After a fuel fuel fill up ( i don't over fill ) when the engin gets hot if seems as if it is beging starved of diesel until the tank is about 3/4 full, then it runs great, I have left the fuel cap loose after the fill up but it still does it, any ideas, please.

 

Hmm, well that's a weird one. I was thinking clogged vent lines, but if it does it with the cap loose then that can't be it. I checked your profile to learn that you have an '04 6.0. Always try to include such important info with your questions.

What does the engine do exactly? Just low on power? Or does it shudder and miss? I'm concerned that the tank level is actually unrelated, but just coincidence makes it seem that way, and that you actually have another issue. Like injectors, FICM, low fuel pressure, incorrect oil, high pressure oil too low, or a bunch of other stuff. Even an EGR valve problem.

I would suggest you try running it until it gets below the 3/4 tank and seems to be running good. Then fuel up WITHOUT shutting the engine down. If it continues to run good with the tank full, then that means it is not really fuel tank related.

Be aware, running a 6.0 with fuel supply issues can damage your injectors very quickly. Don't take too long figuring this out.

And the obligatory question, how long since you changed fuel filters? I have no idea how that could relate to fuel level, but they are extremely critical and should only be OEM filters.

 

I have just replaced the filters and it is not showing any code's, it is as though I am putting my foot on and off the gas pedal but not surging, I am taking it in to the ford dealer on Thursday for a check over, but I would like to have some knowlage before I go.

 

Fuel pump?

 

Surging is often a dirty EGR valve or a sticky turbo. But those won't be related to fuel level in the tank.