If your Hummer water pump fails, your engine will overheat and break down.  The water pump performs one of the most important functions of your engine.  It circulates the coolant through you engine block, heads and radiator.  When the water pump fails the coolant is unable to circulate and begins to boil inside the engine jacket.  The result is overheating and engine breakdown.  There are three common reasons for water pump failure.  The first is leaking shaft seals, the second is noisy shaft seals and the third is impeller slippage on the water pump shaft.  A sure fire way to find out if your shaft seals are leaking is to look for coolant stains around the vent area.  Noisy shaft seals are the result of failing bearings.  The bearings will wear down and the lubricant on the bearings will oxidize; destroying the bearing.  Impeller slippage will most often occur on remanufactured water pumps.  You can avoid these mishaps by replacing your water pump ever 100,000 to 150,000 miles.