FUEL PUMP
Page 3A-8 90-884294 OCTOBER 2001
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
2.5 in. of vacuum (mercury) or higher, proceed to
fuel system leak test
Reading below 2.5 in. of vacuum (mercury) • 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
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
No air leaks, perform vacuum test (following)
Air bubbles seen in clear hose 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
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
Below 2.5 in. of vacuum (mercury)
Reading 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
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