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Inspection procedure 7: Unstable idle (rough idle, hunting)
 
 
 
 
COMMENT
  • In cases such as the above, the cause is probably the air/fuel mixture. Other systems affecting idle quality include the ignition system and compression.
 
 
TROUBLESHOOTING HINTS (The most likely causes for this case: )
  • Fuel pump system failed.
  • Ignition system failed.
  • Injector system failed.
  • Poor compression.
  • Evaporative emission control system failed.
  • EGR system failed.
  • Improper operation of the PCV valve.
  • Charging system failed.
  • Vacuum leak.
  • Contaminated fuel.
  • ECM failed.

 
 
DIAGNOSIS
Required Special Tools:
  • MB992744: Vehicle communication interface-Lite (V.C.I.-Lite)
  • MB992745: V.C.I.-Lite main harness A
  • MB992747: V.C.I.-Lite USB cable short
  • MB992748: V.C.I.-Lite USB cable long
  • MB991958: Scan Tool (M.U.T.-III Sub Assembly)
    • MB991824: Vehicles Communication Interface (V.C.I.)
    • MB991827: M.U.T.-III USB Cable
    • MB991910: M.U.T.-III Main Harness A (Vehicles with CAN communication system)
  • MB992110: Power Plant ECU Check Harness
STEP 1. Check if the EGR valve sounds during initialization.
Is the initialization sound heard in the EGR valve?
Go to Step 2.
Refer to GROUP 17, Emission Control - Exhaust Gas Recirculation (EGR) System - EGR Valve (Stepper Motor) Check .
 
STEP 2. Using scan tool (M.U.T.-III), read the diagnostic trouble code (DTC)
cautionTo prevent damage to scan tool (M.U.T.-III), always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool (M.U.T.-III).
(1) Connect scan tool (M.U.T-III). Refer to HOW TO CONNECT THE SCAN TOOL (M.U.T.-III) .
(2) Turn the ignition switch to the "ON" position.
(3) Read the DTC.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is any DTC stored?
Refer to, Diagnostic Trouble Code Chart .
Go to Step 3.
 
STEP 3. Using scan tool (M.U.T.-III), check actuator test.
(1) Turn the ignition switch to the "ON" position.
(2) Check the following items in the actuator test. Refer to Actuator Test Reference Table .
  1. Item 1: Injectors
  2. Item 10: Evaporative Emission Purge Solenoid
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Are they operating properly?
Go to Step 4.
Repair or replace it. Then confirm that the malfunction symptom is eliminated.
 
STEP 4. Using scan tool (M.U.T.-III), check data list.
(1) Turn the ignition switch to the "ON" position.
(2) Check the following items in the data list. Refer to Data List Reference Table .
  1. Item 5: Intake Air Temperature Sensor
  2. Item 6: Engine Coolant Temperature Sensor
  3. Item 10: Mass Airflow Sensor
  4. Item 76: A/C Switch
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Are they operating properly?
Go to Step 5.
Repair or replace it. Then confirm that the malfunction symptom is eliminated.
 
STEP 5. Check the fuel pressure.
Refer to Fuel Pressure Test .
Is the fuel pressure normal?
Go to Step 6.
Repair it. Then confirm that the malfunction symptom is eliminated.
 
STEP 6. Check the ignition timing.
Refer to GROUP 11C, On-vehicle Service - Ignition Timing Check .
Is the ignition timing normal?
Go to Step 7.
Check for installed conditions of the timing chain. Then confirm that the malfunction symptom is eliminated.
 
STEP 7. Measure the voltage at ECM connector by using check harness special tool MB992110.
(1) Disconnect all ECM connectors. Connect the power plant ECU check harness special tool MB992110 between the separated connectors.
(2) Start the engine and run at idle.
noteVehicles for Canada, the headlight, taillight, etc. remain lit even when the lighting switch is in "OFF" position but this is no problem for checks.
(3) Measure the voltage between terminal ALTG line and ground.
  1. Engine: warming up
  2. Radiator fan: stopped
  3. Headlight: OFF to ON
  4. Rear defogger switch: OFF to ON
  5. Stop light: OFF to ON
    • Voltage should increase.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Did the measured voltage increase?
Go to Step 8.
Replace the generator. Then confirm that the malfunction symptom is eliminated.
 
STEP 8. Check the spark plugs.
Refer to GROUP 16, Ignition System - On-vehicle Service - Spark Plug Check and Cleaning .
Are there any abnormalities?
Replace the spark plug. Then confirm that the malfunction symptom is eliminated.
Go to Step 9.
 
STEP 9. Check the ignition coil.
Refer to GROUP 16, Ignition System - On-vehicle Service - Ignition Coil Check .
Are there any abnormalities?
Replace the ignition coil. Then confirm that the malfunction symptom is eliminated.
Go to Step 10.
 
STEP 10. Check the compression pressure.
Refer to GROUP 11C, On-vehicle Service - Compression Pressure Check .
Is the compression pressure normal?
Go to Step 11.
Repair it. Then confirm that the malfunction symptom is eliminated.
 
STEP 11. Check the positive crankcase ventilation system.
Refer to GROUP 17, Emission Control - Positive Crankcase Ventilation System - Crankcase Ventilation System Check .
Is the positive crankcase ventilation system normal?
Go to Step 12.
Repair it. Then confirm that the malfunction symptom is eliminated.
 
STEP 12. Check the evaporative emission control system.
Refer to GROUP 17, Emission Control - Evaporative Emission Control System - Evaporative Emission Control System Check (purge flow check) .
Is the evaporative emission control system normal?
  • Check the following items, and repair or replace the defective items.
    1. Vacuum leak.
    2. Check if fuel is contaminated.
    3. Check if the injectors are clogged.
  • Then confirm that the malfunction symptom is eliminated.
Repair it. Then confirm that the malfunction symptom is eliminated.