LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> GMC >> 1998 >> Forward Control 7.4 B >> Repair and Diagnosis >> Engine Performance >> Testing & Diagnosis >> Engine Controls - System & Component Testing - 5.7L >> Emission Systems & Sub-Systems >> Fuel Evaporation Control >> EVAP Control System Diagnosis (Non-Enhanced System)
April 5, 2026: LEMON Manuals is launched! Read the announcement.

EVAP Control System Diagnosis (Non-Enhanced System)

  1. If powertrain OBD system check has been performed, go to next step. If powertrain OBD system check has not been performed, perform powertrain OBD system check. See POWERTRAIN OBD SYSTEM CHECK in TESTS W/CODES - 5.7L article.
  2. Start engine and let idle until engine coolant temperature is greater than 113°F (45°C). Ensure vacuum source is okay, repair as necessary. Disconnect outlet line from EVAP purge solenoid. Connect vacuum gauge to EVAP purge solenoid outlet port. Increase engine speed to 2000 RPM. Using scan tool, command EVAP purge solenoid on. If vacuum does not exist, go to next step. If vacuum exist, go to step 6).
  3. Disconnect EVAP purge solenoid harness connector. Connect a test light between EVAP purge solenoid harness connector terminals. Using scan tool, command EVAP purge solenoid on. If test light does not illuminate, go to next step. If test light illuminates, go to step 7).
  4. Using a test light connected to ground, probe EVAP purge solenoid ignition feed circuit (Pink wire) at EVAP purge solenoid harness connector. If test light illuminates, go to next step. If test light does not illuminate, go to step 8).
  5. Using a test light connected to battery voltage, probe EVAP purge solenoid control circuit (Dark Green/White wire) at EVAP purge solenoid harness connector. Using scan tool, command EVAP purge solenoid on. If test light illuminates, go to step 9). If test light does not illuminate, go to step 10).
  6. Check EVAP purge valve vacuum lines. If problem exists, repair as necessary and go to step 13). If problem does not exist, system is okay at this time.
  7. Check for poor connection at EVAP purge valve. If poor connection does not exist, go to step 11). If poor connection exists, repair as necessary and go to step 13).
  8. Repair open in EVAP purge valve ignition feed circuit (Pink wire), between EVAP purge valve and ENG 1 fuse. After repairs are complete, go to step 13).
  9. Repair open in EVAP purge valve control circuit (Dark Green/White wire), between PCM and EVAP purge valve. After repairs are complete, go to step 13).
  10. Check for poor connection at PCM. If poor connection does not exist, go to step 12). If poor connection exists, repair as necessary and go to step 13).
  11. Replace EVAP purge valve and go to step 13).
  12. Replace PCM. Program PCM, perform VTD password relearn procedure and perform crankshaft variation learn procedure. See COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION. After repair is complete, go to next step.
  13. Turn ignition switch to RUN position. Using scan tool, command EVAP purge solenoid on then off. If EVAP purge valve operate as expected, system is okay at this time. If EVAP purge valve does not operate as expected, go to step 2).
scientia non olet · About LEMON Manuals