I have been consistantly getting 12.5 to 13 mpg in town or on the freeway for the past several months, if this isn't bad enough this weekend on a 200 mile trip I got about 11. I used to get around 15 in town and 19 ish on the freeway. This is with a 1997 150 4x4 4.6 auto. The truck has 58000 on it and I put new plug in at 50000, and change the oil and filter every 5000. I have a MAC intake and a flowaster 50 muffler. Does any one have any ideas as to the drop in fuel mileage.

 

clean the pis$ out of your throttlebody! Don't forget to unplug your MAF sensor before you try cleaning it. Disconnect your battery for a couple of min and put everything back togeather. This will reset your computer and it will have to relearn the fuel air mix.

 

mine is doing the same thing with 93000 and check egine light on. gas mileage is around 8 or 9

 

I have had the same problem with my 97 4X4 w/ 5.4, but the gas milage has consistantly been in the neighborhood of 11 to 12.5 in the city. I don't drive hard, but it is the city. I have 114k on it, so I think my next step is O2 sensors

 

So I finally cleaned my throttle body. It was pretty grimy so it needed it. What I did notice when I took off the throttlebody was a 1/2"x1/2" blob of carbon build up coming out of the passanger side port of the EGR. I removed the blob and then took off the intake plenum and found out that that port was clogged full of carbon build up crap from the EGR. I cleaned it out as well as I could and re-installed. Suspected the ERG valve might be clogged due to a lot of post revolving about it and the plugged port. Pulled off the EGR and it was working fine. Reinstalled the EGR valve and the truck is running better, but not sure if the gas mileage is any better yet.
My next question is how the heck did that EGR port get clogged like that and why on only one side?