Checking Fuel Pump Lift (Vacuum); Vacuum Test Troubleshooting - Mercury 150 EFI Service Manual

Checking Fuel Pump Lift (Vacuum)

The square fuel pump is designed to lift fuel (vertically) about 60 in. (1524 mm) if there
are no other restrictions in the system using a fuel hose that is 5/16 in. (7.9 mm) minimum
diameter. As restrictions are added, such as filters, fittings, valves etc., the amount of fuel
pump lift decreases.
Fuel pump vacuum and air bubbles in the fuel supply can be checked with a vacuum
gauge, a t-fitting and a clear piece of fuel hose. Connect the clear hose between the inlet
fitting on the pulse driven fuel pump and the vacuum gauge t-fitting; keeping the t-fitting
as close as possible to the pump. Connect the fuel line from the fuel tank to the remaining
connection on the t-fitting.
a -
b -
c -

Vacuum Test Troubleshooting

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 above 2.5 in. of vacuum (mercury)
90-883728 JULY 2001
a
Clear Hose
T-fitting
Vacuum Gauge
c
b
Below 2.5 in. of vacuum (mercury)
Restriction within the fuel system –
Restricted anti-siphon valve
Restriction within the 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
FUEL PUMP
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