[JimD]

Cavitation could be the problem if it's the pump under the hood that is failing. Anybody know? I don't see how cavitation could fail the electric pump at the tank.

Cavitation could still happen to the mechanical pump under the hood if the pressure of the electric pump at the tank is insufficient to keep the gas liquoid at the temperature it gets to under the hood. Pressurizing it will avoid cavitation if the pressure is high enough, in other words. It would seem like BMW could put a big enough electric pump on with the battery being in the back anyway (makes it easier to get enough electricity to the electric pump).

A long crank as a symptom is also consistent with a heat soak issue - under hood temperature too high. Whether it is causing caviation or not, would depend om the damage found. I would expect missing vanes in the pump if the issue is cavatation. Anybody see the insides of a failed pump?

Turbos use the waste heat from the engine to power the turbo. So they inherently run hot. The turbo itself would be the hottest part. Is one of them close to the fuel pump? If so, if there is space, a simple metal heat shield could make a big difference.

The rumors that ethanol in the gasoline caused me to review the properties of ethanol versus gasoline. The latent heat of vaporization (the amount of energy required to turn the liquod into vapor) is much higher for ethanol. But the boiling point may be lower, the table I found gave a wide range for gasoline from below ethanol to substantially above. So maybe the ethanol in the gas could boil at a lower temperature helping the mechanical pump to see vapor.

Jim