I purchased mt '97 F-150 Super Cab in January. I put in my K&N filter from my other truck right away. Ran great. I have been having problems with a rough idle while in gear or not. About a month or two ago I put on a Flowmaster 40 muffler with duals out the back. I have been getting 2-3 mpg worse gasmilage than before. It has also been very hot here. Don't know if it is from the K&N, exhaust, heat bad idle or what. I had modified my air box to an open style one. I have changed it back to a closed type with just a bigger tube to the fender and cleaned my MAF. My father had basically the same truck. He added the K&N and started getting worse gas mileage.Anyone know what's up???

------------------
Red '97 F-150 SC 4x4 off-road, 16" rims instead of factory 17"'s(borrowed from old '97 reg cab), BFG 265/75 16 all terrain T/A, K&N filter with modified air box, Flowmaster 40 with duals out back, Expedition center console, 3" rear seat lift, after market stereo seen at HTTP://members.sounddomain.com/red97,

 

After adding on performance parts you are now capable of burning more fuel efficiently. Maybe you are stepping on it a bit more to enjoy the benefits of your new performance capabilities. Try driving like a granny and see what happens.


------------------
2000 F150 4X4 5.4 EFI 3.55 limited slip 4spd auto od In other words "STOCK"

 

I have to agree. The problem with having it is USING IT!!!!
Back out some and see what happens.
If you have one of those tall rears, the loss of torque from opening your exhaust can have a pritty bad effect on milage while driving around town.

------------------
Black 1997 F-150 Supercab Flairside. 4.6L with Jocobs DIS, K&N Generation II, Cat back Exhaust (my own, nice rumble). 17" Off Road wheel pkg. Lots of body mods. 4 yrs old and still turns heads...

 

Could it be since the K&N lets more air in, that more fuel is needed in order to keep the same optimal air/fuel mixture that the computer wants?

------------------
Y2K F250 XLT SC 4x2 5.4L 5sp 4.10LS

 

You might check to be sure there is no air leak between the MAF sensor & the throttle body. This can cause a rough idle & poor fuel mileage.

Jay

 

Hey red97
Have you tried to reset the computer after the performance changes?

------------------
Y2K XLT F150 5.4, 3.55 LS, 4x2, tow package, Chestnut color, Lebra low profile soft top and black westin nerf bars. Runs better everyday.

 

Yes, I have tried driving like a granny, atleast I thought, and I have reset my computer. It ran better, no rough idle, for a while then it went right back. I'm trying a few things. I had put a bigger tube from the air box to the fender. I went back to the factory. In physics, when you draw more air through a smaller tube, like in the intake system in our trucks, the air compresses some and is cooled some also. I'm trying that idea out first along with driving like a granny, like that will happen to soon. My tranny also went through a flush and refill because of a shutter. It now seems to hold first gear out longer even when you granny it. I'm experimenting. We'll see and keep it posted.