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My Mercury 20-HP electronic fuel induction (EFI) engine [of unspecified model year] began venting fuel from the vapor-separator tank (VST) in 2024, something not noticed before. I discovered this from the smell of gasoline and a dribble of fuel under the cowling.

I removed the two side panels below the cowling. When the engine was running steady stream of gasoline is spewing from a vent line that comes from the top of the engine. Started it for the first time this year and noticed a strong smell of fuel and then noticed a dribble of fuel coming from underneath the cowling. Based on a diagram, the vent line appears to be coming from the vapor separator tank (VST).

Q1: what condition would cause fuel to spew from the vent of the vapor-separator tank?

Q2: how can that condition be remedied?

I will appreciate your help.

--Joel in Annapolis.

Generally in any system where there is electronic control of fuel induction, the flow of fuel is controlled by some sort of electrically operated valve or sprayer, and that part of the fuel system is maintained at a positive pressure by a pump. The duration of the opening of the valve or sprayer and the pressure of the fuel supplied to that valve or sprayer determines how much fuel is delivered to the air induction path to the combustion chamber.

Also, typical in this type of fuel system, the fuel is circulated at a regulated pressure, and is routed through a tank order to cool the fuel (by having cold seawater pass through the tank as a cold source) and to expel any air in the fuel, often called a vapor-separator tank or VST.

Typically the pressure of the fuel in the pressurized portion is regulated by a pressure-regulator device, and the volume of fuel in the circulating system is maintained at a certain level by a float switch in the VST that operates an electrical switch. The fuel pump runs until the float switch signals it to shut off.

The first thought for having the vapor-separating tank overflow: there is too much fuel or too much fuel pressure in the pressurized portion, more than is being consumed by the engine in its operation, so the fuel is overflowing at the VST.

A simple GOOGLE search found a presentation on youTube from HUEY Marine in Australia in which a similar problem was solved by replacement of the fuel pressure regulator assembly which appears to be part of the VST assembly

See: https://www.youtube.com/watch?v=Wheturv4Q6w

Another cause could be a stuck float in the VST or a bad switch in the float switch mechanism.

Sometimes a stuck float can be unstuck by giving the tank in which the float is immersed a sharp rap with a wood handle of an old screwdriver. The jarring impact may cause the float to unstick. If the problem is chronic, the VST may need to be disassembled and the float inspected. In some instances the VST may be considered an assembly and may not be available as individual parts. In other cases the VST may be possible to disassemble, and a stuck float might be able to be cleaned and made to work properly again without needing to replace the entire assembly.

If this is a recently made 20-HP engine, it was probably made by Tohatsu and just painted black and sold to Mercury. So you might find more about this problem by also searching for 20-HP Tohatsu EFI VST tank venting.

Thanks so much for the reply. I'll try your suggestion.

Upon closer inspection and cross referencing the diagram, I was mistaken on the nomenclature for the tank. According the diagram, the fuel is coming from the Vapor Recovery Tank which is on top of the engine and is the next component downstream of the VST which is lower and connected as to the engine on one side. Thanks again. Hope it works.