Purpose
The function of the fuel metering system is to deliver the correct amount
of fuel to the engine under all operating conditions. Fuel is delivered to
the engine by individual fuel injectors near each cylinder. The PCM uses
the voltage inputs from several sensors in order to determine how much fuel
to give the engine. Each injector is energized individually in the engine
firing order, which is called sequential fuel injection (SFI). However,
if the PCM loses the voltage input from the camshaft position (CMP) sensor,
the PCM will energize the fuel injectors by using the alternating synchronous
double fire (ASDF) method. The main control sensor is the oxygen sensor
(O2S) located in the exhaust manifold. This sensor indicates to the PCM
how much oxygen is in the exhaust gas, and the PCM changes the air/fuel
ratio to the engine by controlling the fuel injectors. The best mixture
to minimize exhaust emissions is 14.7:1 which allows the catalytic converter
to operate most efficiently. Because of the constant measuring and adjusting
of the air/fuel ratio, the fuel injection system is called a Closed Loop
system.
Fuel Metering System
Some failures of the fuel metering system will result in an Engine Cranks
but Won't Run symptom. Refer to
Engine Cranks but Does Not Run
to determine if the problem
is caused by the ignition system, PCM, or fuel pump circuit. Whenever
it is determined to be a fuel problem, the Fuel System Diagnosis table
will be used. This includes the fuel injectors, the fuel pressure regulator,
the fuel pump, and the fuel pump relay. Whenever a problem occurs in
the fuel metering system, it usually results in either a rich or lean
O2S signal, which causes the PCM to change the fuel calculation (injector
pulse width). The change made to the fuel calculation is indicated by
a change in the short and long term fuel trim values (-100 percent
to 100 percent) which can be monitored by using a scan tool.
A momentary change to the fuel calculation is indicated by the short
term fuel trim value, while a prolonged change is indicated by the long
term fuel trim value. A value of 0 percent indicates that the
fuel delivery requires no compensation to maintain the proper air/fuel
ratio. A negative value significantly below 0 percent indicates
that the fuel system is rich and the fuel delivery is being reduced
(decreased injector pulse width). A positive value significantly greater
than 0 percent indicates that a lean condition exists and the
fuel delivery is being increased (increased injector pulse width).
Important: Whenever both fuel trim values are between 0 percent to 100 percent,
see DTC P0171 or DTC P0131 tables for items which can cause a lean system.
Whenever both fuel trim values are between 0 percent to -100 percent,
see DTC P0172 or DTC P0132 for items which can cause the system to run
rich.
Listed below are examples of lean and rich conditions with the system
in control and out of control.
| • | A momentary lean O2S signal (system is in control) will appear
on the scan tool as |
| - | Short term fuel trim value above 0 percent (adding fuel). |
| - | Long term fuel trim value around 0 percent. |
| • | A prolonged lean O2S signal (system is in control) will appear
on the scan tool as |
| - | Short term fuel trim value around 0 percent. |
| - | Long term fuel trim value above 0 percent (added fuel). |
| • | A momentary rich O2S signal (system is in control) will appear
on the scan tool as |
| - | Short term fuel trim value less than 0 percent (reducing
fuel). |
| - | Long term fuel trim value around 0 percent. |
| • | A prolonged rich O2S signal (system is in control) will appear
on the scan tool as |
| - | Short term fuel trim value around 0 percent. |
| - | Long term fuel trim value less than 0 percent (reduced
fuel). |
| • | A prolonged rich O2S signal (system is out of control) will appear
on the scan tool as |
| - | Short term fuel trim value much less than 0 percent (reducing
fuel). |
| - | Long term fuel trim value much less than 0 percent (reduced
fuel). |
