98 with same problem; used to get 425 miles between fill-ups (reset my odo by habit every time I fill up). But now I only get 360-375 tops.

This happened around October when I started hunting down a miss shortly after getting new tires and installing new shocks (the new tires were same size tires/stock, so that should't be it).

The plugs and wires were about a year & a half old along with the fuel filter; tried replacing the wires under warranty, but that didn't solve it. I did not replace the plugs. I replaced the coil pack with an Accell unit just in case...no change.

Since then I've replaced my front brakes, including rotors & bearings and there is no sign of sticking or pulling from the calipers. I have not replaced the rear brakes/drums yet.

My thermostat has just recently broke and is stuck in the open position and until I can replace it, I have cardboard over the radiator to help it warm up in the meantime, but the mileage has not changed for better or worse.

...I'll keep hunting for my missing MPGs.

 

October? That's when they switch over to winter blend gas.

 

Is October when they switch the winter blend fuel? Cause then that wouldnt explain my gas issue. My didnt start till Jan this year. My last tank with some octane booster was 300 and it was purely Highway miles. This is really starting to pull my chain. Im to the point of if i didnt need it for my summer time work i would get rid of it.

 

You PCV valve should be in the throttle body, Yes the throttle body. Seems weird i know but thats where i found mine. Its a heated unit.

 

The exact switchover time varies by region.

Dave, maybe you should go pay the dealer $80 for a full diagnostic.

 

I've been having same problems, only getting 10mpg, alot of city driving and I have a heavy foot sometimes. Truck has 80k miles on it too. Have done MAF and cleaned throtle body, even took it to dealer and had it cleaned. I have replaced plugs since they scorched out. My newest development has been some exhaust rattles under my feet. Checked the shields they seem to be fine and have no movement. Pretty sure it is converter guts. Have been told by some friends that if a converter is going out(or has been going out) that this could mess the O2 reading up and inturn mess fuel/air ratio up. Leaving me with some rough ideling and poor fuel consumption. Also have cracked manifolds and have read that this can suck air in and mess with O2 seasors too. What I want to know is this something that might be worth the money to get new converters and hope its a fix all type thing or do we think something else is going on all together? I figure if I'm getting new converters I'm gonna get a dual catback too, and if I'm doing that I better get some headers. Any ideas here and keep in mind that money is an object to me! Thats why although I love my truck, it is stock as of now.

 

Yeah i have been thinking about paying the stealership for some help, this is beyond me. Just need some extra money to do so.

 

Well when i was outside today something told me to take a look at one of my spark plugs so i tool the number 1 plug out and it was black and smelled horrible of gas. I put those plugs in last Aguhst and they barly have 20000 miles on them. Now this i guess would make since seeing as i had a code for bank one and two running lean when the intake gaskits were bad, and that they were dumping way too much fuek into the engine. Unfortnally i have to wait till i get paid this friday to by the plugs. And yes i know, buy only motercraft. Ran into that issue when i replaced the OE ones 20000 miles ago. Ill update ya'll on the progress.

 

Well ill be damed. The plugs cured the gas mileage. Back to the normal, really irkes me though. Im gona try to post some picks of them sometime. And im glad this battle is finally over. Now if I could just get that stearing gear box fixed i would be in top shape.

 

got a great offer on trade for it, now i have a 2006 FX4, super cab, 4X4, 4.6 V8. more than i really wanted but I got almost half what I had invested in the other one back on trade after driving it six years. Got a fair price on the 06 too.

 

I was sure that fx4 came with 5.4. Guess i was wrong.

Congrats on the new truck!

 

lol, well you think i would know what i bought, It is a STX, sorry, I type worse than I think most of the time.

 

Well, its back to the same old crappy mileage. Dealership time.

 

You said the plugs were fouled and smelled of gas so you changed them? Have you checked them again? The plugs are fouling for a reason.

 

My '99 4.2 w/auto gets about 17mpg hwy, maybe 13 in town - and I drive smoothly and sensibly. Let's face it - these are NOT economy cars!

By the way - stick with Motorcraft platinum plugs - other brands have been known to cause problems.