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 Powertrain 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 Check

1. Check PCM grounds for being clean, tight and
2. proper routing. Refer to PCM wiring diagrams in Electrical Diagnosis.
3. 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 on the scan tool.

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

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Go to Step 7

1. Check items that can cause the engine to run rich. Refer to Diagnostic Aids in Diagnostic Support page of DTC P0172.
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 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 Diagnostic Aids in Diagnostic Support page of DTC P0171
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. Monitor 24X RPM on the scan tool for erratic rpm fluctuations. 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? (Value is displayed in counts).

5-50

Go to Step 11

Go to Step 10

1. Visually/physically inspect for the following conditions:

•	Throttle body tampering (Adjustment screw plug removed).

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

•	Throttle body. Check for objects blocking the IAC passage or throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate.

•	Large vacuum leak. Check for a condition that causes a large vacuum leak, such as a incorrectly installed or faulty crankcase ventilation 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

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 Injector Coil/Balance Test in Fuel Metering 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 13

1. Check for fuel in pressure regulator vacuum hose.
2. If fuel is present, replace the fuel pressure regulator assembly. Refer to Fuel Metering System.
3. If a problem is found, repair as necessary.

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 spark tester J 26792 (ST 125):

•	Clip the spark tester J 26792 to engine ground.

•	Connect J 26792 to an ignition wire then ground spark plug end of ignition wire from companion cylinder. The companion cylinder is the cylinder that shares the same ignition coil.

•	Crank the engine while observing the spark tester J 26792; a crisp blue spark should be observed.

•	Repeat the above steps for each coil.

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

Was a problem found?

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Go to Step 24

Go to Step 15

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

1. Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits. Refer to Electronic Ignition 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 16

1. Check for loose ignition control module ground. Refer to Electronic Ignition 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 17

1. Check ignition coils for cracks or carbon tracking.
2. If a problem is found, repair as necessary.

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

Refer to DTC P0123 for further diagnosis

1. Check Crankcase Ventilation 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 Transaxle Range switch circuit; use a scan tool and be sure the scan tool indicates that the vehicle is in drive with gear selector in drive or overdrive.
2. If a problem is found, diagnose and repair Transaxle Range switch as necessary (Refer to 4T60-E Automatic Transaxle Diagnosis).

Was a problem found?

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Go to Step 24

Go to Step 21

1. Check the following engine mechanical items. Refer to Engine Mechanical for diagnosis procedures:

•	Check compression.

•	Sticking or leaking valves.

•	Worn camshaft lobe(s).

•	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. Refer to Engine Mechanical for inspection of 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.

•	Scan Tool data.

•	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 Service 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

Repair complete