Diagnosis & Repair Procedure
- Perform DTC CONFIRMATION TEST . If DTC P0505 is set, go to next step. If DTC P0505 is not set, problem is intermittent. System is okay at this time. See TROUBLE SHOOTING - NO CODES article.
- Turn ignition off. Connect scan tool. Ensure all wires and hoses are connected, accelerator cable has some play in it, and ignition timing is within specification. Ensure all accessories are off. Ensure air filter is okay and no vacuum leaks are present. Ensure transmission is in Park or Neutral and parking brake is on. Start engine and allow it to reach normal operating temperature. Using scan tool, check idle speed and Idle Air Control (IAC) duty cycle. See IDLE SPEED/IDLE AIR CONTROL (IAC) DUTY CYCLE SPECIFICATIONS
. Go to next step.IDLE SPEED/IDLE AIR CONTROL (IAC) DUTY CYCLE SPECIFICATIONS
Idle Speed (RPM) IAC Duty Cycle (%) A/C Off 700-800 35-45 A/C On 800-900 35-45 - If idle speed and Idle Air Control (IAC) duty cycle are as specified, go to next step. If idle speed and Idle Air Control (IAC) duty cycle are not as specified, go to step 7 .
- Using scan tool, check Vehicle Speed Sensor (VSS) signal on scan tool while driving vehicle. If speedometer and scan tool display match, problem is intermittent. System is okay at this time. See TROUBLE SHOOTING - NO CODES article. If intermittent problem is not found, replace ECM and go to DTC CONFIRMATION TEST . If speedometer and scan tool display do not match, go to next step.
- Remove instrument panel. See appropriate INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. Disconnect instrument cluster 16-pin connector G19. Check for good terminal contact at instrument cluster 16-pin connector G19. If poor terminal contact is found, repair as necessary. If terminal contact is okay, go to next step.
- Turn ignition on. Measure voltage between ground and instrument cluster 16-pin harness connector G19 terminal No. 6 (Purple wire). See Figure . If reading is 4-6 volts, replace speedometer and go to DTC CONFIRMATION TEST . If reading is not 4-6 volts, check for short to ground or open in Purple wire between instrument cluster 16-pin harness connector G19 and ECM 35-pin connector E92 terminal No. 27. See WIRING DIAGRAMS article. See Figure . Repair as necessary. If wire is okay, replace ECM and go to DTC CONFIRMATION TEST .
- Check air intake system for obstruction or leakage. Repair as necessary. If air intake system is okay, measure plunger length at fast idle control cam (measurement "A"). See Fig 1 . Plunger length should be 1.048-1.078" (26.6-27.4 mm). Repair as necessary. If plunger length is okay, go to next step.
- If not using scan tool, go to next step. If using scan tool, turn ignition off. Connect scan tool. Start engine and allow it to reach normal operating temperature. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES under SELF-DIAGNOSTIC SYSTEM. Select MISC TEST mode on scan tool. Using scan tool, open or closed IAC valve. If idle speed changes when IAC valve is opened or closed, go to step 13 . If idle speed does not change when IAC valve is opened or closed, go to step 10 .
- Start engine and allow it to reach normal operating temperature. Backprobing, connect a fused jumper wire between ground and ECM 28-pin connector C20 terminal No. 28, or disconnect IAC valve connector. See Figure . If idle speed changes when fused jumper wire is connected or IAC valve connector is disconnected, go to step 13 . If idle speed does not does not change when fused jumper wire is connected or IAC valve connector is disconnected, go to next step.
- Turn ignition off. Disconnect IAC valve connector. Check for good terminal contact at IAC valve connector. If poor terminal contact is found, repair as necessary. If terminal contact is okay, measure resistance between IAC valve terminals. About 8.6-10.6 ohms should be present at 68°F (20°C). Measure continuity between each IAC valve terminal and valve body. No continuity should be present. If continuity is as specified, go to next step. If continuity is not as specified, replace IAC valve and go to DTC CONFIRMATION TEST .
- Reconnect IAC valve connector. Start engine and allow it to reach normal operating temperature. Disconnect IAC connector. Record idle speed. Using fused jumper wires, connect 12-volt battery to IAC valve. See Fig 2 . Recheck idle speed. If idle speed increases, go to next step. If idle speed does not increase, clean or replace IAC valve and go to DTC CONFIRMATION TEST .
- Check for short or open in Red wire between IAC valve and ECM 28-pin connector C20 terminal No. 28. See WIRING DIAGRAMS article. See Figure . Also check for short or open in Red/Black wire between IAC valve and main relay. If wires are okay, replace ECM and go to DTC CONFIRMATION TEST .
- Check for leaking EGR valve, EVAP system malfunction, engine mechanical problems such as low compression, or excessive accessary engine load. Repair as necessary. If no problem is found, go to next step.
- Ensure engine idle speed is as specified in step 2). If engine idle speed is not as specified, adjust engine idle speed using idle air adjusting screw. If engine idle speed can not be adjusted using idle air adjusting screw, check for plugged idle air passage. If idle air passage is not plugged, replace ECM and go to DTC CONFIRMATION TEST .