I have a 2002 GMC Envoy and it looses power randomly between 1/2 and 1/4 tank full. Can only maintain 25 MPH. If you push on the gas pedal it has no power and will slow down more.

If you fill it up with gas it runs fine. Mostly have problems on the highway at constant speed but is not repeatable and does not happen on every tank full.

Dealer replaced the O2 sensor but still having the same problem. They can't figure it out.

Anyone have this same problem?

Thanks!

Two things I'd be willing to venture a bet on. Either a REALLY REALLLY clogged fuel filter (unlikely since it doesn't happen any other time except when you're running below a half tank) or the fuel pump itself is starting to have problems pulling fuel out of the tank once it drops below that level. The problem isn't necessarily the pump, but the way its designed. See when you've got a full tank, the fuel in the tank acts as a coolant for the fuel pump, the cooler it runs, the better since it is an electric motor and very susceptible (sp?) to heat. Once you drop down below a certain level, the pump is exposed to air heats up, dropping its pumping efficiency. I'd be damn near willing to bet that the fuel pump is starting to go south on you.