One interesting thing that a lot of folks don't understand, is that a cylinder with a miss, such as a bad spark plug or plug wire, low compression, or some other reason that prevents the fuel in the cylinder from burning, will make the closed loop system go full rich, not lean. Even though there's an excess of unburned fuel in the exhaust stream, that's not what the O2 sensor is measuring. There's also excess oxygen, which wasn't consumed by the combustion process. The sensor interprets that as "lean". The O2 sensor sees too much oxygen, and yells "COMPUTER- - - -GIMME MORE FUEL!"- - - - -and the poor henpecked computer does what it's told!
Jerry
