[ossiblue]

First, be certain that the problem is with the tank and not with the gas pump nozzle. If you are using an automatic shut-off, vapor capture nozzle, they have a tendency to shut off due to back pressure from any Porsche tank. General solution is to insert the nozzle upside down in the neck, or pull the spring collar back by hand slightly.

Other than the nozzle problem, if your vent (by the sender in the tank) is truly clear, there should be no back pressure preventing filling the tank. There is a small tube in the filler neck (look under the fender at the filler neck) that connects to the vapor recovery tank via a small hose but even if that's blocked, fuel should flow easily into a properly vented tank.