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.

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

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Go to Step 2

Go to the Powertrain On Board Diagnostic (OBD) System Check

1. Perform a bulletin search.
2. If a bulletin that addresses the symptom is found, correct the condition as instructed in the bulletin.

Was a bulletin found that addresses the symptom?

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Go to Step 13

Go to Step 3

Was visual/physical check performed?

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Go to Step 4

Go to Visual/Physical Checks

1. Check the PCM grounds for being clean, tight and proper routing. Refer to Ground Distribution in Electrical Diagnosis.
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 5

Observe Long Term Fuel Trim Bn1 and Bn2 on the scan tool.

Is Long Term Fuel Trim Bn1 or Bn2 significantly in the negative range (Rich condition)?

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Go to Step 6

Go to Step 7

1. Check items that can cause the engine to run rich. Refer to DTC P0172 Fuel Trim System Rich Bank 1 .
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 9

Is Long Term Fuel Trim Bn1 or Bn2 significantly in the positive range (Lean condition)?

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Go to Step 8

Go to Step 9

1. Check items that can cause the engine to run lean. Refer to DTC P0171 Fuel Trim System Lean Bank 1 .
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 9

1. Check for incorrect idle speed. Ensure that the following conditions are present:

•	Engine fully warm.

•	Accessories OFF.

2. Using a scan tool, monitor IAC position.

Is IAC position within the specified values?

5-50

Go to Step 11

Go to Step 10

1. Visually/physically inspect for the following conditions:

•	Throttle body tampering (throttle shaft stop screw turned).

•	Restricted air intake system. Check for a collapsed air intake duct, restricted air filter element, or foreign objects blocking the air intake system.

•	Throttle body. Check for the following conditions:

-	Excessive deposits in the throttle bore and on the throttle plate.

-	Objects blocking the IAC passage or throttle bore.

-	Excessive deposits in the IAC passage or on the IAC pintle.

•	Large vacuum leak. Check for a condition that can cause a large vacuum leak, such as a incorrectly installed or faulty PCV valve or brake booster hose disconnected.

2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 8

1. Check the injector connections. Refer to

   Fuel Injectiors

   
   
   .
2. If any of the injector connectors are connected to an incorrect cylinder, correct as necessary.

Was a problem found?

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Go to Step 24

Go to Step 12

1. Check for proper operation of the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 13

1. Check for fuel in pressure regulator vacuum hose.
2. If fuel is present, replace the fuel pressure regulator assembly. Refer to Fuel Pressure Regulator Replacement .

Was a problem found?

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Go to Step 24

Go to Step 14

1. Check for proper ignition coil output voltage with a J 26792 spark tester:

-	Clip the spark tester to engine ground.

-	Connect one end of a spark plug wire to the spark tester; connect the other end of the spark plug wire to the coil being tested.

-	Connect one end of another spark plug wire to the other coil tower; connect the other end of the spark plug wire to ground.

-	Crank the engine while observing the spark tester; a crisp blue spark should be observed.

-	Repeat the above steps for each coil.

2. If adequate spark is not present at all coils, replace affected coil(s) as necessary. Refer to Ignition Control Module Replacement .

Was a problem found?

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Go to Step 24

Go to Step 15

1. Remove spark plugs.
2. Check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.
3. If a problem is found, repair as necessary. Refer to On-Vehicle Service in Ignition System.

Notice: Determine the cause of the oil or gas fouled spark plugs in order to avoid damage to the new spark plugs.

Was a problem found?

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Go to Step 24

Go to Step 16

1. Check for loose ignition control module ground. Refer to Ground Distribution in Electrical Diagnosis.
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 17

1. Check the ignition coils for cracks or carbon tracking.
2. If a problem is found, replace affected coil(s) as necessary. Go to Ignition Control Module Replacement .

Was a problem found?

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Go to Step 24

Go to Step 18

Using a scan tool, monitor TP angle with the engine idling.

Is TP angle at the specified value and steady?

0%

Go to Step 19

Go to DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage

1. Check PCV valve for proper operation. Refer to Crankcase Ventilation System
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 20

1. Check Transmission Range switch circuit. Refer to 4L60-E Transmission Diagnosis. .
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 21

1. Refer to Engine General Information and Mechanical Diagnosis to check for the following conditions:

•	Low compression.

•	Sticking or leaking valves.

•	Worn camshaft lobe(s).

•	Incorrect valve timing.

•	Bent push rods.

•	Worn rocker arms.

•	Broken valve springs.

2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 22

1. Check for faulty motor mounts.
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 23

1. Review all diagnostic procedures within this table.
2. If all procedures have been completed and no malfunctions have been found, review/inspect the following:

•	Visual/physical inspection. Refer to Visual/Physical Checks .

•	Scan Tool data. Refer to Engine Scan Tool Data List .

•	Freeze Frame data/Failure Records data.

•	All electrical connections within a suspected circuit and/or system.

3. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 24

Review technical bulletins

Operate the vehicle to duplicate conditions present when the original symptom was noted.

Is symptom still present?

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Go to Step 3

System OK