Always start with the
Powertrain On Board Diagnostic (OBD) System Check
.
The Powertrain OBD System Check table will identify other tables for
specific checks, such as:
• | Idle Speed Control Check |
Diagnostic Aids
If there is no fuel pressure check all component and circuit grounds.
Fuel System Deposits
Fuel system deposits can cause various driveability problems. Deposits
usually occur during hot soaks after key Off. Poor fuel quality or driving
patterns such as short trips followed by long cool down periods can cause
injector deposits. This occurs when the fuel remaining in the injector
tip evaporates and leaves deposits. Leaking injectors can increase injector
deposits. Deposits on fuel injectors affect their spray pattern, which
in turn could cause reduced power, unstable idle, hard starts, and poor
fuel economy.
Intake valve deposits can also be related to fuel quality. While most
fuels contain deposit inhibitors, some do not and the effectiveness of deposit
inhibitors varies by manufacturer. If intake valve deposits occur, fuel
may be suspected. These deposits can cause symptoms such as excessive
exhaust emissions, power loss and poor fuel economy.
The fuel pump relay is located in the Multi-Use Relay Center.
Refer to Driveability and Emissions for the locator view. For diagnosis of
the fuel pump relay circuit, refer to
DTC P0230 Fuel Pump Relay Control Circuit
.
If there is a fuel delivery problem, refer to
Fuel System Pressure Test
. This test diagnoses the
fuel injectors, the fuel pressure regulator, and the fuel pump. If a malfunction
occurs in the fuel metering system, a rich HO2S signal or lean HO2S signal
is a result. This condition is indicated by the HO2S voltage, causing
the PCM to change the fuel calculation (fuel injector pulse width) based
on the HO2S reading. Changes made to the fuel calculation will be indicated
by a change in the long term fuel trim values which can be monitored with
a scan tool. Ideal long term fuel trim values are around 0 percent;
for a lean HO2S signal, the PCM will add fuel, resulting in a fuel trim
value above 0 percent and for a rich HO2S signal the PCM will take
away fuel, resulting in a fuel trim value below 0 percent . Some
variations in fuel trim values are normal due to power train design. If
the fuel trim values are excessively greater or lower then 0 percent
, refer to
DTC P0171 Fuel Trim System Lean Bank 1
,
DTC P0172 Fuel Trim System Rich Bank 1
,
DTC P0174 Fuel Trim System Lean Bank 2
,
or
DTC P0175 Fuel Trim System Rich Bank 2
.
If a malfunction occurs in the fuel metering system, it
usually results in either a rich or lean exhaust condition. This condition
is sensed by the HO2S, and 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 term fuel trim 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
are around 0 percent, but 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 conditions with
the system in control and out of control.
Important: If both fuel trim values are fixed well below 0 percent, refer
to items which can cause the system to run rich. If a driveability symptom
exists, refer to the particular symptom in
Symptoms
for additional items to check.
• | A momentary lean condition (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 condition (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 prolonged lean condition (system is out of control) will appear
on the scan tool as: |
- | Short term fuel trim value well above 0 percent (adding
fuel). |
- | Long term fuel trim value well above 0 percent (added fuel). |
- | If both fuel trim values are fixed well above 0 percent, refer
items which can cause a lean system. Refer to
Symptoms
. |
• | A momentary rich condition (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 condition (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 condition (system is out of control) will appear
on the scan tool as: |
- | Short term fuel trim value much less 0 percent (reducing
fuel). |
- | Long term fuel trim value much less 0 percent (reduced
fuel). |