My 1997 F150 with 5.4L was getting about 10 mpg, give or take. Could generally put about 250 miles or so on a tank. It was also throwing a code that indicated I had a bad oxygen sensor on passenger side, so I figured part of my problem was certainly dumping gas through that pipe. Replaced BOTH 02 sensors. Code has been gone now for about 160 miles, so I figure its gone (when I cleared it before, it would be back within 10 miles).

Filled the gas tank after changing out the sensors. Right now I have about 165 miles on this tank, and I am getting pretty close to empty. Is it possible I am getting WORSE mileage AFTER replacing the sensors? Everyone here suggests changing them to improve mileage.

I don't want this thread to be another "complaining about gas mileage" thread we get here every week, because that is not what I am doing. I am trying to figure why it would go down after changing sensors seems to be the only change?

Other facts - yes, I need to change the fuel filter, but I needed to do that before the sensors too; yes, I probably should change COPs soon, but needed to do that before the sensors; yes, I am likely driving on "winter gas" everyone complains about - but I live in Alaska and it is January, so I am pretty sure I've been driving on "winter gas" for awhile now. I drive the same route to and from work everyday (about 8 miles each way), go about the same speed, +- 5 mph.

I recall some other posts about trucks taking awhile to reset all of its parameters after you clear codes by removing the battery cable for awhile. Could it just be this?

Any ideas on this? This has got me stumped since it seems the only change that has been made is brand new O2 sensors. After this, I will be hard pressed to give the advice of "change your O2 sensors" after this when we do get the weekly "my mpg sucks" threads.

 

you consider cleaning your intake components? maf could be dirty for instance

 

Above 60mph they really suck gas. Tire pressure. You should get 300-360 miles out of your tank .

There's allot of factors that determine mileage. Throttle body/plenum, MAF,air &fuel filters ,exhaust,plugs and yes COP's. A healthy ignition is very important.

BTW - A tuner,efans and under drive pulleys help as well. 4.10's help if you do mostly city driving and of course a CAI is best for converting air to power.

 

not true! i got 19mpg drafting a greyhound at 75-80 lol

 

I have read many posts on here detailing all of the reasons why F150s get crappy mileage, and how to correct the problem. I understand that I need to do additional things to help improve my mileage. 300 miles out of a tank you say jbrew? In my dreams

A couple months before the O2 sensor changes, I cleaned the MAF really well. I still need to replace COPs, fuel filter, do a thorough TB cleaning, etc. I agree, all of those things will improve my mileage. My post, however, is a bit different than that. My TB and all ports were dirty on Friday, my tire pressure was a certain amount, the COPs were there, I used winter gas on friday, it was freezing outside, etc. etc. etc. I rarely, if ever drive over 60 given that most of the roads are icy right now. 50 is about max. But, it was the same before and after the O2 changes.

The only difference I can truly think of between Friday and Monday is that I got new O2 sensors. That's it. At that point, my mileage went from say 10, down to say about 7 or 8. That is what does not make sense to me. I am not inquiring about how to improve my mpg's, as that has been beat to death around here. I am wondering why after changing a specific bad part, a part that is supposed to improve mpg's at least some after replacement, seems to have caused my mileage to drop. Very baffling to me.

 

I've been getting 320 -330 before my light comes on. I have eveything polished inside tho..Plus all new sensors w/ custom intake ,headers and High flows..

 

Any idea how much is left in the tank when the light comes on?

I am going to change plugs/COPs (they're sitting in my garage waiting to be done), fuel filter (also in the garage), clean TB/EGR/IAC stuff, and see what happens from there. I may do an underpulley and e-fans, we'll see what the budget allows and if it will make much difference.

But I am still baffled by this --> change both O2s, and fuel mileage immediately drops.

 

I wanna say about 3 gallons or 30-50 miles..

Give it a little time and check again. You might want to fill up somewhere else and check mileage again. Winter gas stinks .. Plugs open up with age as well . Like I said, there's allot that factors your mileage . You've done good replacing the O2's , but think of it as just a piece of what you need to do to for maximum mpgs. I bet when you get that ignition system tuned up , you'll see a noticeable difference in mileage if you stay off throttle. Discipline discipline discipline..

 

Check for vacuum leaks, they will cause a lean condition and the computer will overcompensate and start dumping fuel and could foul your cats. I was getting 10.1 mpg and after fixing 2 vac leaks and one vacuum hose collapsing in on itself I am now getting 13.3. I still have a CEL for b1/s1 O2 slow ckt and an intermittent misfire on #3 cyl, hoping to break 14 when I got the O2 and COP replaced.

 

That's one fast hound !! Was she trying to get away from yuh? She in heat ? She mustuh been a heffer for you to net that kinduh mileage.

 

Not throwing any codes anfo. The only code was the one being thrown by the bad O2, and it is gone now. The truck runs fine, no misses (except under load --> COPs), so I don't think there are any "problems" necessarily, just some routine maintenance I need to get to.

 

a small vacuum leak won't throw a code and will decrease your milage big time. Also, why would you change your COPs? they are coils. Coils are copper wire wound many times around a core. They either work or they don't.

 

Thats what I was basically trying to say, the O2 is the only code I had and its not related.. in that, yes, the O2 is bad but not the cause of my milage drop off. The vacuum leaks (which didnt cause a code) caused a lean condition (the ford tech measured this in a diagnostic where he hooked up a monitor and drove it around, I didnt have lean codes), the computer detected it going lean and "dumped fuel" to compensate (his words). This caused my sensors and cats to get all fouled up and is what killed the O2, the dealer tech fixed the 2 vacuum leaks and I found the 45deg. elbow under the heater hoses was mushy and collapsing in on itself under vacuum and replaced it. They refused to do the O2 sensor until I drove it for a week (they said to drive hard to clear it out). I filled up on the way home and I got ~12 on that tank and 13.3 on the last tank (before bringing it in I was gettting 10 and just thought I had a bad O2). So, to boil it all down, a vacuum leak will cause a good drop off in milage, just something to check that wont cost you a penny.

 

That's a good idea what anfo is suggesting, I didn't think of that. You know, the easiest way to check all those lines is with a cheap vac tool. $20 on up. I just have a cheap one I purchased from Auto Zone. You can apply vacuum and pressure for a leak down or leak tests. I do that after shrink fitting a new connector to make sure it doesn't leak..

Yea , that's a good idea to run through those lines. You can check your reserves as well - You have two behind the battery ..

 

I didn't see in the thread if you brought the truck in a garage or somewhere warm? I would dump a bottle of Isopropyl (Gas-line anti freeze) in the tank, I haven't had this happen with the truck, but many times with the sled and the quad, both fuel injected four-strokes. They seem to run o.k. but the milage just isn't there(yes I pay close atttention to the sled milage, so I know how far I can go when there's no gas station around) It's cheap and worth a shot. Make sure you use Iso and not Methyl Hydrate, because Iso actually absorbs the water versus the Methyl just turning it from ice to water. Or it maybe just your cops have gotten worse and it's a coincidence. Good luck Either way