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Rough, Unstable or Incorrect Idle, Stalling

WARNING: This page is about a different car, the 2006 Suzuki Verona. However, it is still accessible from the selected car via links, so may be relevant.

DEFINITION: 

The engine runs unevenly at idle. If the condition is bad enough, the vehicle may shake. Also, the idle varies in RPM (called "hunting"). Either condition may be severe enough to cause stalling. The engine idles at incorrect idle speed. 

CAUTION: Before diagnosing the symptom, check service bulletins for updates.
ROUGH, UNSTABLE OR INCORRECT IDLE, STALLING - DIAGNOSIS PROCEDURE

Step Action Value(s) Yes No
1 Were the Important Preliminary Checks performed? - Go to Step 2. Go to Symptom Diagnosis .
2
  1. Connect the scan tool to the Data Link Connector (DLC).
  2. Monitor the Oxygen Sensor (O2S) reading at different throttle positions.

Does the O2S change quickly from rich to lean at the different throttle positions?		 - Go to Step 5. Go to Step 3.
3 Check the O2S for contamination from fuel or improper use of RTV sealant.
Is the O2S contaminated?		 - Go to Step 4. Go to Step 5.
4 Replace the contaminated O2S as needed.
Is the repair complete?		 - System OK -
5
  1. Check for a sticking throttle shaft or binding throttle linkage that may cause incorrect Throttle Position (TP) sensor voltage.
  2. Check the TP sensor voltage reading with the throttle closed.

Is the TP sensor voltage within the value specified?		 0.4 - 0.8 V Go to Step 6. Go to DTC P0123: Throttle Position (TP) Sensor 1 Circuit High Voltage .
6
  1. Check the Engine Coolant Temperature (ECT) sensor voltage reading using the scan tool.
  2. Compare the ECT sensor reading with the ambient temperature when the engine is cold.

Does the ECT sensor temperature reading differ from the ambient temperature by more than the value specified?		 3 °C (37.4 °F) Go to Step 7. Go to Step 9.
7 Check for high resistance in the ECT sensor circuit or the sensor itself.
Is the problem found?		 - Go to Step 8. Go to Step 9.
8 Replace the ECT sensor or repair the circuit as needed.
Is the repair complete?		 - System OK -
9 Check the Manifold Absolute Pressure (MAP) sensor for response and accuracy.
Is the problem found?		 - Go to Step 10. Go to Step 11.
10 Replace the MAP sensor or repair the MAP sensor circuit as needed.
Is the repair complete?		 - System OK -
11
  1. Road test the vehicle at the speed of the complaint.
  2. Monitor the long term fuel trim reading using the scan tool.

Is the long term fuel trim reading within the value specified?		 -20 - 20% Go to Step 14. Go to Step 12.
12 Is the long term fuel trim reading below the value specified? -20% Go to DTC P0172: Fuel Trim System Rich Bank 1 . Go to Step 13.
13 Is the long term fuel trim reading above the value specified? 20% Go to DTC P0171: Fuel Trim System Lean Bank 1 . -
14
  1. Disconnect all of the fuel injector harness connectors at the fuel injectors.
  2. Connect an injector test light between the harness terminals of each fuel injector connector.
  3. Note the test light while cranking the engine.

Does the test light blink for all of the fuel injectors?		 - Go to Step 16. Go to Step 15.
15
  1. Repair or replace the faulty injector drive circuit harness, the connector, or the connector terminals as needed.
  2. If the harness, the connectors, and the terminals are OK, replace the Engine Control Module (ECM).

Is the repair complete?		 - System OK -
16 Measure the resistance of each of the fuel injectors.
Is the resistance within the value specified (the resistance will increase slightly at higher temperatures)?		 11.6 - 12.4 Ω Go to Step 18. Go to Step 17.
17 Replace any fuel injectors with a resistance that is out of specifications.
Is the repair complete?		 - System OK -
18 Perform an FUEL INJECTOR BALANCE TEST .
Is the problem found?		 - Go to Step 19. Go to Step 20.
19 Replace any leaking or restricted fuel injectors.
Is the repair complete?		 - System OK -
20
  1. With the engine OFF, disconnect the fuel pressure regulator vacuum hose.
  2. Thoroughly inspect the fuel pressure regulator vacuum port and the fuel pressure regulator vacuum hose for the presence of fuel.

Is the problem found?		 - Go to Step 21. Go to Step 22.
21 Replace the fuel pressure regulator as needed.
Is the repair complete?		 - System OK -
22
  1. Check the ignition system output voltage for all of the cylinders using a spark tester.
  2. Inspect the spark plugs for excessive wear, insulation cracks, improper gap or heavy deposits.
  3. Inspect the ignition wires for cracking, hardness or improper connections.

Is the problem found?		 - Go to Step 23. Go to Step 24.
23 Repair or replace any ignition system components as needed.
Is the repair complete?		 - System OK -
24
  1. Inspect for vacuum leaks.
  2. Check for proper Positive Crankcase Ventilation (PCV) operation.
  3. Check the Idle Air Control (IAC) valve operation.
  4. Inspect the ECM ground connections.

Is the problem found?		 - Go to Step 25. Go to Step 26.
25 Repair or replace any components as needed.
Is the repair complete?		 - System OK -
26
  1. Check the Exhaust Gas Recirculation (EGR) valve for proper operation.
  2. Inspect the battery cables and the ground straps for proper connections.
  3. Check the generator voltage output. Repair or replace the generator if the voltage output is not within the value specified.

Is the problem found?		 12 - 16 V Go to Step 27. Go to Step 28.
27 Repair or replace any components as needed.
Is the repair complete?		 - System OK -
28
  1. Inspect for broken engine mounts.
  2. Check for proper valve timing.
  3. Perform a COMPRESSION TEST .
  4. Inspect for bent pushrods, worn rocker arms, broken or weak valve springs and a worn camshaft.
  5. Perform repairs as needed.

Are all of the checks and needed repairs complete?		 - System OK -
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