Diagnostic Instructions
Symptoms Description
Symptoms cover conditions that are not covered DTCs. Certain conditions can
cause multiple symptoms. These conditions are listed together under Symptoms Testing.
Conditions that may only cause certain symptoms are listed separately under Additional
Symptoms Test. Perform the Symptoms Testing before using the Additional Symptoms Tests.
Poor Fuel Fill Quality test may be performed separately from the Symptoms Testing
and Additional Symptoms Tests procedures.
Symptoms Definition
Backfire: Fuel ignites in the intake manifold or in the exhaust
system, making a loud popping noise.
Cuts Out, Misses: A steady pulsation or jerking that follows
engine speed, which is usually more pronounced as the engine load increases. This
condition is not normally felt above 1500 RPM or 48 km/h (30 mph).
The exhaust has a steady spitting sound at idle or at low speed.
Detonation/Spark Knock: A mild to severe ping which usually
occurs worse while under acceleration. The engine makes sharp metallic knocks that
change with throttle opening.
Dieseling, Run-On: The engine continues to run after the key
is turned OFF, but runs very rough.
Hard Start: Engine cranks OK, but does not start for a long
time. The vehicle does eventually run, or may start but immediately stalls.
Hesitation, Sag, Stumble: Momentary lack of response as the
accelerator is pushed down. This condition can occur at any vehicle speed. This condition
is usually more pronounced when first trying to make the vehicle move, as from a stop.
This condition may cause the engine to stall in severe conditions.
Lack of Power, Sluggishness, or Sponginess: The engine delivers
less than expected power. Little or no increase in speed when the accelerator pedal
is pushed down part way.
Poor Fuel Economy: Fuel economy, as measured by an actual road
test, is noticeably lower than expected. Also, the fuel economy is noticeably lower
than it was on this vehicle at one time, as previously shown by an actual road test.
Poor Fuel Fill Quality: Difficulty when refueling the vehicle.
Rough, Unstable, or Incorrect Idle and Stalling: The engine
runs unevenly at idle. If severe, the engine or the vehicle may shake. Engine idle
may vary in speed. Either condition may be severe enough to stall the engine.
Surges/Chuggles: Engine power variation under steady throttle
or cruise. Feels like the vehicle speeds up and slows down with no change in the accelerator
pedal position.
Symptoms Verification
Before using the Symptom tables, perform the following inspections:
• | Ensure that the engine control module (ECM) and malfunction indicator
lamp (MIL) are operating correctly. |
• | Ensure that there are no diagnostic trouble codes (DTCs) that are stored. |
• | Ensure that the scan tool data is within a normal operating range. |
• | Verify the customer concern. |
• | Perform the Visual/Physical Inspection in this section. The visual/physical
inspection is extremely important, and can lead to correcting a condition without
additional testing. It may also help reveal the cause of an intermittent condition. |
Identifying Intermittent Conditions
Many intermittent conditions occur with harness or connector movement due to
engine torque, rough pavement, vibration or physical movements of a component. Refer
to the following for a list of issues that may cause an intermittent condition:
• | Moisture and water intrusion in connectors, terminals, and components |
• | Incomplete connector mating |
• | High circuit or component resistance--High resistance can include
any resistance, regardless of the amount, which can interrupt the operation of the
component. |
• | Harness that is too short or tight. |
• | Wire insulation that is chaffed or cut. |
• | High or low ambient temperature |
• | High or low engine coolant temperatures |
• | High underhood temperatures |
• | Heat build up in component or circuit due to circuit resistance, poor
terminal contact, or high electrical load |
• | High or low system voltage |
• | High vehicle load conditions |
• | Electro-magnetic interference (EMI)/circuit interference from relays,
solenoids or other electrical surge |
• | Incorrect installation of aftermarket, add on accessories |
Visual/Physical Check
• | Ensure that the control module grounds are clean, tight, and correctly
located. Refer to and . |
• | Ensure that the vacuum hoses are not split, kinked, and properly connected,
as shown on the Vehicle Emission Control Information label. |
• | Ensure that the air filter is clean and free from restrictions. |
• | Ensure that there is no water intrusion in connectors terminals and components. |
• | Inspect the air intake ducts for the following conditions: |
• | Inspect for air leaks at the throttle body mounting area, the mass air
flow (MAF) sensor and intake manifold sealing surfaces. |
• | Inspect the wiring harness for the following conditions: |
• | Inspect for loose, damaged, unseated, or missing sensors/components. |
• | Inspect the terminals for corrosion and correct contact. |
Symptoms Testing
Backfire, Cuts Out/Misses, Detonation/Spark Knock, Dieseling/Run-On, Hard Start,
Hesitation/Sag/Stumble, Lack of Power/Sluggishness/Sponginess, Poor Fuel Economy,
Rough, Unstable, or Incorrect Idle and Stalling, or Surges/Chuggles
- Test for the following conditions:
• | The fuel system for the following: |
• | The ignition system for the following: |
- | Wet down the secondary ignition system with water from a spray bottle.
Wetting down the secondary ignition system may help locate damaged or deteriorated
components. Look/listen for arcing or misfiring as the water is applied. |
• | The operation of the transmission torque converter clutch (TCC)--The
scan tool should indicate an engine speed drop when the TCC is commanded ON. |
• | The operation of the A/C compressor |
• | Items that can cause an engine to run rich or lean. |
• | Inspect the heated oxygen sensors (HO2S). The HO2S should respond quickly
to different throttle positions. |
Notice: Refer to Heated Oxygen and Oxygen Sensor Notice in the Preface section.
• | Water intrusion in the HO2S connector |
| Important: The embossed arrows on the mass air flow
(MAF) sensor indicate the direction of the intake air flow. The arrows must point
toward the engine.
|
• | Inspect the mass air flow (MAF) sensor connections. |
• | Engine oil contaminated by fuel |
- | Excessive oil in the combustion chamber or leaking valve seals |
- | Incorrect cylinder compression |
- | Sticking or leaking valves |
- | Excessive carbon buildup in the combustion chambers--Clean the chambers
with top engine cleaner. Follow the instructions on the can. |
• | Vacuum hoses for splits or kinks--Verify that the routing and connections
are as shown on the Vehicle Emission Control Information label. |
• | The exhaust system components for the following: |
- | Physical damage or possible internal failure |
- | The three-way catalytic converters for a restriction |
• | Electromagnetic interference (EMI) on the reference circuit can cause
a misfire condition. You can usually detect EMI with a scan tool by monitoring the
engine speed parameter. A sudden increase in the engine speed parameter with little
change in actual engine speed indicates that EMI is present. Inspect the high voltage
components near the ignition control circuit if a condition exists. |
• | Evaporative emission (EVAP) canister purge solenoid that is stuck open |
• | The engine cooling system for the following conditions: |
- If the above conditions do not address the symptom, refer to the additional
symptoms tests.
Additional Symptoms Tests
Detonation/Spark Knock: Test the engine for an overheating condition.
Refer to
Symptoms - Engine Cooling
.
Poor Fuel Economy: Inspect for foreign material accumulation
in the throttle bore, and for carbon deposits on the throttle plate and shaft. Also
inspect for throttle body tampering.
Rough, Unstable, or Incorrect Idle and Stalling: Inspect the
engine mounts.
Surges/Chuggles: Test the heated oxygen sensors (HO2S). The
HO2S should respond quickly to a change in throttle position. If the HO2S do not respond
to different throttle positions, inspect for contamination from fuel, silicon, or
the incorrect use of RTV sealant. The sensors may have a white powdery coating and
result in a high, but false, signal voltage, which gives a rich exhaust indication.
The ECM reduces the amount of fuel delivered to the engine, causing a driveability
condition.
Hard Start
• | Test the engine coolant temperature (ECT) sensor. Compare the ECT sensor
value to the intake air temperature (IAT) sensor value on a cold engine. The ECT and
IAT sensor values should be within ± 3°C (5°F). If the ECT sensor is
out of range with the IAT sensor, test the resistance of the ECT sensor. Refer to
Temperature Versus Resistance
for resistance
specifications. Replace the ECT sensor if the resistance is not within specification.
Refer to
Engine Coolant Temperature Sensor Replacement
. If the sensor is within the specification, test the ECT circuits for
a high resistance. |
• | Test the idle air control (IAC) system. |
Hesitation, Sag, Stumble
• | Test the manifold absolute pressure (MAP) sensor. Refer to
DTC P0106
. |
• | Test the generator. Refer to
DTC P0106
. Repair the charging system if the generator output voltage is less than 9 volts
or more than 16 volts. |
Poor Fuel Economy
• | Heavy loads being carried or towed. |
• | Acceleration rate too much or too often. |
• | Inspect for foreign material accumulation in the throttle bore, and for
carbon deposits on the throttle plate and shaft. Also inspect for throttle body tampering. |
Poor Fuel Fill Quality
Test for the following conditions that is applicable to the current symptom.
Difficult to Fill
• | The evaporative emission (EVAP) vent valve is stuck closed |
• | A condition with the internal components of the fuel tank assembly. |
| For more information, refer to: |
Fuel Odor