Question

In a fuel-injected outboard, what is the specific role of the vapor separator tank (VST) regarding fuel pressure and air removal?

Accepted Answer

The vapor separator tank, or VST, functions as a fuel reservoir that stabilizes pressure and removes air or vapor bubbles before fuel reaches the high-pressure fuel injectors. Modern outboard fuel systems often experience fuel heating, which causes volatile fuel components to turn into gas bubbles, known as vapor lock. This vapor causes engine stalling or erratic performance because injectors are designed to spray liquid fuel rather than compressible gas. Low-pressure fuel from the primary lift pump enters the VST, where a float-controlled needle valve maintains a consistent fuel level. By providing a vented chamber, the VST allows any trapped air or fuel vapor to rise to the top and escape back to the main fuel tank through a return line, ensuring only liquid fuel remains at the bottom. Inside this tank, the high-pressure fuel pump is submerged in the liquid fuel, which serves two purposes: it ensures the pump remains primed with liquid to prevent cavitation, and it helps cool the electric pump. By maintaining this steady supply of pressurized, bubble-free liquid, the VST ensures that the fuel injectors receive a constant pressure, which is critical for precise fuel metering and efficient combustion.

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In a fuel-injected outboard, what is the specific role of the vapor separator tank (VST) regarding fuel pressure and air removal?