[Tommy]

What follows is mostly theory rather than practical help. But maybe it can help you sort this out.

The temperature at which gasoline (a mixture of chemicals) boils varies according to what the producing refinery made. For example, some parts of the mixture may boil at 140 F and other parts at 200 F. If any part boils in sufficient quantities, it can produce enough bubbles to cavitate a pump.

Let's take a ride with a drop of gasoline starting in the tank. It's a hot day so the gasoline in the tank is already a warm 95 F. The electric fuel pump is running and producing heat that is dissipated into the gasoline. After a little while, the temperature of the gas in the tank is 96 F. As the drop approaches the warm fuel pump, it gets even hotter as it is sucked in and pushed out by the pump at 150 PSI of pressure. Raising the pressure significantly raises the temperature at which gas boils, so the fuel is comfortably in its liquid state.

On the way to the engine, the fuel in the line may pick up more heat from the hot road surface radiating up, or hot engine or exhaust components. It might also lose a little heat if the air around it is cooler than the gasoline in the line.

Arriving at the pressure regulator, some of the gas is allowed to flow to the FI system to be immediately consumed, while the rest is sent back to the tank, picking up heat much like it did on its way forward. Arriving back at the tank, this warmed gas mixes with the rest of the gas, warming the overall mixture to 100 F. And then the process begins again.

The point of this Disney-like description is to say that the pressurized fuel in the FI system is not likely to vaporize and be the source of the problem. The temperature of the fuel at the fuel pump likely is the problem. And the source of that problem may be heat that the fuel is absorbing during its round trips from the pump to the pressure regulator and back. If I were working on your problem, I'd look for ways to insulate the fuel lines wherever they are near hot things like roads, engines and exhausts. And if the fitting for the return line is close to the engine, maybe you can move it to a cooler place.