1991 Subaru XT AutomaticFACTORY OEM SERVICE PROCEDURE

By-Pass Air Control Valve (TBI)

  1. With engine running, disconnect electrical connector. Engine speed should drop (a little with engine warm, a lot with engine cold). Reconnect electrical connector. Ensure engine speed returns to previous speed.
  2. If not, turn engine off. Turn ignition switch to ON position. Ensure 10-12 volts are available at Black/White wire of air control valve electrical connector.
  3. Turn ignition off. Ensure resistance of Air Control Valve (ACV) is 7.3-13 ohms at -4 to 176°F (-20 to 80°C). If not, replace ACV. Check ACV solenoid for any short to ground. If present, replace ACV.
  4. Attach electrical connector to ACV. Disconnect connector at ECU. Turn ignition on. Ensure 10-12 volts are present at terminal No. 45 (Green/Red wire) of ECU. If not, check harness between ACV and ECU.
  5. Turn ignition off and reconnect ECU connector. Turn ignition on. Backprobe terminal No. 45 (Green/Red wire) of ECU. There should be one volt or less for about one minute after ignition switch is turned to ON position. After one minute, there should be at least 10 volts. If not, check for a bad ground, problem with ECU circuits (sensors) or ECU.
Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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