Definition: The engine runs unevenly at idle. If the condition is severe, the engine or vehicle may shake. The engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Did you perform the On-Board Diagnostic (OBD) System Check?

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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 present, 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

Did you perform a visual/physical check?

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Go to Step 4

Go to Symptoms for the Visual /Physical Check

1. Check the ECMPCM grounds for being clean, tight and for proper routing.
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 5

Observe the Long Term Fuel Trim on the scan tool.

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

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Go to Step 7

1. Check the items that can cause the engine to run rich.
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 9

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

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Go to Step 8

Go to Step 9

1. Check the items that can cause the engine to run lean.
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 9

1. Check for an incorrect idle speed. Before checking, make sure that the following conditions are met:

•	The engine is at the normal operating temperature.

•	All of the accessories are off.

•	The transmission is in P/N

2. Monitor the IAC Long Air Trim with a scan tool.

Is the IAC Long Air Trim slewed excessively positive or negative?

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Go to Step 11

Go to Step 10

1. Visually/physically inspect for the following conditions:

•	Throttle body tampering (Adjustment screw plug removed).

•	A restricted air intake system. Check for the following items:

•	A possible collapsed air intake duct

•	A restricted air filter element

•	Any foreign objects that could be blocking the air intake system

•	Any objects that could be blocking the IAC passage or the throttle bore

•	Excessive deposits in the IAC passage and in the IAC valve

•	Excessive deposits in the throttle bore and on the throttle plate

•	A large vacuum leak. Check for a condition that causes a large vacuum leak, such as an incorrectly installed or malfunctioning crankcase ventilation valve or a disconnected brake booster hose.

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

Was a problem found?

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Go to Step 24

Go to Step 11

Check the injector connections. 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. Perform the Fuel Injector Solenoid Coil Test .
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 13

1. Check for any fuel in the pressure regulator vacuum hose.
2. If there is fuel present, replace the fuel pressure regulator assembly. Refer to Fuel Pressure Regulator Replacement .
3. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 14

1. Check for a proper ignition coil output voltage with spark tester J 26792 (ST 125) by performing the following procedure:

1.1.	Clip the spark tester J 26792 to engine ground.

1.2.	Connect one end of a spark plug wire to the spark tester J 26792 . Leave the other end of the spark plug wire connected to the coil being tested.

1.3.	Disconnect the spark plug wire from the other spark plug on the same coil being tested, and ground the wire.

1.4.	Crank the engine while observing the spark tester J 26792 . Look for a blue spark.

1.5.	Repeat the above steps for each coil.

2. If an adequate spark is not present at all of the coils, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 15

1. Remove the spark plugs.
2. Check the spark plugs for the following conditions:

•

Wet plugs

•

Cracks

•	Excessive wear

•	An improper gap

•	Burned electrodes

•	Carbon tracking on the insulator

•	Heavy deposits. Refer to Ignition System Check .

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

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

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 the primary connections at the ignition coil assembly. Refer to Engine Electrical, and Troubleshooting Procedures for the proper procedure. Also, refer to Checking Terminal Contact in Engine Control Module Diagnosis .
2. If a problem is present, 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 any cracks or for any carbon tracking.
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 18

Use a scan tool in order to monitor the TP Signal with the engine idling.

Is the TP angle at the specified value and steady?

0%

Go to Step 19

Refer to DTC P0120 Throttle Position (TP) Sensor Circuit

1. Check the crankcase ventilation valve for a proper operation.
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 20

1. Check the Transmission Range switch circuits with a scan tool. Be sure that the scan tool indicates that the vehicle is in drive with the gear selector in drive or overdrive.
2. If a problem is present, diagnose and repair the Transmission Range switch as necessary. Refer to Transmission Fluid Check .

Was a problem found?

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Go to Step 24

Go to Step 21

1. Check for the following Engine Mechanical problems. Refer to Engine Mechanical :

•	Low compression

•	Sticking valves

•	A hydraulic valve lifter problem

•	Leaking cylinder head gaskets

•	Worn or incorrect camshaft

•	Incorrect cam timing

•	A damaged timing belt

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

Was a problem found?

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Go to Step 24

Go to Step 22

1. Check for a damaged engine mount. Refer toEngine Mechanical for inspection of mounts.
2. If a problem is present, repair as necessary.

Was a problem found?

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Go to Step 24

Go to Step 23

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

•	Perform a visual/physical inspection.

•	The scan tool data.

•	The Freeze Frame data.

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

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

Was a problem found?

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Go to Step 24

Go to Step 3

Operate the vehicle under the same conditions (as closely as possible) as when the original symptom occurred.

Is the original symptom still present?

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Go to Step 3

The repair is complete. If another driveability symptom exists, refer to Symptoms .