Ok last winter is when I started to keep track of my mpg. I have a 97 f150 supercab flareside 4x4 with the 4.6 and 3.55ls. I have 99k miles on it now. Last winter I was getting around 13-14.5 mpg in 50/50 city/highway driving using super. That was without my chip installed and only an airfoce 1 intake.
Over the summer I would get 16mpg with intake, chip and flowmaster true duals.

Now this winter. I have my af1 intake, chip installed and true dual magnaflow exhaust with x pipe. I have been getting 10.5 mpg with the same 50/50 driving. I know the winter gas is different but i can't figure out why i've dropped nearly 3mpg. Driving habits are the same. Truck is well maintained. I have a clean airfilter on my intake, actually brand new. I've kept the maf, temp sensor and iac valve spotless. I've cleaned the tb every 6k.
Front 2 O2 sensors are brand new, fuel filter was changed in august the same time as the o2 sensors. pcv valve as just changed today.

What else could've caused this sudden drop off? I've though about clogged cats maybe. I've got highflows waiting to go in.

I cannot deal with this.

 

do this-i checked my cats this way- buy a cheap early 80's non heated 02 sensor-18mmx1.5
bust sensor of end and duill through it ,cut wire off, tig weld or braze steel or brass pipe thread 1/8'npt to it- get 1/8" npt to 1/4 compression fitting- attach this to 1'of 1/4"copper-on other end put 1/4" compression to 1/8" barb fitting- run this via 1/4 vacuum line to pressure gauge-0-30 psi [2" round" in cab max psi just after y is 7psi
measure in 2nd gear while driving- there is hardly any backpressure in neutral-if each side is 4psi or under while in gear up to redline- they are fine- on my 03- they were 2psi on each side- i spent maybe 15 bucs total versus the 70$ that jc whitney charged for the kit-phil

 

when was last time fuel filter was changed- i changed mine with only 9k miles and throttle response improved IMMENSELY!

 

who did the chip?

I would pull the plugs and take a look, while I am looking at them, I would clean the maf sensor and reset the computer.

JH

 

Ok im goin to give a rundown of fixes.

both front o2 sensors aug. '03
fuel filter aug. '03
New air filter for airforce 1 intake aug. '03
New plugs/wires Dec. '02
New coil packs May '03
New pcv valve 3 days ago

I actually just cleaned the maf last night and it did seem to help alot. I got 95 miles on the first 1/4 tank of gas with the new much heavier and larger tires. I used to get 100 on stockers.

TB and elbow are spotless that I can see.

Only thing i do need to do is an alignment. Other then that i can't figure out how it dropped nearly 3mpg.

Last wintes mileage was all done on just an intake. I had the chip out and still had stock exhaust.

 

what about a bad fan clutch?

 

When you did all the mods/work did you reset your computer by disconnecting the negative battery cable for at least 10 minutes?? The only reason I'm asking is b/c I've read many posts where the person didn't reset the computer and their trucks ran like crap. Try it, if you haven't, it could help things. It did for me.
Good luck

 

I've reset the computer everytime. It even happened back accident. I had a loose negative battery cable.

 

pull the chip out and see where the mpg goes.

Thats where i would start.

You don't have anything that requires teh chip, so you can just pull it out (different maf, etc)

JH

 

i am having the same problem with my 2002 with the 4.6. it is going to the dealer when i gets back from the body shop.

 

Winter gas ,warming the car up longer ,air intake temp adjust computer, tire pressure,injector pulse width changed ,all of the above has effect on the change your seeing.

You said you changed to bigger tires how much bigger from stock, what have got now for an end ratio .if you changed the diameter of he tire greatly this will effect above also

Chk it when it warms up around the northeast

Rich

 

OH, MUCH LARGER AND HEAVIER TIRES. Did you re=calibrate the speedo for that modification? I lost 10 miles for every 100 miles driven and the speedo is off by 6 miles an hour(95 dollar ticket told me so) . I believe this is where your problem is and a speedo shop or Fords can re-calibrate the computer to your TIRE SIZE. Do a search here and find speedo calibrations. Let us know and good luck.

 

ken payne at www.motorhaven.com

 

its not the tires. I've had this problem before I went to larger tires. But cleaning the maf has helped out alot.

 

well i filled up tonight and after converting old miles to actual miles i got 11.5 mpg. Not bad. Im a little happier but still not satisfied.