Look for a vacuum leak or exhaust leak ahead of the catalytic converter. A vacuum leak in a hose but more commonly in an intake gasket will let in air that didn't go through the mass air flow sensor to get measured. Since the Engine Computer doesn't know about that air, it doesn't include that in its fuel metering calculations. The extra unburned air is detected by the oxygen sensor. The computer responds by commanding more fuel to enrichen the mixture to all cylinders. That extra air usually goes to just the closest cylinder to the leak so while one cylinder is running lean, all the rest are running rich. No matter how much fuel is added, there will always be that unburned oxygen in the exhaust. As it keeps detecting that extra oxygen, it adds more and more fuel. That's what you see and smell at the tail pipe, but O2 sensors don't detect unburned fuel; just oxygen.
When there's an exhaust leak, the momentum from the pulses of exhaust flow creates little pulses of vacuum that can suck in outside air through that leak. Here again, the O2 sensor will detect that unburned oxygen and the computer will add more fuel. No matter how much fuel it adds, there will still be that unburned oxygen.
GM has also had a lot of trouble with fuel pressure regulators leaking raw fuel into the intake manifold through the regulator's vacuum hose. If you have that on your truck, pull the hose off and look for signs of wetness.
The fastest way to diagnose this is with a scanner that can display live sensor data. In particular, look at the short term fuel trims (STFT). If the numbers are high positive, the computer is requesting more fuel than normal in a misguided attempt to correct what it thinks is a lean mixture. You have to find why it thinks the mixture is lean. If the numbers are high negative, the computer knows the mixture is too rich and is not having success in trying to correct that. That's where you look for the cause of too much fuel going into the engine. When the computer has lost control of fuel metering, it's usually a mechanical problem such as that leaking regulator, a severely leaking injector, or even fuel pressure that's too high.
Saturday, July 30th, 2011 AT 5:12 AM