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 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 more 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 0-100 percent,
see DTC P0171 or DTC P0131 tables for items which can cause
a lean system. Whenever both fuel trim values are between 0 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 one of the following conditions: |
| - | 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 one of the following conditions: |
| - | 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 one of the following conditions: |
| - | 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 one of the following conditions: |
| - | 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 one of the following conditions: |
| - | Short Term FT value much less than 0 percent (reducing
fuel) |
| - | Long Term FT value much less than 0 percent (reduced fuel) |
