Your Hummer fuel pump does more than ensure that the fuel is pushed through the combustion system -- it maintains the proper pressure levels throughout the entire fuel cycle. After all, it's easier for pumps to push liquid through a line than to pull on it. The higher pressure levels keep the fuel's boiling point higher, which reduces the opportunity for vapor lock. A pressure regulator ensures that the levels are up to spec. To test the pressure regulator, pinch the return fuel line to the fuel tank to close it. Check the pressure on the line; a bad pressure regulator will boost the fuel pressure briefly when the return fuel line is blocked. There is also a separate 'rest pressure' that the fuel system maintains when the engine is turned off. If the rest pressure is not up to spec, you will notice that it takes longer for your engine to crank when it starts. A clogged fuel filter will cause excess pressure in the system, which will strain your fuel pump into working overtime. When you finish installing the new Hummer fuel pump, make sure you check the pressure levels against the manual's specs in every segment of the fuel line.