Greetings,
I have a 1993 Ford F150 w/a 4.9 EFI 5spd, dual tanks. This truck had been sitting for at least ten years and was parked due to a shredded rear diff. I swapped that out, changed the oil/filter, did a once over checking everything..put fuel in the tank that didnt stink like sour fuel...the front tank, and drove off. I was half way home 60mi away when my wife told me it smelled like something burning...sure enough the damn orchard rats had stashed all their nuts and crap under the plenum and it was on fire...luckily i had a half bucket of water and doused it without any damage except the hard vacuum lines that run behind the engine melted..so i patched them with rubber lines. It ran pretty decent the rest of the way home...considering i was burning off a little crap fuel but it was diluted with fresh.
Since then it began to act as though it was starving for fuel..and I made the mistake of hitting the tank switch momentarily. So, I was at a friend's 50mi away from home..I changed the fuel filter. It continued to struggle..it idles great and will all day but try to drive and it bucks and stumbles.
I then siphoned off the sour fuel and put ten gallons of fuel in the rear tank.
I started driving and it started to stumble...so i switched tanks. The rear tank is not working and the engine immediately quits...BUT, after that, while still rolling, I switched back to the front tank and it ran great...for a while.
Upon driving it the 50mi back home..it would start starving for fuel, or seeming to, but it would happen like a switch was thrown...while rolling and in gear i switch from the front to the rear tank...open the throttle and then switch back to the front and it drives perfect again. Sorry this is so long. ...whatdoya think Ford peoples?

 

Bad gas is nasty stuff. Did you know it can gum up valves so they get stuck?


Time to drop the tanks, pull the sending units and I think they have pumps on the ends. The fuel inlet of the pumps or fuel pick up has a filter sock. This could be plugged up and when you stop pulling from that tank the crud falls away from the filter and you can run again till it happens again.


Drop the tanks, clean the insides and if they show a lot of rust replace them. Also bad gas can coat the tanks sides. This can fall off in sheets and cover the fuel pickup and cause the issue you have.


It might be best to get new tanks, pick ups/pumps and the switching valve as it may be gummed up too.
Dave ----


edit - welcome to the forum