Vapor bubbles in a fuel line

What is the symptom or problem? Does the engine run properly? How do you know there are vapor bubbles in a fuel line? Most are made of steel and black rubber. Clear, transparent fuel lines are very rare.

There are two ways you really can get air bubbles in the fuel system. The most common is when a hot engine is stopped, "hot soak" occurs where engine heat is able to migrate upward to sensors and other parts instead of being blown away from normal air flow while driving. That heat can cause fuel to start to vaporize in the lines near the engine, but that is not a concern when you have the fuel pressure regulator on the fuel rail on the engine, and a return line going back to the tank. As soon as the engine is running, the fuel pump moves a huge volume of fuel to the engine, through the regulator, and back to the tank. Only a tiny percentage of that fuel veers off and goes through the injectors. Any air bubbles will immediately wash into the tank with the circulating fuel.

There have been instances where a rubber fuel hose inside the gas tank becomes dry-rotted and allows air to be sucked in. These hoses are between the pick-up screen and the fuel pump. This happened quite often on some Ford models years ago. The clue was the engines ran fine whenever the fuel level in the tank was over half full. The level was high enough that fuel got sucked in through the porous hose, but once the level dropped enough, the hose was exposed above the fuel, so air got sucked in. If that leakage became bad enough, the engine would stall and would not restart until gas was added. A lot of people incorrectly thought they had run out and the gauge was reading wrong.