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 14.6:1 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 FT 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 FT value, while a prolonged change is indicated by the Long Term
FT 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 FT value above 0 percent (adding fuel). |
| - | Long Term FT value around 0 percent. |
| • | A prolonged lean O2S signal (system is in control) will appear
on the scan tool as |
| - | Short Term FT value around 0 percent. |
| - | Long Term FT value above 0 percent (added fuel). |
| • | A momentary rich O2S signal (system is in control) will appear
on the scan tool as |
| - | Short Term FT value less than 0 percent (reducing fuel). |
| - | Long Term FT value around 0 percent. |
| • | A prolonged rich O2S signal (system is in control) will appear
on the scan tool as |
| - | Short Term FT value around 0 percent. |
| - | Long Term FT 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 FT value much less than 0 percent (reducing
fuel). |
| - | Long Term FT value much less than 0 percent (reduced fuel). |
