I've got a 1997 F150 4X4 with a 5.4.and auto trans. K&N filter kit and opened up exhaust with a dyna flow muffler. 103,000 miles. I bought the pickup 5 years ago with 70,000 miles on it. fuel mileage was about 18 mpg. on the highway. Now my fuel mileage has dropped to about 13-14 mpg. I've changed the fuel filter,put in new plugs,changed the coils and the mileage is still 13-14mpg. Anybody got any ideas on what might be the cause of drop in fuel mileage? I've done everything I know to do!!

 

Here are a number of reasons your mileage is lower.
1. Cold temperatures are detected by the Air intake sensor. This richens the fuel delivery..
2. The gas formulation is different in winter than in summer, from the refineries.
3. The engine cold starting and high idle take more fuel.
4. Are you idling the keep heat in the cab more often?
5. Little known is that as you run down the road, the air inlet temperature the IAT sees stablizes colder and adds fuel to the motor as opposed to slow running where the intake tract get the effects of engine bay heat more and leans out the fuel more.
Basicly, cold air is more dense and needs more fuel to keep the ratio in the proper range.
Add all these togather and you can get quite a difference that you can't do anything about.
Good luck.

 

Thanks Bluegrass 7, I was hoping it was going to be something simple that I overlooked. Oh well, it"s been a great pickup so I guess I"ll take the bad with good! Thanks again.

 

Clean your intake and valves. Run some seafoam through the vacuum hose on your brake booster and that will make a big difference. That stuff gets dirty in there and if it doesn't get cleaned out then your engine won't breathe. Just be careful and don't let it suck to much at one time. You should notice white smoke coming out the back. The best way is to take a dropper and let it run a little at a time run about half the bottle. then drive the **** out of it to blow that crud out the back.

 

I know how you feel. when I first got my '99 F150 4.6, the gas mileage was up around 17/18. 4 1/2 yrs and 40k miles later it's down to 14/15. it's not worth spending all that money changing the fuel filter, new plugs and the coils to pick up a couple of mpg. it runs too good to do anything to it, I'm just gonna drive it like that.