Adjusting Idle Speed & Mixture – Civic ’92-95

by NinjaX on June 21, 2010

Adjusting Idle speed and mixture applies to Honda Civic 1992-1995.

Idle Air Control (IAC) Valve

Engine idle speed is controlled by IAC and fast idle valve. IAC varies amount of air by-passing throttle plate (into intake manifold) in response to signals from ECM. After engine starts, IAC opens for a short time to increase idle speed. Activation time is dependent upon engine coolant temperature. When coolant temperature is low, IAC is held open to obtain proper fast idle speed. After engine reaches normal operating temperature, IAC is activated only to maintain minimum idle speed.

Idle Air Control (IAC) Thermal Valve

Formerly called fast idle valve, IAC thermal valve allows additional air to by-pass throttle plate into intake manifold. Increased idle speed prevents engine from running erratically during warm-up. Valve is controlled by a thermowax plunger, which contracts when cold and expands when hot.

Idle Mixture

Idle mixture is computer controlled and is not adjustable. Maximum CO level is .1%.

Idle Speed

  1. Start and run engine under no-load condition at about 3000 RPM until cooling fan comes on. Let engine slow to idle. Turn engine off. Connect a tachometer. Disconnect 2-wire Idle Air Control (IAC) valve connector.
  2. Start and run engine at about 1000 RPM until engine speed stabilizes. Slowly release pedal until engine idles. Check idle speed under no-load conditions. Ensure all accessories, cooling fan and lights are off. If necessary, adjust idle speed by turning idle adjusting screw.
  3. Turn ignition off. Reconnect 2-wire IAC connector. Remove BACK-UP fuse from underhood relay box for 10 seconds to reset ECM. Restart engine. Check idle speed (after ECM reset) with all accessories off and transmission in Neutral or Park. Idle speed should be stable and within specification. Turn engine off.
  4. Start engine and idle engine for one minute with various accessories on. Check idle speed. Idle speed should be stable and within specification. If idle is not within specification, check vacuum hose connections. Check idle adjuster screw, fast idle valve and Idle Air Control (IAC) valve for proper operation.
  5. If idle components are okay, check MAP sensor, Electronic Load Detector (ELD), fuel injector circuit, A/C signal circuit, EGR system and lock-up control solenoid (if equipped) for proper operation. On Civic VX there has been a revision as per TSB Number HSN-0894-02 dated August 1994. Idle engine for one minute with headlights (low) ON, and check idle speed.


  • Honda Civic 1992-1995
    • Before ECM Reset (1): 370-470 RPM
    • After ECM Reset:
      • No Load:
        • Auto tranny: 650-750 RPM
        • Manual tranny:
          • Except D15Z1 MT: 620-720 RPM
          • D15Z1 MT: 550-650 RPM
      • Under Load:
        • Except D15Z1 MT: 700-800 RPM
        • D15Z1 MT: 650-750 RPM
  • Civic Del Sol
    • Before ECM Reset (1): 370-470 RPM
    • After ECM Reset:
      • No Load:
        • Automatic Tranny: 650-750 RPM
        • Manual Tranny:
          • Except B16A3 MT: 620-720 RPM
          • B16A3 MT: 650-750 RPM
      • Under Load: 700-800 RPM

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