the normal combustion temperatures are around 2000 to 2500 degrees centigrade any increased combustion temperature will heat the exhaust valve to above what it was designed to operate at. Inlet valves in the same engine have the advantage of the cooling effect of the intake air. When the inlet valve is open and off its seat it is subject to cool intake air. When the exhaust valve is open and off it's seat it is subject to combustion heat. If any problem exists such as weak valve springs, tight tappets or worn guides the cooling effect that takes place as the exhaust valve is seated will be reduced and the valve will overheat and burn.