Gas Mileage

[gull]

You may need to replace the o2 sensor. If it goes bad, the fueling defaults to full rich.

This also happens if the wiring harness fails (BTDT). It's hard to miss, though. You get black smoke at idle once the car goes into closed-loop mode. The idle also goes to pot, cycling up and down, but that symptom may not be there if someone bumps up the idle speed to mask it. If engaging the full throttle switch makes the car run better, this is another sign that the O2 sensor may be suspect -- the full throttle switch temporarily overrides the O2 sensor.