Rough, Unstable, or Incorrect Idle, Stalling

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.

1. Was the "On-Board Diagnostic (OBD) System Check" performed?

[YES] Go to Step 2.
[NO] Go to OBD System Check. Description of On-Board Diagnostics

2. a. Perform a bulletin search.
b. If a bulletin that addresses the symptom is found, correct the condition as instructed in the bulletin.
c. Was a bulletin found that addresses the symptom?

[YES] Verify repair.
[NO] Go to Step 3.

3. Was a visual/physical check performed?

[YES] Go to Step 4.
[NO] Go to Visual/Physical Check. Visual/Physical Check

4. a. Check the Powertrain Control Module (PCM) grounds for cleanliness, tightness, and proper routing. Refer to PCM wiring diagrams.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 5.

5. a. Observe the long term fuel trim on the scan tool.
b. Is the long term fuel trim significantly in the negative range (rich condition)?

[YES] Go to Step 6.
[NO] Go to Step 7.

6. a. Check items that can cause the engine to run rich. Refer to Diagnostic Aids in DTC PO172.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 9.

7. Is the long term fuel trim significantly in the positive range (lean condition)?

[YES] Go to Step 8.
[NO] Go to Step 9.

8. a. Check items that can cause the engine to run lean. Refer to Diagnostic Aids in DTC PO171.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 9.

9. a. Check for incorrect idle speed. Ensure that the following conditions are present:
- The engine is fully warm.
- The accessories are "OFF."
b. Using a scan tool, monitor the IAC position.
c. Is the IAC position within the specified values?

[VALUES] Between 10 and 50 counts.

[YES] Go to Step 11.
[NO] Go to Step 10.

10. a. Visually/physically inspect for the following conditions:
- 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 Idle Air Control (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 an incorrectly installed or faulty crankcase ventilation valve or a disconnected brake booster hose.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 8.

11. a. Check the injector connections. If any of the injectors are connected to an incorrect cylinder, correct as necessary.
b. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 12.

12. a. Perform the "Injector Coil/Balance Test". Injector Balance Test
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 13.

13. Check for fuel in the pressure regulator vacuum hose.
b. If fuel is present, replace the fuel pressure regulator assembly.
c. If a problem is found, repair as necessary.
d. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 14.

14. a. Check for proper ignition voltage output with spark tester J 26792 (ST-125).
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 15.

15. a. Remove spark plugs. Check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.

NOTE: If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 16.

16. a. Check for a loose ignition control module ground.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 17.

17. a. Check ignition coils for cracks or carbon tracking.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 18.

18. a. Using a scan tool, monitor the throttle position (TP) angle with the engine idling.
b. Is the TP angle at the specified value and steady?

[VALUES] 0%

[YES] Go to Step 19.
[NO] Refer to DTC P0123 for further diagnosis.

19. a. Check the positive crankcase ventilation (PCV) valve for proper operation.
b. It a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 20.

20. a. Check the transmission range switch circuit. Use a scan tool and be sure the scan tool indicates that the vehicle is in drive with the gear selector in drive or overdrive.
b. If a problem is found, diagnose and repair the transmission range switch as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 21.

21. a. Check the following engine mechanical problems:
- Exhaust Gas Recirculation (EGR) valve mounted backward. Compare with a known good vehicle.
- Low compression.
- Sticking or leaking valves.
- Worn camshaft lobe(s).
- Camshaft drive belt slipped or stripped.
- Incorrect valve timing.
- Worn rocker arms.
- Broken valve springs.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 22.

22. a. Check for faulty motor mounts.
b. If a problem is found, repair as necessary.
c. Was a problem found?

[YES] Verify repair.
[NO] Go to Step 23.

23. a. Review all diagnostic procedures within this table.
b. 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 buffer.
- All electrical connections within a suspected circuit and/or system.
c. Was a problem found?

[YES] Verify repair.
[NO] Contact Technical Assistance.