Ok, I have a 1987 F-150 with the SD 5.0, I have oversize tires and I know i was getting around 10-12mpg. Well I noticed I have stared using more fuel and now I'm getting 6-9mpg. I was wondering what ya'll thought it could be. I'm thinking O2 sensor? Could it be anything else?

 

your fule is dumping in your front tank ( if you have one)

 

fuel* sorry and by front i mean if you have dual tanks its going in the other tank.... well wait does it smell rich?

 

well I have thought that, because my rear tank(the one I don't use) is full now. And yes it smells rich as well.

 

well then thats your problem the gen 8's with the dual tank have the dumbest set up i actually too out both tanks used a bronco tank in the rear and a ranger tank as the midship (front) bypassed a bunch of the junk and ended up wiring the toggle switch to just turn on the in tank pumps it.... worked sometimes.....
+ full up i could hold like 40 gallons

and replace your o2... should probaly be done anyways

 

how could I stop my front take from transferring fuel to my rear tank? And I plan on changing it as soon as I can.

 

i just pluged it for a temp fix... a tight fitting bolt and a hose clamp