You would notice bad plug wires as a rough running engine, and the Engine Computer would detect the misfires, memorize a diagnostic fault code, and turn on the Check Engine light.
Is it possible the cold winter temperatures are responsible for the lower fuel mileage?
Your mechanic can connect a hand-held computer called a scanner to read the short and long term fuel trims. If those numbers are high positive, it means the Engine Computer is requesting more fuel. If the numbers are high negative, the computer is trying to reduce the amount of fuel entering the engine.
One thing that can be hard to find is an exhaust leak before the catalytic converter. In between the pulses of exhaust gas, the momentum creates a tiny pulse of vacuum. That vacuum can draw in fresh air. The oxygen sensor will detect the unburned oxygen and the Engine Computer will mistakenly add fuel in an attempt to stabilize the mixture. No matter how much fuel is added, there will always be that unburned oxygen. Oxygen sensors only respond to oxygen, not fuel.
Friday, March 12th, 2010 AT 2:47 AM