Electronic fuel injection (EFI) comes in several basic varieties.
Back in the mid to late 1980s, many cars were equipped with
"throttle body injection (TBI). This type of EFI system uses a single
throttle body with one or two fuel injectors. The throttle body sits
on the intake manifold in place of a carburetor, and sprays fuel and
air into the intake manifold. By the early 1990s, TBI was obsolete.
Another type of electronic fuel injection system is "Multiport"
injection (MPI). This type of system has a separate fuel injector for
each of the engine's cylinders. Each injector sprays fuel directly into
the intake valve port. This provides better cylinder-to-cylinder fuel
distribution than throttle body injection systems and improves engine
performance, emissions and fuel economy.
Some General Motors vehicles have a "Central Port Injection (CPI)
fuel injection system, or its newer version "Central Multi-Port
Injection" (CMPI). Here, a central "Maxi" fuel injector feeds fuel
through individual lines to mechanical poppet injector valves at each
cylinder. When fuel flows through the lines and reaches a certain
pressure (37 to 40 psi), it pushes the poppet valves open and sprays
out of the individual injectors.
Most older multiport fuel injection system energize all the injectors
at the same time (once every revolution of the crankshaft or every other
revolution depending on the application). Most newer multiport fuel
injection systems, by comparison, energize each injector individually just
before the intake valve opens. This is called "Sequential" fuel injection
(SFI). Energizing each injector one at a time allows faster changes in the
air/fuel mixture and better fuel control to reduce emissions and improve
fuel economy and performance.
HOW IT WORKS
Fuel injectors do not spray fuel continuously, but are "pulsed" on
and off by the powertrain control module (PCM). On most engines, the
injectors receive power (battery voltage) when the ignition is on. But
the injectors do not open until the PCM driver circuit completes the
ground connection to the injector solenoid.
When the PCM commands the injector on, the driver circuit closes the
ground connection allowing current to flow through the solenoid coil in
the top of the injector. This creates a powerful magnetic field in the
coil that pulls the injector pintle or ball valve open. When the injector
valve opens, pressure in the fuel supply to the injector forces fuel out of
the injector nozzle. Fuel sprays into the intake port, mixes with air
and is sucked into the cylinder when the intake valve opens.
When the injector has delivered its dose of fuel, the PCM opens the
driver circuit and stops the flow of current through the injector. The
magnetic field collapses and spring pressure forces the injector valve
to snap shut. This stops the flow of fuel and the injector ceases to
spray fuel until the next pulse from the PCM.
DIAGNOSING INJECTOR PROBLEMS
One way to tell if the injectors are working, therefore, is to listen
for a buzzing noise when the engine is running. Another is to hold the
tip of a steel screwdriver next to each injector to see if it is attracted
by a magnetic field.
Injectors can sometimes fail if the solenoid coil inside develops an
open or a short. One way to check the electrical condition of the injectors
is to measure the resistance across the injector terminals. This is done
by unplugging the injector's wiring connector (key off) and measuring the
resistance between the injector terminals with an ohmmeter.
Injectors typically fall into one of two categories: high resistance
(12 to 17 ohms) or low resistance (2 to 7 ohms). If resistance is out of
specifications (refer to a service manual for the exact specifications),
the injector is defective and should be replaced.
If you don't have the exact specifications, another method is to
measure and compare the resistance readings of all the injectors. If one is
more than a few ohms higher or lower than the rest, it is probably defective
and should be replaced.
NOTE: It doesn't take much of a change in resistance to affect injector
performance. A shorted injector may pull so much current through the driver
circuit that it may prevent other injectors from opening or cause the engine
to stall or not start.
The amount of air entering a fuel injected engine is controlled by the
throttle body. The throttle is connected by a cable to the gas pedal, but
on some newer vehicles there is no direct connection. The throttle is
controlled by the PCM using a small electric motor and inputs from a gas
pedal position sensor.
To maintain the correct air/fuel ratio under changing operating
conditions, the amount of air entering the engine must be measured or
estimated by the PCM. In "Mass Air Flow" EFI systems, a "Vane" (VAF) or
"Hot Wire" (MAF) airflow sensor is located ahead of the throttle body to
measure incoming air. In "Speed-density" EFI systems, no airflow sensor is
used. The PCM estimates airflow using inputs from the "throttle position
sensor" (TPS), "manifold absolute pressure "(MAP) sensor and
"manifold air temperature" (MAT) sensor.
The air/fuel ratio is controlled by increasing or decreasing the
amount of fuel delivered by the injectors as engine speed and load change.
The PCM does this by increasing or decreasing the on-time of the
injectors. Increasing each injector pulse allows more fuel to spray into
the engine while decreasing each injector pulse reduces fuel delivery.
When the engine warms up, the PCM goes into a special fuel control mode
called "Closed Loop" operation. The PCM starts to use the rich/lean signal
from the "oxygen" (O2) sensor in the exhaust manifold to fine tune the fuel
mixture. If the O2 sensor reads lean (too much air, not enough fuel), the
PCM responds by increasing the duration of the injector pulses. You can
see this change on a scope that displays waveforms. When the O2 sensor
then reads rich (too much fuel, not enough air), the PCM decreases the
duration of the injector pulses. This constant flip-flopping back and forth
from lean to rich and back again results in an average fuel mixture that is
properly balanced for best fuel economy and emissions.
When a cold engine is first started, the PCM is in "Open Loop" fuel
control mode and delivers a relatively rich fuel mixture. If the PCM
remains in open loop once the engine is warm, it will hurt fuel economy
and emissions. A defective coolant sensor or oxygen sensor can prevent
the PCM from going into closed loop. Loop status (open or closed) can be
read on a scan tool.
The amount of fuel delivered by the injectors is also affected by
fuel pressure. A "fuel pressure regulator" maintains system pressure at
a preset level. A diaphragm inside the regulator is connected to
engine vacuum so the regulator can increase or decrease fuel
pressure in proportion to engine load. On some newer vehicles, the
pressure regulator is not on the engine but is part of the fuel pump
assembly inside the fuel tank. This is called a "returnless" system
because there is no fuel return line from the engine back to the tank.
Fuel pressure is generated by a high pressure electric pump usually
located in or near the fuel tank. When the ignition is turned on, the PCM
energizes a relay that routes voltage to the pump. A no-start condition
due to no fuel can be caused by a bad fuel pump relay, a defective fuel
pump or wiring problems in the fuel pump circuit.
For the engine to start and run properly, fuel pressure to the injectors
and the volume of fuel delivered by the pump must both be within
specifications. A difference of only a few pounds may be enough to cause
starting and performance problems.
Pressure can be checked with a fuel pressure gauge connected to the test
valve on the fuel supply rail, or teed into the fuel supply line. Fuel volume
can be measured by disconnecting a fuel line, putting the hose into a container
and running the pump briefly (CAUTION: KEEP SPARKS AND FLAME AWAY FROM FUEL!)
A good pump should usually deliver about a pint of fuel in 15 seconds.
Low fuel pressure and/or low volume can result in a lean
air/fuel mixture, hard starting, misfire, detonation, increased emissions
and poor performance. Low fuel pressure can be caused by a defective fuel
pressure regulator, a weak fuel pump, a plugged fuel fuel filter, or a
restricted fuel line.
Too much fuel pressure can cause a rich air/fuel mixture, rough idle,
surging, increases fuel consumption and emissions. Too much fuel pressure
can be caused by a defective pressure regulator or a plugged fuel return
Older fuel injection systems often have an additional "cold start"
injector that serves the same function as the choke on a carburetor for
starting (fuel injection systems do not have a choke). The cold start
valve sprays extra fuel into a cold engine when it is first cranked for
easier starting. A "warm-up regulator" provides additional fuel enrichment
while the engine is warming up. Newer EFI systems do not use either of
these devices and rely instead on longer injector pulses from the PCM for
fuel enrichment during starting and warm-up.
IDLE SPEED CONTROL
Idle speed in fuel injected engines is not controlled by the
opening of the throttle, but rather by an air bypass circuit. On
most EFI applications, idle speed is computer-controlled by an
"idle air control" (IAC) valve. A small electric motor opens and
closes the bypass circuit valve so air can bypass the throttle
Problems here can occur if the motor fails or the intake system has
an air leak downstream of the throttle body. Air leaks typically increase
idle speed so the idle control system will try to compensate for the leak
by closing the idle air bypass. This may overload and burn out the motor.
If idle speed is not within specifications, there is an air leak or
a problem with the idle speed control circuit, valve or motor.