1999 Chevrolet Cutaway 7.4 JFACTORY OEM SERVICE PROCEDURE

Diagnostic Procedures

  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
  2. Turn ignition on. Using scan tool, monitor MAP sensor reading. If MAP sensor reading is less than 65 kPa, go to next step. If MAP sensor reading is not less than 65 kPa, go to step  6.
  3. Turn ignition off. Remove MAP sensor from intake manifold, leaving harness connected. Turn ignition on. Using a hand-held vacuum pump, apply 20 in. of vacuum, one inch at a time to MAP sensor. Each one inch of vacuum applied should result in a 3-4 kPa drop in MAP sensor value. If MAP sensor value changes smoothly through entire range, go to next step. If operation is not as specified, go to step  5.
  4. Apply 20 in. of vacuum to MAP sensor, and then release vacuum. If MAP sensor value is about 34 kPa with vacuum applied, and then returns to barometric pressure when vacuum is released, go to step  12. If operation is not as specified, go to next step.
  5. Replace MAP sensor. After repairs, go to step  13.
  6. Monitor TP angle reading on scan tool while slowly opening throttle. If TP angle reading increased steadily throughout entire range, see DIAGNOSTIC AIDS . If operation is not as specified, go to next step.
  7. Disconnect TP sensor. If TP sensor reading is near zero volts, go to next step. If TP sensor reading is not near zero volts, go to step  9.
  8. Using a fused jumper wire, jumper 5-volt reference circuit and signal circuit together at TP sensor harness connector. If TP sensor reading is 5 volts, go to step  11. If TP sensor reading is not 5 volts, go to step  10.
  9. Check TP sensor signal circuit for a short to voltage and TP sensor ground circuit for poor connection or high resistance between TP sensor and PCM. Repair as necessary. After repairs, go to step  13. If circuits are okay, go to step  12.
  10. Check TP sensor signal circuit or 5-volt reference circuit for a poor connection or high resistance between TP sensor and PCM. Repair as necessary. After repairs, go to step  13. If circuits are okay, go to step  12.
  11. Replace TP sensor. After repairs, go to step  13.
  12. Replace PCM. Program replacement PCM using required equipment. After repairs, go to next step.
  13. Using scan tool, clear DTCs. Turn ignition off for 15 seconds. Start and warm engine to normal operating temperature. Operate vehicle within conditions required for setting this DTC. Select SPECIFIC DTC function then enter DTC P0121. If scan tool indicates this test ran and passed, go to next step. If scan tool does not indicate this test ran and passed, go to step  2.
  14. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If any undiagnosed DTCs are displayed, diagnose affected DTCs. If no undiagnosed DTCs are displayed, system is okay.
Factory Service Tools

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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.

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