Fuel pressure can be checked by attaching a gauge to the Schrader valve
fitting on the fuel injector rail, or by teeing the gauge into the fuel
supply line if the fuel system lacks a test fitting.
The operating pressure of the fuel system must be within the range
specified by the vehicle manufacturer. The numbers will typically read
5 to 9 PSI lower when the engine is idling than when the engine is off
with the key on. Some vehicle manufacturers only list KOEO (Key On
Engine Off) fuel pressure specs, while others also list the pressure
specs for when the engine is running.
The volume of fuel delivered by the fuel pump is also important,
because a weak pump may generate enough pressure to read normally at
idle, but not keep up with the engine’s fuel requirements at higher
speeds. This may cause a loss of power at highway speeds or when
accelerating hard.
A dirty or clogged fuel strainer indicates that contaminants within
the fuel tank are impeding fuel flow and might have damaged the pump. It is important to also inspect the fuel filter for clogging, which
can cut off fuel supply to the engine.
A good fuel pump should usually deliver about a pint of fuel in 15
seconds (half a gallon per minute), but some engines need more than
this. Fuel flow can be measured by disconnecting the fuel line from the
injector fuel rail and routing it into a container, then energizing the
pump.
A more accurate means of measuring fuel delivery is to connect a fuel
flow meter to the fuel system. The meter has a floating ball that
indicates fuel flow in gallons per minute (gpm).
A “good” reading on most applications would typically be 0.3 to 0.7
gpm. The flow meter can be hooked up to the supply line that runs to
the fuel rail to measure flow.
Why do fuel injected cars use electric fuel pumps instead of mechanical fuel pumps?
Because electronic fuel injection (EFI) requires a higher fuel
system operating pressure than a carburetor (35 to 80 PSI versus 4 to 6
PSI). An electric pump also provides instant pressure to the injectors
when the key is turned on (something a mechanical pump can’t do until
the engine is cranked), and the speed and flow rate of an electric can
be varied electronically to match the engine’s fuel needs.
On an older engine with a carburetor, vacuum created in the intake
manifold by the pumping action of the pistons pulls fuel from the
carburetor into the engine. With fuel injection, fuel is sprayed into
the intake ports under high pressure.
The amount of pressure behind the injectors is critical for proper fuel
mixing and atomization, so low fuel pressure (or too much fuel
pressure) can upset the air/fuel mixture and cause driveability and
emissions problems. Fuel pressure is usually regulated by a mechanical
regulator on the injector fuel rail. On “returnless” EFI systems, the
pressure regulator is mounted on the fuel pump assembly in the fuel
tank.
