Boy, where to start. What have you checked so far? Dragging brakes can be caused by brake fluid contaminated with a petroleum product, a sticking caliper, or rear parking brake cable rusted in the partially applied position.
Engine misfires are common and not always felt by the driver. GM has a huge problem with injectors that don't flow the same rates, so a lean cylinder will introduce extra unburned oxygen into the exhaust where it will be detected by the oxygen sensor. That will tell the Engine Computer to add fuel delivery to all cylinders even though three or five of them are not running lean.
Engine not reaching proper temperature due to a faulty thermostat will prevent the system from going into "closed loop", and it will prevent the fuel from vaporizing properly. Liquid gasoline doesn't burn; it just goes out the tail pipe, wasted.
First rule out mechanical and engine performance things, then look at the short and long-term fuel trims on a scanner to see if the computer is requesting extra fuel beyond the pre-programmed amounts or if it sees there's too much fuel going in and is trying to correct that without success. Look at the coolant temperature sensor's reading to verify the engine is warming up, and be sure the MAP / barometric pressure sensor is reporting the correct value.
Vacuum leaks will cause the oxygen sensor to again report a lean condition that the computer will try to correct.
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Wednesday, December 7th, 2011 AT 9:33 PM