Nothing you did so far is going to affect fuel mileage except for the spark plugs. If the Check Engine light is not on and there are no stored fault codes, look for something causing the need for an increase in power. Most commonly that is a sticking brake caliper. Stop on a slight incline, shift to neutral, then release the brakes and see if the vehicle creeps downhill on its own. If it doesn't, have the brakes checked first.
The next place to start is by having your mechanic connect a scanner that can read live data to see what the oxygen sensors are doing. If there's a vacuum leak, the unburned oxygen will be detected as a lean condition and the Engine Computer will command more fuel to all the cylinders. A single-cylinder misfire will do the same thing. The unburned fuel and air go into the exhaust but only the unburned oxygen is detected, again, as a lean condition.
An exhaust leak ahead of the oxygen sensor can do that too. Between the pulses of exhaust flow, the momentum creates little pulses of vacuum that can draw in outside air. The oxygen in that air gets detected as a lean condition.
Friday, February 17th, 2012 AT 10:45 PM