Checks
| Action
|
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.
|
Preliminary Checks
|
| • | Refer to
Symptoms
,
Important Preliminary Checks Before Starting. |
| • | Check the PCM grounds for being clean, tight and in their proper
locations. |
| • | Remove and check the air filter element for dirt, or for being
restricted. Replace as necessary. |
|
Fuel System
|
| • | Check for a restricted fuel filter. |
| • | Check for incorrect minimum idle speed. |
| • | Check that each injector harness is connected to the correct injector/cylinder
according to the firing order. Relocate injector harnesses as necessary. |
| • | Check for injectors that are shorted internally. Compare injector
resistance. |
| • | Check for fuel in pressure regulator vacuum hose. If fuel is present,
replace the regulator assembly. |
| • | Check for foreign material accumulation in the throttle bore,
coking on the throttle valve or on the throttle shaft. Also inspect for throttle
body tampering. |
| • | Check items which can cause an engine to run rich (long term fuel
trim near -99%). |
| • | Check items that can cause an engine to run lean (long term fuel
trim near 100%). |
|
Sensor/System
|
| • | Check the crankcase ventilation valve for proper operation. Place
a finger over the inlet hole in the valve end several times. The valve should
snap back. If not, replace valve. |
| • | Using a scan tool, monitor TP sensor voltage. The TP sensor voltage
should be less then 0.9 volts at rest. If the TP sensor voltage is greater
than 0.9 volt, inspect the throttle body and accelerator cables for binding. |
| • | Using a scan tool, monitor the KS system for excessive spark
retard activity. |
| • | Check the EGR system for proper operation. |
|
Ignition System
| Remove the spark plugs and check for the following:
| | An improper spark plug gap will cause a driveability problem. The spark
plug gapping should be done using a wire gauge gap tool (J 41319). |
If spark plugs are gas or oil fouled, the cause of the fouling
must be determined before replacing the spark plugs.
Check the plugs/coils for signs of arcing, cracks or carbon tracking.
| • | Check for loose ignition control module grounds. |
| • | Check the spark plugs for proper heat range. |
|
Engine Mechanical
| Check for the following conditions:
| • | Excessive oil in combustion chamber may indicate leaking valve
seals. Refer to Oil Consumption Diagnosis
. |
| • | Low cylinder compression. Refer to Engine Compression Test
. |
| • | Incorrect basic engine parts. Inspect the following: |
Refer to Base Engine Misfire Diagnosis
.
|
Additional Checks
|
| • | Visually/physically check: Vacuum hoses for splits, kinks, and
proper connections and routing as shown on Vehicle Emission Control Information
label. |
| • | Check for 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 erratic sensor values or excessive
changes in indicated RPM with little change in actual engine RPM. This
may indicate EMI is present. If a problem exists, check for improper routing
of the harness near high voltage components (Ignition Components) or high
current components (alternator, motors, solenoids etc.). |
| • | Check the charging system. a bad diode in the alternator can
allow AC noise to enter the electrical system. This may affect the engine
controls system operation. Refer to
Charging System Check
. |
| • | Check the Trans Range switch circuit and be sure the scan tool
indicates vehicle is in drive with gear selector in drive or overdrive. |
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