I have a manual 1990 F-150 with a 4.9L V6. I drive it everyday with no issues unless I try to drive somewhere far away. After about an hour of driving on the highway when I get back onto slower roads my truck seems hesitant to accelerate. Sometimes it is bad enough where I can no longer use the tank I was driving with untill i let it sit for awhile. However, since i have dual tanks, the other tank still works normally. Any ideas what might be happening?

 

This is the case no matter which tank you start out with? If so I would try changing the fuel filter.

 

It sounds like only one tank does this? If so, I would think that the in tank pump is starting to fail.

The 4.9L is an inline 6 not a V6

 

@My4Fordtrucks and @88n94 thank you for your responses. I only drive using the rear tank because the front tank doesn't drive as nicely as the rear tank. So i am not sure if the front tank would do the same. I have recently replaced the fuel filter as well. If it was a fuel filter issue it seems like neither pump would work once the issue began because there is only one fuel filter for both tanks. Could it be as simple as a failing pump even if this only occurs after driving for extending periods of time and all other times it works great??

 

More than likely the tank that works normally has a bad fuel module in it, bad valves in it.

 

@subford is there a way to know that this is the issue aside from replacing the module and hoping for the best?

 

@subford is there a way to know that this is the issue aside from replacing the module and hoping for the best?

 

While running on the not so good tank you can keep checking the fuel gauge for the other tank and see if it is going up toward full.

 

@subford I'm not sure I understand what you mean...there is only one gauge that toggles between tanks depending on which one is selected for use. Also what would it mean if the gauge was goin up toward full?

 

If you check the gauge for the rear tank and then if you run on the front tank for a while then check the rear tank and the gauge is higher than before it means the valves in the rear tank are not holding. This is reducing the fuel pressure and making the engine run lean and starved for gas.

This is also call fuel cross-flow.

 

@subford is there any connection between this and that fact that it only seems to have issues after driving far distances?

 

No the distance you drive should not make any differnce.

 

If you start out with a half tank in front & a half tank in back, then you drive 50 (or ever how much to notice) miles (regardless of which tank you were using to start) then check the level of the tank you WEREN'T using & its got MORE that a half a tank...

Then you have cross flow between tanks caused by a faulty shuttle valve in one of the tanks.

It is the most common fault of the 2 pump set-up of the 92-96 OBS trucks.

Subford is the best here in explaining why & how.

 

I have never noticed that. But if it was happening how would that affect my ability to drive off one tank unless it ran out of gas?

 

Subford answered this already.

if the check valve is faulty in one tank, then the other tank is bleeding off its pressure into the faulty tank vs going where it's supposed to; to the injectors. The drop in fuel pressure reduces power and drivability. This may or may not be your issue.

Check fuel pressure.