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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 4.3 W, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS TESTS W/CODES - DIESELDIAGNOSTIC TESTSDTC P0236: TURBOCHARGER BOOST SYSTEM ("C", "G", "K" & "P" SERIES)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 4.3 W, Standard

Diagnostic Procedures

1999 GMC Forward Control 4.3 W, StandardSECTION Diagnostic Procedures
  1. If OBD system check has been performed, go to next step. If OBD system check has not been performed, go to ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK  under SELF-DIAGNOSTIC SYSTEM.
  2. Check if DTC P1656 is also set. If DTC P1656 is set, diagnose DTC P1656 first. If DTC P1656 is not set, go to next step.
  3. Disconnect vacuum hose from turbocharger wastegate actuator. Install vacuum gauge to vacuum hose. Start and idle engine. Observe vacuum reading. If vacuum is greater than or equal to 15 in. Hg, go to next step. If vacuum is less than 15 in. Hg, go to step  6.
  4. Disconnect wastegate solenoid harness connector. With engine running, check vacuum gauge reading. If vacuum reading is greater than one in. Hg, go to step  12. If vacuum reading is one in. Hg or less, go to next step.
  5. Turn engine off. Connect a hand-held vacuum pump to wastegate actuator. Apply 5 in. Hg of vacuum to actuator. If actuator holds vacuum, go to step  7. If actuator does not hold vacuum, go to step  14.
  6. Check all vacuum lines from vacuum pump to wastegate actuator for leaks, deformities or pinches. Repair as necessary. After repairs, go to step  15. If vacuum hoses are okay, go to step  8.
  7. Ensure engine is off. Disconnect vacuum lines to wastegate actuator. Using a pair of pliers, grip wastegate actuator rod. Move actuator rod back and forth. If actuator rod moves freely, go to next step. If actuator rod does not move freely, go to step  14.
  8. Check vacuum pump for proper output. Repair as necessary. After repairs, go to step  15. If vacuum pump output is okay, go to next step.
  9. Install scan tool. Start and idle engine. Observe boost pressure on scan tool display. Increase engine speed to 1500 RPM, and then return to idle. If scan tool displays a change in boost pressure, go to next step. If scan tool does not display a change in boost pressure, go to step  11.
  10. DTC is intermittent. If no additional DTC(s) are present, see DIAGNOSTIC AIDS . If additional DTC(s) are present, diagnose affected DTC(s).
  11. Replace boost sensor. After replacing sensor, go to step  16.
  12. Check for plugged wastegate solenoid filter. Repair or replace as necessary. After repairs, go to step  16. If filter is okay, go to next step.
  13. Replace wastegate solenoid. After replacing solenoid, go to step  16.
  14. Replace turbocharger actuator. After replacing actuator, go to step  16.
  15. Replace vacuum pump. After replacing pump, go to step  16.
  16. Using scan tool, select DTC, CLEAR INFO. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, and then enter DTC P0236. 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.
  17. Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTC(s) are displayed, go to applicable DTC test.
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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.