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Home >> GMC >> 1997 >> Forward Control 4.3 W, L35, Automatic >> Repair and Diagnosis >> External Pages >> Different variant/trim >> Section 11 (Engine Controls - Tests W/Codes - 7.4L (P Series)) >> Diagnostic Trouble Codes >> DTC P0172 - Fuel Trim System Rich Bank 1 >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different variant/trim than selected.
  1. Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. See the F - BASIC TESTING - 7.4L article. After performing OBD system check, go to next step.
  2. Check for the following conditions:
    • Air intake duct collapsed or restricted.
    • Air filter plugged.
    • Throttle body coking or objects blocking IAC passage, if a low or unsteady idle is being experienced.
    • Fuel pressure regulator for proper operation. See the F - BASIC TESTING - 7.4L article.
    • TP sensor for loose or missing mounting bolts. Monitor TP sensor voltage using scan tool while slowly opening throttle. Voltage should increase steadily and evenly from closed throttle voltage (.5 volt) to wide open throttle voltage (4.5 volts). If voltage is not as specified, replace TP sensor.

    If a problem was found, go to step 8). If a problem was not found, go to next step.

  3. Connect a fuel pressure gauge to fuel rail fitting. Turn ignition off for 10 seconds. Turn A/C off. Turn ignition on. Fuel pump should run for about 2 seconds. It may be necessary to cycle ignition on more than once to obtain maximum pressure. Note pressure with pump running. On CSI models, pressure should be 60-66 psi (4.2-4.6 kg/cm2 ). On SFI models, pressure should be 56-62 psi (3.9-4.4 kg/cm2 ). On all models, when pump stops, pressure may slightly vary, but should hold steady. If pressure is as specified, go to next step. If pressure is not as specified, go to step 11).
  4. Perform injector balance test. See the SYSTEM/COMPONENT TESTS article. If a problem was found, go to step 9). If a problem was not found, go to next step.
  5. Remove HO2S and inspect for silicone contamination (a white powdery deposit on portion of sensor exposed to exhaust stream). If contamination is found, go to step 12). If contamination is not found, go to next step.
  6. Perform canister purge solenoid check. See the SYSTEM/COMPONENT TESTS article. If a problem was found, go to next step. If a problem was not found, see DIAGNOSTIC AIDS.
  7. Repair EVAP system. Install scan tool. Operate vehicle in closed loop while monitoring LT and ST fuel trim values. LT fuel trim value should increase to greater than 100. ST fuel trim value should increase to greater than 94. If values are as specified, go to step 10). If values are not as specified, see DIAGNOSTIC AIDS.
  8. Repair problem as necessary. Operate vehicle in closed loop while monitoring LT and ST fuel trim values. LT fuel trim value should increase to greater than 100. ST fuel trim value should increase to greater than 94. If values are as specified, go to step 10). If values are not as specified, go to step 3).
  9. Replace fuel injector. Operate vehicle in closed loop while monitoring LT and ST fuel trim values. LT fuel trim value should increase to greater than 100. ST fuel trim value should increase to greater than 94. If values are as specified, go to next step. If values are not as specified, go to step 5).
  10. Rich condition is not present. See the TESTS W/O CODES article.
  11. Diagnose fuel system. See the F - BASIC TESTING - 7.4L article. After repairs, go to step 13).
  12. Replace HO2S. After replacing sensor, go to next step.
  13. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0172. Operate vehicle within the conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step 2).
  14. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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