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
|
| • | Check for a restricted fuel filter. |
| • | Check 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. |
| • | Check for fuel injectors that are shorted internally. Compare
the injector resistances. |
| • | Check for fuel in the pressure regulator vacuum hose. If any fuel
is present, replace the regulator assembly. |
| • | Check 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. |
| • | Check the items that can cause an engine to run rich. |
| • | Check the items that can cause an engine to run lean. |
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Sensor and System
|
| • | Check 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 throllte 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. |
| • | Check the exhaust gas recirculation (EGR) system for the correct
operation. |
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Ignition System
| Remove the spark plugs and check for the following conditions:
| | 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, the cause of the fouling
must be determined before replacing the spark plugs.
Check the plugs and coils for signs of arcing, cracks, or carbon tracking.
| • | Check for any loose ignition control module grounds. |
| • | Check the spark plugs for the correct heat range. |
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Engine Mechanical
| Check 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 parts: |
Refer to
Symptoms - Engine Mechanical
in Engine Mechanical.
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Additional Checks
|
| • | Visually and physically check the vacuum hoses for splits, kinks,
and the correct connections and routing as shown on the Vehicle Emission Control
Information label. |
| • | Check the exhaust system for a restriction. Refer to
Restricted Exhaust
in Engine Exhaust. |
| • | Check 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 check 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, check
for an improper routing of the harness near the high voltage components
or the high current components. |
| • | Check 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. |
| • | Check the transaxle range switch circuit. Ensure the scan tool
indicates the vehicle is in drive with the gear selector in Drive or in Overdrive. |
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