Checks
| Action
|
Definition: The engine runs unevenly at idle. If severe enough, the engine
or vehicle may shake. The engine idle speed may vary in RPM. Either condition
may be severe enough to stall the engine.
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Preliminary Check
| Refer to
Symptoms
.
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Sensor Checks
|
| • | Check the Heated Oxygen Sensor (HO2S). |
| | Check for silicon contamination
from fuel or improperly used sealant. The sensor will have a white powdery coating.
The sensor will result in a high but false signal voltage (rich exhaust indication).
The Control Module will reduce the amount of fuel delivered to the engine causing
a severe driveability problem. |
| • | Check the Throttle Position (TP) sensor. If a sticking throttle shaft
or binding linkage causes a high TP sensor open throttle indication, the Control
Module will not control the idle. Monitor the TP sensor voltage. A scan tool
and/or voltmeter should read less than 0.85 volts with the throttle closed. |
| • | Check the Engine Coolant Temperature (ECT) sensor using the scan tool
to compare the engine coolant temperature with the ambient air temperature on
a cold engine. If the coolant temperature reading is more than 5 degrees
greater than or less than the ambient air temperature on a cold engine, check
for a high resistance in the coolant sensor circuit or the sensor itself. |
| • | Check the Manifold Absolute Pressure (MAP) sensor response and accuracy. |
|
Fuel System Checks
|
| • | Check to determine if a Rich or Lean system causes the condition. Drive
the vehicle at the speed of the complaint. Monitoring the Fuel Trim will help
identify the problem. |
| - | Lean - The Long Term Fuel Trim is greater than 150. |
| - | Rich - The Long Term Fuel Trim is less than 115. |
| • | Check the fuel injector driver circuit. |
| 1. | Disconnect the injector harness connector at the injectors. |
| 2. | Connect an injector test light between the terminals of each injector
connector and note the light while cranking. |
| 3. | If the test light fails to blink at any connector, it is a faulty injector
drive circuit harness, connector, or terminal. |
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Ignition System Checks
|
| • | Check for the following conditions: |
| • | Check the spark plug wires by connecting a J 39200
DMM to the ends of each wire in question. If the meter reads over
30,000 ohms, then replace the wires. |
|
Additional Checks
|
| • | Check for vacuum leaks. Vacuum leaks can cause a higher than normal idle
and low Idle Air Control (IAC) counts. |
| • | Check the Control Module grounds for being clean, tight, and in their
proper locations. |
| • | Check the battery cables and ground straps. They should be clean and
secure. |
| • | Check the Crankcase Ventilation Valve for proper operation by placing
a finger over the inlet hole in the valve end several times. The valve should
snap back. If not, replace the valve. Refer to Crankcase Ventilation
System Diagnosis
. |
|
Engine Mechanical Check
| Check for the following:
| • | Broken or weak valve springs. |
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