Chevrolet Repair: gas mileage, poor fuel economy, air fuel mixture

Question
hello wayne, i have a 2000 silverado with the 5.3, auto, air, 4x4, latley i have been noticing poor fuel economy. i have no black smoke, seems to perform well, no check engine lamp, i replaced the plugs about a year ago, the air and fuel filters about 4 months ago, the trans, diff, tansfer case, and engine oil levels are all good, tire pressures are good, brakes aren't dragging, not stuck in 4x4, i am running out of ideas as to what it would be, any advice would be awesome, thanks, mike.

Answer
Hi,

 Two of the most common contributors to fuel economy is mis calibrated oxygen sensors and dirty MAFF sensors.  The Maff is a very important sensor responsible for the main fuel calculation based on engine intake air volume, mass and temperature.  This calculation is called speed density and is measured in grams per second.  
 The oxygen sensors are responsible for fuel trim.  Lets say for instance that -15 is lean, 0 is mean and 15 is rich.  If the oxygen sensor senses that the air fuel mixture post combustion is rich it will drive the computer lean until the threshold is crossed and visa versa.  If a sensor is weak or wrong it can play major havoc with the fuel economy.  If you find a shop that has a gas analyizer or at least equipment that can measure CO (carbon monoxide) they can tell you how close to stoichiometric the fuel ratio is.  14.7 would be considered perfect.  13-1 would be rich and 17-1 would be lean.  Air fuel mixtures can be calculated by most 5 gas analyzers in milliseconds.