Abrupt drop in mileage!

[plumbob]

Hey everyone nice site. My story is that I have a 1997 Chevy full size extended cab 4X4 with a 350 vortec 5.7 liter V8. I drive it around 900 miles a week to work and back. For the first few months I was getting around 16 mph and about a month ago it dropped suddenly to 12 mph. I filled up and checked today and it is now below 12 mph. Since I have had a fuel pump put on, then I gave it a tune up with AC delco plugs and wires, and this weeekend I put two oxygen sensors onto it. I should say that the service engine soon light is on and it is showing misfire but the truck does not misfire. Does anyone have any ideas that might be my problem because the cost is getting unbearable. The truck has 144000 miles on it and I give it frequent oil changes with high mileage pennzoil. Also when the fuel pump went out I thought it could have been my air filter which is a K & N FIPK kit so I took it off and blew it out with an air hose. Since I figured out that was wrong but I don't think this could cause such a sever decline, am I wrong? HeHe one more thing, the truck was fine until the fuel pump went out.

[cautionwatch]

Well, you probably got it fixed by now, but if not, check to see if the catalytic converter is plugging up. I have had this happen, and the vehicle got to the point where it wouldn't run anymore.

[Mopar JunkE]

Maybe your chevy is losing its mind but yeah, check your converter and add a bottle of SeaFoam and or Valvoline's Complete fuel system cleaner. you might have dirty injectors and intake valves. and if thats not the problem you might have to spend some money. air intake, higher quality plugs and wires. better cap & rotor. synthetic oils in engine, transmission, and rear axle. run with higher tire pressure. my truck was getting around 15mpg new. i switched to full synthetic motor oil and full synthetic ATF and put in some nice champion spark plugs, now i get around 19mpg. a build-up of the simple things makes a difference. good luck.