the real ess

Standish, something is wrong with your truck. My '99 4.2L. manual 5 sp. gets between 17-18 mpg, mostly highway driving 42 miles each way to work & back. That's why I bought it, and it was the only non-white truck on the lot. It sounds like you've done everything except change the coil pack (probably not the problem unless the motor is noticeable skipping) & having an exhaust man check the catalytic converter. If it's coming apart inside, it can clog. I'm not sure without going outside in the cold & crawling under mine if you have an O2 sensor behind the catalyst or just in front. If you have none behind, it wouldn't give you a code.

A friend told me his EGR stopped working & it got awful mileage. He disabled it & started getting around 20 mpg. I guess it was never getting out of EGR mode, recirculating gases until it got warm. I don't know how you disable it.

Have you put big or little wheels/tires on? Carrying a load in the back? Driving 100 mph everywhere? Changed the rear gear? Done any performance upgrades? Are you driving in snow? Convert to ethanol?

aerosurfer

Also Try disconnecting the battery for 15-20 mins, with all the recent maintenance being done the computer may need to be reset to allow it learn the new changes, also check to make sure your tire pressures are up to par. Have you tried any snake oil...ahem fuel system treatments? Winter driving sucks on economy, but I had a 98 4.2, and used to see 17 or better all year round.

Younggun

If I do all city driving and really short trips mine will get about 14, however when I get on the highway I have gotten up to 22, averaging 19, the automatic wont help though, also if its four wheel drive and a hub is locked it will cause it to decrease.