GFI on a 3 prong plug keeps tripping
Outside outlet possible moisture in the box causing the trip since the pump works in other gfci protected locations and you have replaced the device, there may be a minor ground fault in the wiring and the pump together they add up to enough to trip but seperatly they are ok. The removing the wires and tape has me scratching my head but this would be my guess that there are multiple minor ground faults that adds to enough to cause a trip.