Hi guys,

Just purchased a 1996 F150 w/ 4.9l. I am currently chasing down a lean condition, but that's not my current question. When I drive the pickup and switch from the front to rear tank (and vice versa), the motor stumbles slightly. Is this normal?

 

well i have a 93 f150 4.9 2wd and mine doesnt skip a beat when i switch tanks.

i am also chasing what i believe to be a lean condition little high idle hard to keep running for the first few mins on cold start and slight off idle hesitation,
replaced
iac
tps
intake gasket
all vacume lines
blocked off egr
seafoamed
there is probably a few things im forgetting but im thinking i have a slight vacum leak somewhere but i cant find it. if you find anything on yours let me know

 

I have the same problem. In my case when I switch to the rear tank on a COLD engine it dies immediately. I can use it going down the road to cruise, but it always sets a lean code. It's because my rear fuel pump is weak, or the ground, or the wiring is chafed. I've not had the time to pull the tank to look yet.

 

Knowing that the tank being switched to has not been cycled since it was the primary tank I always assumed it needed a little bit if time to pressurize so I always let off the gas pedal when I switched. I wouldn't expect both tank's systems to be standing by in full pressurized readiness and keep the throttle open and not expect a stumble. This has been my standard operating procedure on all of my gas F-trucks.

Except the diesel, which has the two tanks filling a common fuel bowl that then is pressurized into the injectors. This one you can change tanks whenever you want without affecting the pressure to the injectors.

 

Thanks for the replies! It makes sense it would need a second to pressurize. I do suspect I have a weak fuel pump or a vacuum leak causing me to throw lean codes. This may exacerbate the stumbling when switching. Again, thanks!

 

If the pump is weak, it'll throw a lean. I never had that problem until the front pump started to go bad and it would do it quite often.

 

I had a stumble when switching, after some time the truck quit running when switching from rear to front, had to cycle the key to on to make the pump run then key off, then key on again to run pump repeat this 4 or 5 times to pressurize system, then start it. After some more time the front pump was dead. New fuel pumps front and rear, now I can switch at 70mph with my foot on the gas with no stumble. I imagine its the check valve in the pump that goes bad and causes the pressure loss that leads to the stumble during a switch, new pumps and all is well.

 

I have had this problem in the past and a new fuel filter always takes care of it. Bought the truck with 74000 miles from my dad and it was doing it when I bought it. It did it again at about 130000 miles and everytime a filter has fixed it. the truck now has about 171000 on it with the original fuel pumps in both tanks. I never run my tanks below 1/4 because I had a Ford mechanic tell me that staying above 1/4 makes the pumps last alot longer. By the way it is a 1995 F250 4X4 5.8L C6 RCLB, and I try to do the fuel filter every 30000.

 

Turns out it was a weak rear fuel pump. So when switching from the front tank to the rear, the rear fuel pump couldn't keep the pressure where it needed to be. Thanks for the replies.