Code Fix

Tiny
NELSONJJ
  • MEMBER
  • 1998 GMC ENVOY
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 140,000 MILES
PO171 and PO174 codes. 1998 gmc envoy 4.3 liter Tuneup and o2 sensors have all just been replaced
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Sunday, May 22nd, 2011 AT 3:13 AM

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Tiny
KHLOW2008
  • EXPERT
Diagnostic Procedures
1. Check exhaust system for corrosion, loose or missing hardware. Ensure HO2S is installed securely, and connector and harness are not contacting exhaust manifold or ignition wires. Check vacuum hoses for splits, kinks and proper routing. Check throttle body, intake manifold, and EGR valve for vacuum leaks. Check IAC, if high or unsteady idle condition exists. Check crankcase ventilation valve, spring and "O" ring for proper installation. Check for fuel contamination. Check for good VCM and sensor grounds. If any of these items isolate or require repair, go to step 7). If all items are okay, go to next step.

2. Turn ignition off. Connect fuel pressure gauge to fuel rail test fitting. Turn ignition on, engine off (fuel pump should run for about 2 seconds). Cycle ignition off and on until maximum pressure is obtained. Note fuel pressure while pump is running (pressure may vary slightly when pump stops). If pressure holds at 60-66 psi (4.2-4.6 kg/cm2 ), go to next step. If pressure does not hold at 60-66 psi (kg/cm2, go to step 10).

3. Warm engine to normal operating temperature. If fuel pressure drops 3-10 psi (0.2-0.7 kg/cm2 ), go to next step. If fuel pressure does not drop 3-10 psi (0.2-0.7 kg/cm2 ), go to step 10).

4. Perform INJECTOR BALANCE TEST. If an injector problem is found, go to step 8). If injectors are okay, go to next step.

5. Perform evap canister purge solenoid check. If a solenoid problem is found, go to next step. If solenoid is okay, see DIAGNOSTIC AIDS.

6. Repair EVAP system. Operate vehicle in closed loop mode while using scan tool to read LT and ST FUEL TRIM values. LT FUEL TRIM value should decrease to less than 158, ST FUEL TRIM value should decrease to less than 180. If FUEL TRIM values are as specified, go to step 10). If values are not as specified, see DIAGNOSTIC AIDS.

7. Repair items as necessary. Operate vehicle in closed loop mode while using scan tool to read LT and ST FUEL TRIM values. LT FUEL TRIM value should decrease to less than 158, ST FUEL TRIM value should decrease to less than 180. If FUEL TRIM values are as specified, go to step 9). If values are not as specified, go to step 2).

8. Replace fuel injector(s) as necessary. Operate vehicle in closed loop mode while using scan tool to read LT and ST FUEL TRIM values. LT FUEL TRIM value should decrease to less than 158, ST FUEL TRIM value should decrease to less than 180. If FUEL TRIM values are as specified, go to next step. If values are not as specified, go to step 5).

9. Lean condition does not exist.

10. Check fuel system.

11. Using scan tool, select DTC, CLEAR INFO function. Start engine and allow it to reach operating temperature. Using scan tool, select DTC, SPECIFIC function and enter this DTC. Operate vehicle within conditions required to set DTC. If scan tool indicates that this DTC RAN AND PASSED, go to next step. If scan tool does not indicate that this DTC RAN AND PASSED, repeat step 2).

12. Using scan tool, select Read and Record INFO, REVIEW INFO function. Check if any undiagnosed DTCs are displayed. Diagnose affected DTCs. If no additional DTCs are displayed, system is okay.
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Sunday, May 22nd, 2011 AT 3:45 AM

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