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Idle Mixture

NOTE: Idle mixture must be checked with air cleaner installed, all air intake system hoses and vacuum lines connected, EFI system wiring connectors tight, transmission/transaxle in Neutral, all accessories and cooling fan off (if equipped) and engine at normal operating temperature.
  1. Ensure idle speed is within specification. Install jumper wire between "VF1" or "VF2" and "E1" terminals of check engine connector. Connect positive (+) probe of voltmeter to terminal "VF1" for left bank or "VF1" for right bank and negative (-) probe to terminal "E1". See Fig 1 & 7.
    Fig 1: Checking LS400 Idle Mixture (Right Bank)
    G90G20772Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
    Fig 2: Checking LS400 Idle Mixture (Left Bank)
    G90I21319Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
  2. Start engine and maintain engine RPM at 2500 RPM for approximately 2 seconds. With engine RPM at 2500, count the amount of times that voltmeter fluctuates between the specified voltage. See IDLE MIXTURE TEST SPECIFICATIONS TABLE in this article.
  3. If the amount of fluctuations is less than the minimum amount, check for air leaks at the air induction system, defective airflow meter or throttle body.
  4. If fluctuations were within specification, accelerate and hold engine at 2500 RPM for approximately 2 minutes. Install exhaust gas analyzer probe in tailpipe at least 1.3 feet. Wait one minute and check idle mixture readings. If readings are not within specification, see IDLE MIXTURE TROUBLE SHOOTING TABLE in this article.
IDLE MIXTURE TEST SPECIFICATIONS

Model Voltage Minimum Fluctuations
All Models 0-5 8 Per 10 Seconds
IDLE MIXTURE TROUBLE SHOOTING

Problem CO HC Probable Cause
Rough Idle Normal High Incorrect Ignition Timing, Faulty Ignition Components, Incorrect Valve Clearance, Defective EGR Valve, Leaking Intake/Exhaust Valves, Defective Cylinder
Rough Idle (1) Low High Vacuum Leaks, Lean Mixture Causing Misfire
Rough Idle (2) High High Restricted Air Filter, Plugged PCV, Faulty Pressure Regulator, Clogged Fuel Return Line, Defective Coolant Temp or Air Temp Sensor, ECU, Fuel Injector, Cold Start Injector, TPS, Airflow Meter
(1) Fluctuating HC reading.
(2) Black smoke from exhaust.
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