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 mounted
in the intake manifold near each cylinder.
Fuel Metering System Components
The fuel metering system consists of the following parts:
| • | The fuel pump electrical circuit |
| • | The fuel supply components (fuel tank, pump, lines) |
| • | The fuel rail assembly which includes the following components: |
| - | The fuel pressure regulator |
| - | The electrical wiring harness |
| • | The lower manifold assembly which includes MAP sensor. |
| • | The throttle body assembly including the throttle position (TP)
sensor. |
Fuel Metering System
| • | The fuel pump relay is mounted in the dashboard electrical center
located under the glovebox. This table determines if the problem is caused
by the ignition system, PCM, or fuel pump circuit. Perform one or
more of the following procedures: |
| - | Some failures of this system will result in an Engine Cranks
But Will Not Run symptom. |
| • | The Fuel Metering System includes the following components: |
| - | The fuel injector assembly |
| - | The fuel pressure regulator |
| • | The fuel system wiring schematic diagram is covered in Fuel
Pump Circuit Diagnosis. |
| • | If a problem occurs in the fuel metering system, it usually results
in either a rich or lean exhaust condition. This condition is sensed by the
HO2S. This condition 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 which
can be monitored by 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. Average Fuel Trim values
will measure around 128. The averages may vary slightly from engine
to engine. |
| | Important: When using a scan tool to observe Fuel Trim values, remember that if
the system is in control, no action is required unless a driveability symptom
is present.
|
| • | Listed below are examples of lean and rich HO2S signals with the system
in control and out of control: |
| - | A momentary lean HO2S signal (system is in control) will appear
on the scan tool as the following items: |
| • | Short term Fuel Trim value above 128 (adding fuel) |
| • | Long term Fuel Trim value around 128 |
| - | A prolonged lean HO2S signal (system is in control) will appear
on the scan tool as the following items: |
| • | Short term Fuel Trim value around 128 |
| • | Long term Fuel Trim value above 128 (added fuel) |
| - | A prolonged lean HO2S signal (system is out of control) will appear
on the scan tool as the following items: |
| • | Short term Fuel Trim value well above 128 (adding fuel) |
| • | Long term Fuel Trim value well above 128 (added fuel) |
| - | If both Fuel Trim values are fixed well above 128, refer to DTC
P0131 for items which can cause a lean system. |
| - | A momentary rich HO2S signal (system is in control) will appear
on the scan tool as the following items: |
| • | Short term Fuel Trim value less than 128 (reducing fuel) |
| • | Long term Fuel Trim value around 128 |
| - | A prolonged rich HO2S signal (system is in control) will appear
on the scan tool as the following items: |
| • | Short term Fuel Trim value around 128 |
| • | Long term Fuel Trim value less than 128 (reduced fuel) |
| - | A prolonged rich HO2S signal (system is out of control) will appear
on the scan tool as the following items: |
| • | Short term Fuel Trim value much less than 128 (reducing fuel) |
| • | Long term Fuel Trim value much less than 128 (reduced fuel) |
| - | If the Fuel Trim values are fixed well below 128, see DTC P0132
for items which can cause the system to run rich. |
| • | If a driveability symptom exists, refer to
Symptoms
. |
Fuel Control
Fuel delivery is controlled by the control module system.
The diagnosis of fuel control starts with Engine Cranks But Will Not
Run. This table will test the fuel system to determine if there is a problem.
Fuel Injector
Testing of the fuel injector circuit is located in the Fuel Injector
Circuit Diagnosis Table.
A fuel injector which does not open may cause a no-start condition.
An injector which is stuck partially open could cause loss of pressure after
sitting, resulting in extended crank times on some engines. Also, dieseling
could occur because some fuel could be delivered to the engine after
the key is turned OFF.
Pressure Regulator
If the pressure regulator supplies pressure which is too low, poor performance
could result. If the pressure is too high, exhaust odor may result.
Fuel Pump Circuit
The relay and the fuel pump operation can be tested by commanding on
the relay by the Tech 2. By commanding on the relay, it can be determined
if the fuel pump will operate. This command will also prime the fuel
line to the fuel injection unit.
An inoperative fuel pump will cause a no start condition. A fuel pump
which does not provide enough pressure can result in poor performance.
An inoperative fuel pump relay can result in long cranking times, particularly
if the engine is cold.
Engine Coolant Temperature (ECT) Sensor
The engine coolant temperature sensor is a thermistor (a resistor which
changes value based on temperature) mounted in the engine coolant stream.
Low coolant temperature produces a high resistance (100,000 ohms
at -40°C/-40°F) while high temperature causes low resistance
(70 ohms at 130°C/266°F).
The PCM supplies a 5 volt signal to the engine coolant temperature
sensor through a resistor in the PCM and measures the voltage. The voltage
will be high when the engine is cold, and low when the engine is hot.
By measuring the voltage, the PCM calculates the engine coolant temperature.
Engine coolant temperature affects most systems the PCM controls.
The scan tool displays engine coolant temperature in degrees. After
engine start-up, the temperature should rise steadily to about 90°C
(194°F) then stabilize when thermostat opens. If the engine has not
been run for several hours (overnight), the engine coolant temperature
and intake air temperature displays should be close to each other.
A fault in the engine coolant sensor circuit should set DTC P0117
or DTC P0118.
Intake Air Temperature (IAT) Sensor
The intake air temperature (IAT) sensor is a thermistor which changes
value based on the temperature of air entering the engine. Low temperature
produces a high resistance (100,000 ohms at -40°C/-40°F),
while high temperature causes low resistance (70 ohms at 130°C/266°F).
The PCM supplies a 5 volt signal to the sensor through a resistor
in the PCM and measures the voltage. The voltage will be high when
the incoming air is cold, and low when the air is hot. By measuring
the voltage, the PCM calculates the incoming air temperature.
The IAT sensor signal is used to adjust spark timing according to incoming
air density.
The scan tool displays temperature of the air entering the engine, which
should read close to ambient air temperature when engine is cold, and rise
as underhood temperature increases. If the engine has not been run
for several hours (overnight) the IAT sensor temperature and engine
coolant temperature should read close to each other. A failure in
the IAT sensor circuit should set DTC P0112 or DTC P0113.
Manifold Absolute Pressure (MAP) Sensor
DTC P0107 or DTC P0108 indicates a failure in the MAP sensor circuit,
which may effect fuel metering.
Heated Oxygen Sensor (HO2S 1)
The exhaust heated oxygen sensor (HO2S 1) is mounted in the exhaust
manifold where it can monitor the oxygen content of the exhaust gas stream.
The oxygen content in the exhaust reacts with the sensor to produce
voltage output. This voltage should constantly fluctuate from approximately
100 mV (high oxygen content -- lean mixture) to 900 mV
(low oxygen content -- rich mixture). The heated oxygen sensor
voltage can be monitored with a scan tool.
By monitoring the voltage output of the heated oxygen sensor, the PCM
calculates what fuel mixture command to give to the injector (lean mixture
-- low HO2S 1 voltage=rich command, rich mixture -- high
HO2S 1 voltage=lean command).
The heated oxygen sensor circuit, if open, should set a DTC P0134 and
the Scan tool will display a constant voltage between 350-550 mV. A
constant voltage below 250 mV in the sensor circuit should
set DTC P0131, while a constant voltage above 750 mV in the
circuit should set DTC P0132. DTC P0131 and DTC P0132 could also be
set as a result of fuel system problems.
HO2S 2 (Catalyst Monitor)
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide
(CO) and Oxides of Nitrogen (NOx), a three-way catalytic converter is
used. The catalyst within the converter promotes a chemical reaction
which oxidizes the HC and CO present in the exhaust gas, converting
them into harmless water vapor and carbon dioxide. The catalyst
also reduces NOx, converting it to nitrogen. The PCM has the capability
to monitor this process using HO2S 2. The HO2S 2
is located in the exhaust stream past the three-way catalytic
converter. The HO2S 2 produces an output signal which indicates
the oxygen storage capacity of the catalyst. This in turn indicates the
catalyst's ability to convert exhaust emissions effectively,
depending on the specific condition. If the catalyst is functioning
correctly, the HO2S 2 signal will be far less active than
that produced by HO2S 1.
Throttle Position Sensor
Important: The throttle position (TP) sensor cannot be serviced.
The throttle position (TP) sensor is mounted on the throttle body assembly.
The sensor is actually two individual throttle position sensors within one
housing. Two separate signal, ground and 5 volt reference circuits
are used to connect the TP sensor assembly and the throttle actuator
control (TAC) Module. The two sensors have opposite functionality.
The TP sensor 1 signal voltage increases as the throttle opens, from
below 1.1 volts at 0 percent throttle to above 3.7 volts
at 100 percent throttle. The TP sensor 2 signal voltage decreases
from above 3.9 volts at 0 Percent throttle to below
1.2 volts at 100 percent throttle. Note also that
the signal circuit for TP sensor 1 is pulled up to 5.0 volts
and that the signal circuit for TP sensor 2 is pulled to ground
within the TAC module. The TAC module converts these different
signals to a common scale and continuously compares them to each other
to verify proper system operation.
When the PCM detects a malfunction with the TP sensor circuits, the
following DTCs will set:
| • | DTC P1120 Throttle Position TP Sensor 1 Circuit |
| • | DTC P1220 Throttle Position TP Sensor 2 Circuit |
| • | DTC P1221 Throttle Position TP Sensors 1, 2 Performance |
Crankshaft Position Sensor (CKP)
The crankshaft position sensor (CKP) provides the PCM with crankshaft
speed and crankshaft position.
The PCM also monitors the CKP sensor signal circuit for malfunctions.
When the PCM detects a CKP sensor that is out of normal operating range, the
PCM will set a DTC P0335 or a DTC P0336.
Camshaft Position Sensor
The camshaft position sensor (CMP) is mounted through the top of the
engine block at the rear of the valley cover. The camshaft position sensor
works in conjunction with a 1X reluctor wheel on the camshaft.
The reluctor wheel is inside the engine immediately in front
of the rear cam bearing. The PCM provides a 12 volt power
supply to the CMP sensor as well as a ground and a signal circuit.
The camshaft position sensor is used to determine whether a cylinder
is on a firing or exhaust stroke. As the camshaft rotates, the reluctor wheel
interrupts a magnetic field produced by a magnet within the sensor.
The sensors internal circuitry detects this and produces a signal
which is read by the PCM. The PCM uses this 1X signal in combination
with the crankshaft position sensor 24X signal to determine crankshaft
position and stroke. This diagnostic for the camshaft position
sensor checks for a loss of camshaft position sensor signal. The
PCM also monitors the CMP sensor signal circuit for malfunctions.
The following DTCs set when the PCM detects a CMP sensor that is
out of the normal operating range.
| • | DTC P0341 Camshaft Position Sensor (CMP) Circuit Performance |
| • | DTC P0342 Camshaft Position Sensor (CMP) Circuit Low Voltage |
| • | DTC P0343 Camshaft Position Sensor (CMP) Circuit High Voltage |
Fuel Pump Electrical Circuit
When the ignition is turned ON (before engaging the starter), the PCM
energizes the fuel pump relay for 2 seconds causing the fuel
pump to pressurize the fuel system. If the PCM does not receive
the ignition reference pulses (engine cranking or running) within 2 seconds,
the control module shuts off the fuel pump relay, causing the fuel
pump to stop.
An inoperative fuel pump relay can result in long cranking times, particularly
if the engine is cold.
Fuel Rail Assembly
