Mercury 200 OptiMax Jet Drive, Service Manual, Page: 159 / 391

Fuel Pump
Page 3A-6 90-8M0050731 MAY 2011
Anti‑Siphon Valves
While anti‑siphon valves may be helpful from a safety standpoint, they clog with debris, they may be too small, or they may have
too heavy a spring. The pressure drop across these valves can, and often does, create operational problems and/or powerhead
damage by restricting fuel to the fuel lift pump and, subsequently, the high‑pressure fuel pump. Some symptoms of restricted
(lean) fuel flow, which could be caused by use of an anti‑siphon valve, are:
•
Severe fuel rail pressure fluctuation
• Loss of fuel pump pressure
• High‑speed surging
• Outboard cuts out or hesitates upon acceleration
• Outboard runs rough
• Outboard quits and cannot be restarted
• Outboard will not start
• Vapor lock
Since any type of anti‑siphon device must be located between the outboard fuel inlet and fuel tank outlet, a simple method of
checking if such a device (or bad fuel) is a problem source, is to operate the outboard with a separate fuel supply which is known
to be good, such as a remote fuel tank.
If, after using a separate fuel supply, it is found that the anti‑siphon valve is the cause of the problem, there are two solutions to
the problem; either 1) replace the anti‑siphon valve with one that has a lighter spring tension, or 2) replace it with a
solenoid‑operated fuel shutoff valve.
NOTE: If the fuel tank is pressurized, a fuel demand valve is required. Refer to
Section 1D - Fuel Demand Valve Requirements
or
Integrated Fuel Demand Valve Requirements
.
Fuel Lift Pump Description/Operation
The fuel lift pump is an electric fuel pump that supplies fuel from the fuel tank to the low‑pressure boost pump, the water separating
filter, then to the high‑pressure fuel pump inside the VST. Electrical power is supplied to the fuel lift pump from the starter solenoid
positive terminal (starter side terminal).
The fuel lift pump is designed to lift the fuel (vertically) approximately 152 cm (60 in.) using a 7.9 mm (5/16 in.) minimum diameter
fuel hose with no other restrictions. As filters, fitting, and valves are added, restricting fuel flow, the lift capability of the pump
decreases.
a - Fuel filter
b - Lift pump
c - Low‑pressure boost pump
d - Water separating filter
e - High‑pressure fuel pump (inside the VST)
Testing the Fuel Lift Pump Vacuum
1. Separate the engine fuel line and the boat fuel line.
2. Install a T‑fitting on the boat fuel line.
3. Install a clear fuel line on the opposite side of the T‑fitting.
4. Connect the clear fuel line to the engine fuel line.
5. Install a fuel vacuum gauge (obtain locally) on the T‑fitting.
6. Clamp all fuel line connections securely to prevent vacuum leaks.
7. Start engine.
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a
b
c
d
e