F250 , 1997, 460. Put new plugs, cap, rotor, wires, o2 sensor, air cleaner. Was getting and always did around 11. Now only 4.5-4.9 mpg. No check engine light and no codes . Fuel isn't leaking nor transferring to other tank.

 

Welcome to FTE.

Next door kid helping himself at night? They do that now and then.
You did plug the under hood sensors back in? An unplugged mas airflow sensor
will keep you in an open loop default. Double check your wires are seated and
on the right plugs. How does you oil look? That fuel is going somewhere.

We need more info. Why change the O2 sensor? Is it running worse or the same?



Sean

 

It all started after simple tune up. Mechanic hooked up to machine and ran truck for over an hour, did not throw one code. Drove it and o2 temp was erratic. He blamed it on kn air filter being clogged. That was not it. He then changed out o2 sensor. I filled it up, drive 38 miles at avg of 60, and it came to 4.85 mpg. Everything is hooked up, engine running great, oil is due to change but not smelling of gas.

 

Did you change out the O2 sensor and put the K&N back on?
If so, clean the sensor and install a dry paper filter.
That fixed a crappy running car I had.

 

Yes new o2 sensor, old one doesn't appear bad at all. And put in a new paper filter with factory box.

 

The oil in K&N filters have been known to contaminate the mass air flow sensors also. When he checked the codes did he reset everything ? Did he disconnect power to the battery when he did the work ? If so, it'll take a while for your vehicle to recalculate the mileage to an accurate reading, you may just need to drive it for a while.

 

The truck runs fine, under acceleration no problems. I will look at replacing the MAF, I can Easily swap it with one from another truck.

 

I used to have a 77 Ford E-250 4X4 van with a 460 in it, I was lucky if I got 7 mpg. It would take a full tank of gas to drive 140 miles. Needless to say I don't have it anymore.

 

Don't start throwing parts at it. The mechanic sounds like he already did that.
You have to investigate the problem. Do you have access to a
tool that will read live data so you can do some diagnostics?
Being a 97 you may not have too many options for live data. But that is
where I would start.

Sean
NOTE : Just a thought. Check your brakes. If you have a jack get the tires off the ground and spin
them by hand. Dragging brakes will kill your MPGs.

 

I got to admit I kind of miss the old van sometimes though, I had some pretty good times in the back of that old beast.

 

Doing what??? Waiting for someone to bring you more gas? Although that kind of mileage is typical for a 460 ford it still amazes me that they could push that much gasoline through an engine without an external explosion LOL!......... I had an 85 GMC 4x4 with 305ci in it that I couldn't get over 8 mpg with so I kinda now how you feel....I say kinda only because you had a gas guzzler with HP, where as my 85 GMC wouldn't pull a sick 'lady of the night' off a 'slop jar' I NEVER miss that truck.

 

Moved to the correct forum.

 

you don't hear of 'em getting this bad that often.
if you got 11 mpg right before the tune up,then just start tracing out everything that was replaced.somethings got to be real bad or real wrong.for example,since it's less than half the mpg the truck should have,you need to check for loose wires or perhaps one or more is broken/defective or perhaps plugged on the wrong cylinders.double and triple check the firing order.don't have the old parts to start swapping back on do ya?
look the cap over real good.could be cracked.even new doesn't matter.you know most crap is all made in China these days,even good branded stuff.something like this has to be up to cause 4.8 mpg lol.i mean really.it's got to be something real wrong.

 

How are you calculating mpg? The exhaust should stink like fuel for it to be drinking that much.

 

Pull the K&N and clean the air sensor. Drive it and see if you have any difference. As someone else said, those oiled filters can contaminate an air sensor. Might not be the whole problem, but it seems like a good starting place.