Page Number - 18
A clogged inlet strainer can starve the fuel pump of fuel, causing
it to cavitate. Cavitation will create rapid changes in fuel density,
causing the float in the FST flowmeter to bounce up and down.
It can also cause the needle on the pressure gauge to bounce.
In addition, as the pump tries to pull gas through the clogged
strainer, it creates a pressure drop that may cause air bubbles to
form and become visible.
Always inspect the inlet strainer whenever a fuel pump is being
serviced. If it is rusty, brown, or dark colored, proper action must
be taken to clean and service the tank. Never reuse old filters or
strainers, especially when installing a new fuel pump.
Fuel Filter
Fuel filters ensure that the fuel sent to the injectors is clean.
They are installed in the fuel supply line, such that fuel flows
from the tank, through the filter on its way to the fuel rail. Filters
are typically mounted under the car along the frame, or in the
engine bay where they are relatively accessible for periodic
replacement.
A clean filter should have little or no affect on the pressure or
flow of fuel. A clogged filter becomes a restriction to flow. As
blockage in the filter increases, flow decreases. Depending on
the type of fuel delivery system and the level of restriction, a
clogged fuel filter will cause pressure to increase between the
pump and the filter, and/or decrease between the filter and the
fuel rail. Because fuel systems are designed to deliver a greater
volume and pressure than the engine will require, small
blockages may not be noticeable. However, as the restriction
increases, it will cause the engine to under-perform and/or stall
or hesitate under load. In addition, it causes the fuel pump to
work harder, which can lead to premature failure.
The FST Pro can accurately diagnose a clogged fuel filter by
comparing flow and pressure levels at idle with those at
simulated load conditions.
Pressure Regulator
The pressure regulator is a component of mechanically regulated
fuel delivery systems. It creates and maintains the pressure
required by the fuel injectors to perform at their optimum
efficiency. While it’s the fuel pump that creates the flow of fuel,
it’s the regulator that creates the pressure by restricting the flow.
The pressure regulator uses a spring to control the amount of
restriction based on the pressure in the fuel system. When the
engine is off and there is no flow of fuel, the regulator spring
completely closes the restriction, trapping pressure in the fuel
rail. When the engine is started, the restriction causes the
pressure to increase in front of the regulator. When fuel system
pressure builds to the optimum level, it overcomes the force of
the regulator spring, and the regulator opens to allow fuel to flow
through. As the fuel requirements of the engine differ from idle,
the regulator automatically increases or decreases the restriction
to maintain the optimum pressure at the injectors.
Pressure regulators used in return type fuel systems are
mounted on the fuel rail, and are either vacuum or atmospheric
modulated. Vacuum modulated pressure regulators use manifold
vacuum to maintain a constant pressure differential across the
fuel injectors. This means that fuel rail pressure will vary
depending on manifold vacuum. At idle, manifold vacuum is
high. This causes the regulator to pass more fuel back to the
tank, lowering fuel pressure in the rail. When the throttle is
opened, manifold pressure drops. This causes the regulator
to close, allowing less fuel flow back to the tank, so fuel rail
pressure increases. Vacuum modulation ensures the injectors
are receiving the proper fuel pressure for peak engine
performance, depending on the engine’s requirements.
Atmospheric modulated pressure regulators modify fuel
pressure with changes in atmospheric pressure. This allows
fuel systems to compensate for changes in altitude.
Atmospheric modulated fuel systems rely more on the ECM to
control fuel injection performance using other feedbacks such
as the O2 sensor, and controlling fuel injector duty cycle.
Mechanically regulated returnless fuel delivery systems use a
constant pressure regulator located in the fuel tank, as part of
the fuel pump module. It maintains constant pressure due to
a fixed spring rate. Fuel pressures in returnless systems are
typically higher than return systems, to compensate for the lack
of modulation.
When diagnosing a potential pressure regulator failure, consider
what happens in the fuel delivery system when the regulator fails
to maintain the proper fuel pressure. If the regulator sticks in the
open position, such that there is little or no restriction to flow,
fuel will flow freely through the regulator and back to the fuel
tank. Fuel system pressure may drop drastically below what
is required by the injectors. Depending on how the low the
pressure drops, the malfunction may not be noticeable when
the car is idling or running under a minimal load. But if a heavy
load condition is introduced on the engine, such as a hard
acceleration, the lack of pressure will cause the engine to
stumble and lack power.
If the sticking regulator was over-restricting the flow of fuel back
to the tank, it would cause fuel system pressure to increase
above normal. Because the fuel pump produces much greater
flow than what the engine will typically require, this malfunction
may not be evident at idle or at minimal load conditions.
However, fuel economy could drop severely as the high
pressure causes too much fuel to be injected into the engine.
Fuel Lines
Fuel lines are manufactured from steel, rubber or plastic. They
can affect fuel delivery performance if they become clogged,
pinched, kinked, dented or ruptured. Because fuel delivery
systems keep fuel under pressure, a ruptured fuel line will be
evident by leaking fuel. Likewise, most other restrictions can
identified with a simple visual inspection.
A restricted fuel supply line due to damage or blockage will have
the same affect as a clogged inline filter. If FST Pro test results
indicate a restriction to the supply of fuel, always make a visual
inspection of the lines before performing additional diagnostics
or replacing a suspect component.
A restricted fuel return line due to damage or blockage will have
the same effect as an over-restricting pressure regulator. If FST
Pro test results indicate a restriction to the return of fuel, always
make a visual inspection of the lines before performing addition-
al diagnostics or replacing a suspect component.
