Hi. I have a 99 5.4L 2v with a 151,000 miles.Fuel pump went bad,replaced it and now it stumbles and hesitates only when at half tank or lower.When full or at 3/4 runs fine.

 

It may not be from the fuel pump replacement. Have u changed the fuel filter, plugs, wires. Maybe clean the throttle body.

 

do you ahve a aftermarket gas cap?

 

replace fuel filter a week before fuel pump went out. replace plugs and #4 cop at 120,000.have not clean throttle body yet and no aftermarket gas cap.

 

Got a warranty on that pump still? I'd R&R it and get a new one especially if I could get one free.

 

Did you have to install any clamps on your fuel lines? You may want to recheck those or replace the pump again.

 

how does clamps cause the problem I have?

 

bingo (unless it's a faulty pump)


because the fuel in the tank is providing enough pressure to keep the fuel pump feeding enough to the fuel lines and when it gets below a certain level it can't do that...


either way you are dropping the tank

 

I'd check for a pinched evap line or vent tube (caused by R&R of the fuel tank). It could be causing a vacuum lock once the tank gets to a lower fuel level.

 

A fuel pump does not rely on fuel to be making it's own pressure, fuel is just a liquid. I know temperature builds pressure, but no fuel systtem relys on that. That pressure buildup in the tank is vented via a release valve and is far below that which the fuel injection system get's from the pump.

The pump is working, providing sufficient fuel from 1/2 tank and above. Something is affecting it's ability to supply fuel below that level.

Did youi compare pump and pickup depth into tank between old and new side by side?

I'ld suspect a fuel pickup line that the pump pulls fuel from. The part with a sock like strainer on it.

Are there any connections in the pickup between strainer and pump that could allow vapor to be sucked in when fuel level drops below that point or maybe where the pickup attaches to the pump itself?

That's where I'ld look.

Take a drink straw, cut a very minor slit in it about 2" from bottom end. Insert it into a milk shake. Drink milkshake, and see what happens when that slit suddenly comes uncovered in milkshake .... you get less milkshake if any at all. Try it with water even.