Rough, Unstable, or Incorrect Idle, Stalling
Checks
| Action
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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 Oxygen Sensor (O2S) sensor 1. 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 a scan tool and
compare the coolant temperature with the ambient air temperature on a cold engine.
If the coolant temperature reading is 5°C (41°F) more or less than the ambient
air temperature, check for high resistance in the coolant sensor circuit or a shifted
sensor. Refer to
Temperature Versus Resistance. |
| • | 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. |
| • | Inspect for the correct operation of the exhaust gas recirculation (EGR)
system. Excessive EGR flow can cause incorrect air fuel ratios resulting in inefficient
combustion and misfire at low engine speeds.. Refer to
Exhaust Gas Recirculation (EGR) System Diagnosis. |
| • | 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. |
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