1998 Isuzu Oasis LSFACTORY OEM SERVICE PROCEDURE

Intake Air By-Pass (IAB) Control System Testing (Prelude)

  1. Start engine and allow to idle. Disconnect No. 13 vacuum hose from IAB control diaphragm. See VACUUM DIAGRAMS article. Check for vacuum at hose. If vacuum does not exist, go to next step. If vacuum exists, go to step 7).
  2. Disconnect No. 12 vacuum hose from IAB vacuum tank. See VACUUM DIAGRAMS article. Check for vacuum at IAB vacuum tank. If vacuum exists, go to next step. If vacuum does not exist, repair blockage or vacuum leak between IAB vacuum tank and intake manifold.
  3. Turn ignition switch off. Disconnect IAB control solenoid valve 2-pin connector. Measure voltage between IAB control solenoid valve harness connector terminals No. 1 (Black/Yellow) and No.2 (Red/Blue). If battery voltage is not present, go to next step. If battery voltage is present, replace IAB control solenoid valve.
  4. Measure voltage between ground and IAB control solenoid valve harness connector Black/Yellow wire. If battery voltage is present, go to next step. If battery voltage is not present, repair open in Black/Yellow wire between IAB control solenoid valve connector and fuse No. 14 (ECU CRUISE CONTROL) located in under-dash fuse/relay box. See WIRING DIAGRAMS article.
  5. Turn ignition off. Reconnect IAB control solenoid valve 2-pin connector. Turn ignition on. Connect a fused jumper wire between PCM 32-pin connector "A" terminal A9 (Brown/Black wire) and terminal A26 (Red/Blue wire). See Figure . If IAB control solenoid valve does not click when jumper wire is connected, repair open in wire between IAB control solenoid valve and PCM (A26). See WIRING DIAGRAMS article. If IAB control solenoid valve clicks when jumper wire is connected, substitute a known-good PCM and recheck. If symptom or indication goes away, replace original PCM.
  6. Increase engine speed to more than 5000 RPM. Check for vacuum at hose No. 13. See VACUUM DIAGRAMS article. If vacuum does not exist, IAB control system is okay. If vacuum exists, disconnect IAB control solenoid valve 2-pin connector. Check for vacuum again. If vacuum does not exist, go to next step. If vacuum exists, replace IAB control solenoid valve.
  7. Turn ignition off. Disconnect PCM 32-pin connector "A". See Figure . Check for continuity between ground and IAB control solenoid valve 2-pin connector terminal No. 2 (Red/Blue). If continuity exists, repair short in Red/Blue wire between PCM and IAB control solenoid valve 2-pin connector. See WIRING DIAGRAMS article. If continuity does not exist, substitute a known-good PCM and recheck. If symptom or indication goes away, replace original PCM.
    CAUTION: DO NOT  adjust IAB control valve full-close screw. It is factory preset.
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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.