When I first fill my tank up, why does the FP drop? I just started noticing this. When I get below 3/4, my FP goes up to 17 at idle and 13 or so at WOT. But when I have more than 3/4 tank, I get 15 idle and 12.5 at WOT. What gives? Is it because the pump doesn't have to work as hard to pull fuel or something? Fuel filter is brand new and pump wrorks great. Just curious.

 

I am going to bet on fuel Temperature.<br><br>Ever try to cahnge a fuel filter 10 minutes after the truck has been shut off? The Fuel is HOT. Hot fuel, like Motor oil, is much thinner than cold fuel. When you pump fresh fuel in, it's coming out of the underground holding tanks, usually the fuel coming out is around 70*.<br><br>The Lift pump is going to have a hard time movin' thick fuel than thin fuel.<br><br>That's just my guess, as I haven't done any research on the matter.<br><br>Merrick Cummings Jr

 

air bubbles

 

Is the lower fuel pressure with the fuller tank present at both hot (after running) and cold fuel temps?

I wonder where the return line returns to the tank? If the tank is full(er) and return line is into fuel instead of into air???? Just a wild guess.....

 

It only does it hot. Hot being a relative term. Avg. temps here are getting around 90*.