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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 260 (ENGINE CONTROLS - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0446: EVAP VENT VALVE PERFORMANCEDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2003 Cadillac Seville and 2003 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check-Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Are any DTCs other than DTC P0446 set? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Inspect the EVAP system for the following conditions:
    • A damaged EVAP vent valve.
    • A pinched EVAP vent hose.
    • A damaged EVAP canister.

    Did you find and correct the condition? If yes, go to step  16. If no, go to next step.

  4. Turn ignition off. Disconnect the purge line from the EVAP canister purge solenoid. See EVAP CANISTER PURGE SOLENOID under MOTORS & SOLENOIDS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Turn ignition on, with engine off. Is the FUEL TANK PRESSURE SENSOR parameter within -1 to 1 in. H2O? If yes, go to next step. If no, go to step  10.
    5. NOTE: DO NOT exceed the specified value in this step. Exceeding the specified value may produce incorrect test results.
  5. Perform the following:
    • Turn ignition off.
    • Connect the EEST power supply clips to a known good 12-volt source.
    • Install the Fuel Fill Cap Adapter to the fuel fill pipe.
    • Connect the fuel fill cap to the Fuel Fill Cap Adapter.
    • Connect the EEST nitrogen/smoke supply hose to the Fuel Fill Cap Adapter.
    • Turn ignition on, with engine off.
    • Command the EVAP vent valve closed with a scan tool.
    • Turn the nitrogen/smoke valve on the EEST control panel to NITROGEN.
    • Pressurize the EVAP system to 5 in. H2O with the remote switch.
    • Observe the FUEL TANK PRESSURE SENSOR parameter with a scan tool.
    • Command the EVAP vent valve open with a scan tool.

    Is the FUEL TANK PRESSURE SENSOR parameter less than 1 in. H2O? If yes, go to next step. If no, go to step  8.

  6. Perform the following:
    • Connect the nitrogen/smoke hose to the EVAP service port.
    • Remove the Fuel Fill Cap Adapter.
    • Install the fuel fill cap to the fuel fill pipe.
    • Start the engine.
    • Allow the engine to idle.
    • Use the PURGE/SEAL function to seal the system with a scan tool.
    • Command the EVAP purge solenoid to 30 percent.
    • Observe the vacuum/pressure gage on the EEST and the FUEL TANK PRESSURE SENSOR parameter on the scan tool.
    • Allow the vacuum to increase on the gage of the EEST, until it reaches approximately 16 in. H2O.
    • Use the PURGE/SEAL function to seal the system with a scan tool.

    Is the FUEL TANK PRESSURE SENSOR parameter on the scan tool within 1 in. H2O of the vacuum/pressure gage on the EEST, until the vacuum reached the abort limit on a scan tool. If yes, go to next step. If no, go to step  10.

  7. Did the FUEL TANK PRESSURE SENSOR parameter on a scan tool display more than 3.2 volts? If yes, see DIAGNOSTIC AIDS . If no, go to step  10.
  8. Disconnect the EVAP vent hose from the EVAP vent valve. Is the FUEL TANK PRESSURE SENSOR parameter less than 1 in. H2O? If yes, go to step  14. If no, go to next step.
  9. Disconnect the EVAP vent hose from the EVAP canister. Is the FUEL TANK PRESSURE SENSOR parameter less than 1 in. H2O? If yes, go to step  12. If no, go to step  15.
  10. Test for an intermittent and/or poor connections at the Fuel Tank Pressure (FTP) sensor. Did you find and correct the condition? If yes, go to step  16. If no, go to next step.
  11. Test the FTP sensor low reference circuit for an open or high resistance. Did you find and correct the condition? If yes, go to step  16. If no, go to step  13.
  12. Repair the pinched or restricted EVAP vent hose. Did you complete the repair? If yes, go to step  16.
  13. Replace the FTP sensor. See FUEL TANK PRESSURE SENSOR under SENSORS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to step  16.
  14. Replace the EVAP vent valve. See EVAP CANISTER VENT VALVE under MOTORS & SOLENOIDS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to step  16.
  15. Replace the EVAP canister. See EVAP CANISTER under EMISSION SYSTEMS & SUB-SYSTEMS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to next step.
  16. Perform the Service Bay Test. See SERVICE BAY TEST  under DRIVE CYCLES. Does the scan tool indicate that the Service Bay Test passed? If yes, go to next step. If no, go to step  2.
  17. Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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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.