Fuel Pump Capability Test; Fuel System Leak Test; Vacuum Test - Mercury 225 EFI Service Manual

FUEL PUMP

Fuel Pump Capability Test

Before proceeding with the system vacuum test, confirm that the pulse fuel pump is capa-
ble of supplying the required vacuum. To do this, start the engine, pinch off/restrict the fuel
supply hose between the vacuum gauge and fuel tank. The vacuum gauge should rise
to or exceed the maximum normal reading of 2.5 inches vacuum (mercury). If it fails to
reach this minimum number, the pump needs servicing or there is a lack of crankcase
pressure to operate the pump.
Normal Reading
Reading below 2.5 in. of vacuum (mercury)

Fuel System Leak Test

This test is done with the engine running, and the tubing clamp removed. The clear hose
that was installed previously is used to view the fuel flow to the pulse pump.
No air bubbles seen in clear hose
Air bubbles seen in clear hose

Vacuum Test

The vacuum test is normally performed at an idle speed. As engine RPM increases, there
will be a slight increase in vacuum; this increase should not exceed normal readings at
any RPM.
Normal Reading
Reading below 2.5 in. of vacuum (mercury)
Page 3A-8
2.5 in. of vacuum (mercury) or higher, proceed to
fuel system leak test
Pump check valves defective, replace
valves
Pump diaphragm defective, replace
diaphragms
Air leak in pump, rebuild pump with new
gasket, check fitting for leaks
Low crankcase pressure, check for
crankcase leaks or plugged pulse pump
pressure/vacuum passageways
No air leaks, perform vacuum test (following)
Air leak on intake side of fuel system –
Pickup tube in fuel tank leaking
Outlet fitting at fuel tank leaking
Fuel inlet hose not properly clamped at
fitting
Leaking fuel tank valve
Fuel line from kicker engine connected into
fuel line of main engine
Below 2.5 in. of vacuum (mercury)
Restriction within the fuel system –
Restricted anti-siphon valve
Restricted or malfunctioning primer bulb
Kinked or collapsed fuel hose
Plugged water separating fuel filter (in the
boat)
Restriction in fuel line thru-hull fitting
Restriction in fuel tank switching valves
Plugged fuel tank pick-up screen
90-884294 OCTOBER 2001