2004 Colorado Fuel Cap light

[fallin22]

my 04 colorado 2wd with 133k miles automatic keeps having the fuel cap light coming on. At first I thought it was because I didnt have enough gas in the tank but noticed it comes on with a full tank. I had a GM mechanic look at it a few months ago and he even put a new fuel cap on it yet it still comes on. ANy suggestions?

[Tech II]

I guess GM called it a fuel cap error, because more often than not, that was the problem....you get gas, and you or the attendant fail to click it properly.....so while it is called a fuel cap error, it is actually a small leak in the system.....

This emissions test only runs between 1/4 and 3/4 of a tank.....what happens is, the purge solenoid will open, the vent solenoid will close, thus creating a vac in the system....when it reaches a certain level, then the purge closes and you have a vac in the gas tank that is read by the pressure sensor attached to it....over a prescribed period of time, there should be no drop in vac.....if a small leak happens, this code will set.....this also turns on the check the gas cap message, like I said, because more often than not, that is the problem.....

However, if the cap has been replaced, and the mating surface of the fill pipe is not rusted, you have a leak elsewhere......the only way to easily find the leak, is to seal the system with a scan tool, and to pressurize it with a smoke machine....where you see smoke, you see the leak....