Well, we lost engine due to this
Yep, my buddy was flying our Cherokee along, hot day in Arizona but high up. He wasn't paying attention to the fuel gauge, and the engine shut off. Luckily he was high up. Turned on the electric booster pump, and the engine started right back up. We'd had the fuel pump replaced about 15 flight hours earlier, because we were getting no fuel pressure from it. Everything seemed to work great until this new incident. Lately we've had problems when we shut the mechanical fuel pump off. Sometimes the pressure drops, sometimes it doesn't. Checked the tank vents. They're all clear (you can blow into the bottom vent and get air out the top with no resistance). But lately this problems seems to be related to altitude. If you fly lower than about 4000', no problem. But up above that, consistently a problem. This really has got us stumped. Anybody got any ideas why this would be related to altitude when the vents seem to work okay?