fuel injection systems come in several versions. Throttle
Body Injection (TBI) was used in the 1980s on many vehicles
as an intermediate step from electronic carburetion to
multiport fuel injection. TBI uses one or two fuel injectors
mounted in a throttle body to fuel the engine.
Multiport fuel injection (MFI), which is used on almost all
late-model engines, has a separate fuel injector for each
cylinder. The injectors are usually mounted in the intake
manifold and spray fuel into the intake ports, but on some
new direct injection applications, the injectors spray
directly into the combustion chamber. These include some new
GM, Mazda and VW engines. Direct injection requires much
higher injection pressures because the injector has to
overcome the pressure inside the cylinder during the
compression stroke. But the advantages include finer fuel
atomization, cleaner combustion (lower emissions), better
fuel economy and better performance. Because of this, more
and more new engines will be going to direct injection.
First-generation MFI systems fire all the injectors
simultaneously or each bank separately, but most of the
newer MFI systems are sequential fuel injection (SFI)
systems that fire each injector separately just as the
intake valve is about to open. The advantage with the latter
approach is that the air/fuel mixture can be adjusted almost
instantly (from one cylinder firing to the next), and even
adjusted for individual cylinders.
Central Point Injection
One notable variation is General Motor's Central Point
Injection (CPI) system.
Here, a centrally-located Maxi injector routes fuel to
mechanical poppet valve injectors at each cylinder. The
system was used on GM's 4.3L V6 Vortec engines from 1992
through 1995. The Maxi injector routes fuel into six nylon
fuel lines that have poppet-style spray nozzles on the end.
When the pressure in the lines reaches (43 psi), the poppet
valves open and fuel sprays into the intake ports. All the
nozzles spray simultaneously when the Maxi injector opens
(three times per crankshaft revolution).
GM redesigned the CPI system in 1996 and called it Central
Sequential Fuel Injection (CSFI). The single Maxi injector
was replaced with six or eight electronic injectors, but
each injector still routes fuel through a plastic line to a
mechanical poppet valve.
The fuel injectors in most fuel injection systems are
electronic and have a solenoid valve at the top to open the
nozzle. The Powertrain Control Module (PCM) determines the
on-time (pulse width modulation) of each injector to
regulate fuel delivery. The PCM uses various sensor inputs
(airflow, temperature, oxygen, throttle position, engine
speed and load) to control fuel delivery. The injectors
typically receive voltage when the ignition is on, and open
when the injector driver circuit in the PCM grounds the
injectors are a common problem. Injector nozzles can become
clogged with fuel varnish over time, causing a loss of
engine performance and misfiring. Contributing factors
include short trip driving and using gasoline that does not
contain adequate levels of detergent to keep injectors
clean. Symptoms include rough idle (lean misfire),
hesitation or stumble when accelerating, and reduced
Fuel injectors can also leak fuel, causing an increase in
fuel consumption and emissions. An injector failure will
result in a dead cylinder and power loss. This will usually
turn on the Check Engine Light in 1996 and newer vehicles
with OBD II, and set a misfire code. Fuel pressure is
critical with fuel injected engines. Operating pressures may
range from 35 to 85 PSI depending on the application, but
the specifications typically allow a very limited range in
pressure (say from 45 to 55 psi). If fuel pressure is
outside specifications, it may cause hard starting,
driveability or other performance issues. That's why fuel
pressure checks are a must when diagnosing fuel-related
fuel injection systems use an electric pump mounted inside
the fuel tank. Pump designs also vary and include single- or
double-vane, roller vane, turbine or gerotor-style pumps.
Most have a one-way check valve to maintain pressure in the
fuel system when the engine is shut off.
Fuel pumps can fail for a variety of reasons: old age, loss
of voltage, ground at the power relay, wiring connections
and pump motor or bearing damage. Running the fuel tank
empty may damage the pump because it relies on fuel for
Note: Many fuel pumps are replaced unnecessarily because of
misdiagnosis. A no-fuel condition might indicate a bad pump,
but could also be a plugged fuel filter, bad electrical
connection, bad fuel pump relay or leaky fuel pressure
regulator. New fuel pump bench testers are now available for
testing customers' old fuel pumps, and should be used to
reduce fuel pump warranty returns.
Replacement fuel pumps must have the same pressure rating as
the original. The pump is usually part of the fuel sending
unit and may be replaced separately or as a complete
assembly. The fuel inlet strainer sock should also be
replaced when the pump is changed.
When fuel reaches the engine, it enters a fuel rail and goes
to the injectors. A fuel pressure regulator on the fuel rail
maintains a certain operating pressure. Inside is a
spring-loaded diaphragm attached to a source of intake
vacuum. As engine load (vacuum) changes, pressure is
adjusted as needed to maintain proper fuel delivery. Excess
fuel is routed back to the fuel tank through a return line.
Many newer vehicles have returnless systems that do not have
a regulator on the engine fuel rail. The regulator is in the
fuel tank with the pump.
Problems with the fuel pressure regulator can cause hard
starting, idle roughness, hesitation or stumble when
accelerating, and engine performance problems. Fuel leaking
into the regulator vacuum supply hose indicates the vacuum
diaphragm inside the regulator has failed, and the regulator
needs to be replaced.
Airflow into the engine is regulated by a throttle body
attached to the intake manifold. Air first flows through an
air filter, then through the throttle body before passing
through the manifold and into the engine. The PCM must
monitor the amount of air entering the engine, so many fuel
injected systems have a vane or mass airflow sensor ahead of
the throttle body. A dirty or defective airflow sensor is a
common cause of fuel-related driveability problems.
Other engines use a speed-density system that estimates
airflow based on throttle position, RPM, temperature and
engine load. There is no airflow sensor on a speed-density
system, so bad inputs from the TPS sensor or MAP sensor can
cause problems on these engines.