Anti-Siphon Valves; Testing Fuel Lift Pump Vacuum - Mercury 25 EFI Service Manual

Troubleshooting and Diagnostics
Function
Start up
Engine over temperature
Low oil pressure
Engine overspeed
Water temp or MAP sensor
out of range
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 shut off valve.

Testing 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.
8. Observe clear fuel line for air bubbles.
1. Stop engine to cancel warning notification.
Page 3B-6
Warning System
Sound
One beep for 2
On for 5 seconds Normal system test
seconds
Continuous
1.
Continuous
Continuous
Intermittent short
1.
beep
© 2016 Mercury Marine
Warning Light
On Engine overheat
On Low oil pressure
Engine speed exceeds maximum
On
allowable ‑ 6300 RPM
Engine speed will limited ‑ 2800
Flashes RPM. Consult your dealer for
assistance
Engine Speed
Description Limited to 2800
90-8M0105568 eng MARCH 2016
RPM
X
X
X