[DJ]

I suspect that your oxygen sensors are defective.

A short story explains why I might think that. Some years ago, my older brother was driving from central Missouri back to central Oklahoma after a deer hunting trip. His Ford pickup was still in warranty. Suddenly, it lost much of its power and gas mileage dropped remarkably. He called my other brother, who has been in the automobile business for decades, and who told him, "It's your oxygen sensors." The rest of the trip, he averaged about six mpg. The next day, he took it to the dealer, who changed the oxygen sensors under warranty. The truck was immediately back to normal, at about 18 mpg.

Now, why would oxygen sensors cause this problem? It's actually pretty simple.

The optimum ratio of air to fuel that is burned in the cylinders is about 14.6 to 1, meaning 14.6 parts of air to one part of fuel. If the ratio is higher, then air is being pumped through the cylinders without contributing to combustion, which wastes the horsepower required to pump the air, and so wastes fuel. If the ratio is lower, then fuel is being pumped through the cylinders without being burned, which wastes fuel. Either way, the result is less miles per gallon.

The engine uses an "MAF", or Mass Air Flow sensor, to measure the rate air flows into the engine. It opens and closes each fuel injector in time with the engine to inject and vaporize fuel into the incoming air at each cylinder. It varies the amount of time each injector is open to vary the amount of fuel each injector supplies, and it does so based on the measured rate of air flow.

So far, that's an "open loop" system. The fuel mixture would be correct if the MAF is calibrated accurately, the fuel system pressure is correct, the injectors are clean, and so on. But, what if things aren't "correct"?

That's where the oxygen sensors come into play. The engine control system senses the amount of oxygen in the exhaust gas at the headers and uses the measured oxygen content to "close the loop". If the oxygen level is too high, then the air/fuel mixture is too lean, and so it opens the injectors for a longer duration. If the oxygen level is too low, then the air/fuel mixture is too rich, and so it opens the injectors for a shorter duration.

This works nicely, as components can deteriorate and yet the system will keep the mixture correct. But, it works only so long as the oxygen sensors accurately measure the oxygen content. If the oxygen sensors measure more oxygen in the exhaust than there really is, then the system will supply more fuel to make the mixture richer. If the oxygen sensors are way off, then the system will supply way more fuel than is necessary, which results in really low gas mileage.

I think that's what's happening in your truck. If it were my truck, I'd replace the oxygen sensors, regardless of whether or not warranty covers it.

Incidentally, I made that same trip to Missouri and back 10 days ago in my '00 Tundra and I got 18.5 mpg.