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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 98 (ENGINE CONTROL SYSTEM TESTING & ENGINE COMPONENT TESTING)ENGINE SENSORS & SWITCHESMANIFOLD ABSOLUTE PRESSURE SENSOR (3.1L & 3.4L)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2002 Saturn SL2, 2002 Saturn SL1, 2002 Saturn SL, 2002 Saturn SC2, and 2002 Saturn SC1. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. After performing Diagnostic System Check - Engine Controls, go to next step.
  2. Install a scan tool. Turn ON the ignition, with the engine OFF. With the scan tool observe the MAP sensor pressure. Compare this value to a value observed in a known-good vehicle. Is the difference between the two vehicles less than 3 kPa? If yes, go to next step. If no, go to step  7.
  3. Observe the MAP sensor pressure on the scan tool. Start the engine. Does the MAP sensor value change? If yes, go to next step. If no, go to step  7.
  4. Turn OFF the ignition. Remove the MAP sensor from the engine vacuum source. Leave the MAP sensor connected to the engine harness. Connect a Vacuum Pump (J 23738-A) to the MAP sensor. Turn ON the ignition, with the engine OFF. Observe the MAP sensor pressure as you SLOWLY apply vacuum 1 in. Hg at a time. Each one inch of vacuum should result in a 3 to 4 kPa drop in the MAP sensor pressure. Does the scan tool indicate that the MAP sensor is operating normally? If yes, go to next step. If no, go to step  15.
  5. Observe the MAP sensor pressure on the scan tool with 20 inches Hg of vacuum applied to the sensor. Is the pressure less than 34 kPa? If yes, go to next step. If no, go to step  7.
  6. Disconnect the MAP sensor from the J 23738-A. The MAP sensor pressure should return to the value observed in step  2. Does the MAP sensor pressure return to the value observed earlier? If yes, system is okay. If no, go to step  15.
  7. Turn OFF the ignition. Disconnect the MAP sensor from the engine harness. Turn ON the ignition, with the engine OFF. Observe the MAP sensor VOLTAGE display on the scan tool. Is the voltage more than 0.1 V? If yes, go to step  11. If no, go to next step.
  8. Connect a test lamp to a good ground. Probe the 5-volt reference circuit of the MAP sensor with the test lamp. Perform a voltage drop test across the test lamp with a DMM. Does the voltage measure more than 4.4 V? If yes, go to next step. If no, go to step  12.
  9. Jumper the MAP sensor 5-volt reference circuit to the MAP sensor signal circuit using a fused jumper wire. Does the scan tool indicate that the MAP sensor voltage is near 5 V? If yes, go to next step. If no, go to step  13.
  10. Remove the jumper. Disconnect the electrical connector of the Engine Coolant Temperature (ECT) sensor. Probe the MAP sensor low reference circuit with a test lamp connected to battery positive voltage. Does the test lamp illuminate? If yes, go to step  15. If no, go to step  14.
  11. Test the signal circuit for a short to voltage. Did you find and correct the condition? If yes, go to step  19. If no, go to step  17.
  12. Test the 5-volt reference circuit of the MAP sensor for high resistance or an open. Did you find and correct the condition? If yes, go to step  19. If no, go to step  17.
  13. Test the signal circuit for the following conditions:
    • High resistance.
    • An open.
    • A short to ground.

    Did you find and correct the condition? If yes, go to step  19. If no, go to step  17.

  14. Test the MAP sensor low reference circuit for high resistance or an open. Did you find and correct the condition? If yes, go to step  19. If no, go to step  17.
  15. Inspect the MAP sensor connector for poor connection or terminal contact. Did you find and correct the condition? If yes, go to step  19. If no, go to next step.
  16. Replace the MAP sensor. After repairs, go to step  19.
  17. Inspect the PCM for poor connection or terminal contact. Did you find and correct the condition? If yes, go to step  19. If no, go to next step.
  18. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE under PROGRAMMING in appropriate SELF-DIAGNOSTICS article. After repairs, go to next step.
  19. Use the scan tool in order to clear the DTCs that may have set. Turn OFF the ignition for 30 seconds. Start the engine. Did you correct the condition? If yes, system is okay. If no, go to step  2.
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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.