Checks
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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. The engine idles at
an incorrect speed.
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Preliminary Check
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Sensor Checks
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• | Check the Heated Oxygen Sensor (HO2S) sensor 1 and 2. Refer to
DTC P0130 through DTC P0141. |
• | Check the Throttle Position (TP) sensor. A sticking throttle shaft
or a binding throttle linkage will causes a high TP sensor voltage (open throttle
indication). Under these conditions the PCM may not control the idle. Monitor
the TP sensor angle with the scan tool. The indicated angle should be aproximately
11% with throttle closed. |
• | Check the Engine Coolant Temperature (ECT) sensor -- Using
the scan tool, compare the engine coolant temperature with the ambient air
temperature on a cold engine. If the engine coolant temperature is more
than 5°C (41°F) greater or less than the ambient air temperature,
check for a high resistance in coolant sensor circuit or the sensor itself.
Refer to
Temperature Versus Resistance
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• | Check the MAP sensor for a quick response and accuracy. |
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Fuel System Checks
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• | Check if a Rich or Lean system can cause the condition. Drive
the vehicle at the speed of the complaint. Monitoring the Fuel Trim may help
identify the problem. |
- | Lean -- The Long Term Fuel Trim will be greater than 20%.
Refer to Diagnostic Aids. |
- | Rich -- The Long Term Fuel Trim will be less than -20%.
Refer to Diagnostic Aids. |
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Ignition System Checks
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• | Check for the proper secondary voltage output with a J 26792
or an equivalent. |
• | Check the spark plugs for any of the following conditions: |
- | Wet (fuel fouled) plugs. |
- | A terminal screw that is bent or loose (rotate and pull). |
- | A ceramic insulator that has cracks, carbon tracking or deposits
(red or black dust) |
- | A tip insulator that has cracks, carbon tracking, deposits (oil,
carbon, glazing, fuel additive), or is loose (rattles). |
- | A center electrode that is loose (wiggles side to side) |
- | An incorrect spark plug gap that is too large or too small. |
- | Platinum pads that are missing. |
• | Check the spark plug cables by connecting an ohmmeter to the ends
of each cable. Replace any spark plug cables with a resistance greater than
25,000 ohms. |
• | Observe the Spark parameter and the IAC Duty Cycle parameter on
the scan tool at idle, while operating the rear defogger, the A/C system,
and the headlights. The Spark parameter should increase 2-3 degrees
and the IAC Duty Cycle parameter should increase 4-7% when operating
the engine loads. If the Spark and IAC Duty Cycle parameters remain fixed,
check for a short to ground in the Diagnostic Request circuit from PCM terminal
C2-15 to the underhood diagnostic link connector. |
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Engine Mechanical Check
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• | Check for any of the following engine mechanical problems: |
- | Faulty hydraulic lifter assemblies |
- | Broken or weak valve springs |
- | Sticking or leaking valves |
- | Incorrect or worn camshaft |
• | Refer to Engine Mechanical. |
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Additional Checks
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• | Check for vacuum leaks. Vacuum leaks can cause a higher than normal
idle speed. |
• | Check the PCM grounds for clean, tight, and proper connections.
Refer to PCM Wiring Diagram. |
• | Check the battery cables and ground straps. They should be clean
and secure. |
• | Check the A/C refrigerant pressure for being too high or for a
faulty high pressure switch. |
• | Check for the proper operation of the IAC system. Use a scan tool
to command the IAC valve to increase and decrease engine speed. |
• | Check that the PCM is increasing the idle speed when the P/S system
pressure is increased on turns. Refer to
PSP Switch Diagnosis
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