Checks
| Action
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DEFINITION: Engine runs unevenly at idle. If severe, the
engine or vehicle may shake. Engine idle speed may vary in RPM. Either
condition may be severe enough to stall the engine.
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Preliminary
Checks
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| • | Verify that the powertrain control module (PCM) grounds are clean,
tight, and in the correct locations. |
| • | Inspect the air filter element for dirt and for restrictions.
Replace the element as necessary. |
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Fuel System
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| • | Inspect for a restricted fuel filter. |
| • | Inspect for an incorrect minimum idle speed. |
| • | Verify that each fuel injector harness is connected to the correct
injector and cylinder according to the firing order. Relocate any fuel injector
harnesses as necessary. |
| • | Inspect for fuel injectors that are shorted internally. Compare
the injector resistances. |
| • | Inspect for fuel in the pressure regulator vacuum hose. If any
fuel is present, replace the regulator assembly. |
| • | Inspect for any foreign material in the throttle bore, and for
coking on the throttle valve or on the throttle shaft. Also inspect for any
throttle body tampering. |
| • | Inspect the items that can cause an engine to run rich. |
| • | Inspect the items that can cause an engine to run lean. |
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Sensor and System
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| • | Inspect the crankcase ventilation valve for the correct operation.
Place a finger over the inlet hole in the valve end several times. The valve
should snap back. If not, replace the valve. |
| • | Using a scan tool, monitor the throttle position (TP) sensor voltage.
The TP sensor voltage should be less than 0.9 volt at rest. If the
TP sensor voltage is more than 0.9 volt, inspect the throttle body
and the accelerator cables for binding. |
| • | Using a scan tool, monitor the knock sensor (KS) system for any
excessive spark retard activity. |
| • | Inspect the exhaust gas recirculation (EGR) system for the correct
operation. |
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Ignition System
| Remove the spark plugs. Inspect
for the following:
| | An improper spark plug gap will cause a driveability problem. The spark
plug gapping should be done with a wire gauge gap tool. |
If the spark plugs are gas or oil fouled, determine the cause of
the fouling before replacing the spark plugs.
Inspect the plugs and coils for signs of arcing, cracks, or carbon tracking.
| • | Inspect for any loose ignition control module grounds. |
| • | Inspect the spark plugs for the correct heat range. |
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Engine Mechanical
| Inspect for the following conditions:
| • | Any excessive oil in the combustion chambers may indicate leaking
valve seals. Refer to
Oil Consumption Diagnosis
in Engine Mechanical. |
| • | Any incorrect basic engine parts. Inspect the following: |
Refer to
Symptoms - Engine Mechanical
in Engine Mechanical-3.5L.
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Additional Checks
|
| • | Inspect the vacuum hoses for splits, kinks, and the correct connections
and routing as shown on the Vehicle Emission Control Information label. |
| • | Inspect the exhaust system for a restriction. Refer to
Restricted Exhaust
in Engine Exhaust. |
| • | Inspect for any improper harness routing. An engine miss condition
can be caused by electromagnetic interference (EMI) on some engine controls
circuits. Use the scan tool to inspect for any erratic sensor values
or excessive changes in the indicated RPM with little change in the
actual engine RPM. This may indicate EMI is present. If a problem exists,
inspect for an improper routing of the harness near the high voltage
components or high the current components. |
| • | Inspect the charging system. A bad diode in the alternator can
allow A/C noise to enter the electrical system. This may affect the engine
controls system operation. Refer to
Diagnostic Starting Point - Engine Electrical
in Engine Electrical. |
| • | Inspect the transaxle range switch circuit. Ensure that the scan
tool indicates the vehicle is in drive with the gear selector in Drive or
in Overdrive. |
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