Black Exhaust Pipe, Poor Mileage

[Tim Mc]

Help!
I need some help My '99 cylinder, 5 speed, 4WD Taco just isn't responding to tender loving care With 120K I gave it a complete tune-up. Plugs and Filters. I use it as a daily commuter on the open highway about 50 each day. I can't seem to break the 20MPG figure. The exhaust pipe is soot black which makes me feel it is running rich. Would a bad oxygen sensor cause this?? If the sensor is bad shouldn't it be throwing a code??
Short of taking it to the local dealership (Last Resort) I'm note sure where to go next.


Tim

[Tim Mc]

Make that a "4 Cylinder"

Tim

[kathyricks]

The pre-cat oxygen sensor wears enough to degrade fuel economy by 1-2 MPG BEFORE the check engine light comes on. So you are due for one (the post-cat sensor typically lasts 200+K miles and doesn't have an adverse impact on fuel economy so I wouldn't bother changing it).

Review the Engine & Drivetrain forum for throttle body and MAF sensor cleaning procedures. Cleaning could gain you an additional 1-2 MPG.

Using the factory original wheel rims and tire size could gain you another 1-2 MPG
as could using a low rolling resistance passenger car tire like the Goodyear Integrity tires (the tire Toyota uses on the Prius and Chevy uses on it's Cobalt enhanced fuel economy model)

The best spark plugs to use for fuel economy are the Denso's listed in your owners manual especially if you purchase them from a Toyota dealer as they come pregapped to a spec which is optimal for fuel economy whereas Denso's purchased from an auto parts store often have wider gaps which degrade fuel economy.

I'd also look into checking the valve clearances on your engine as they must be within factory specs for optimal fuel economy.