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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 77 (ENGINE CONTROLS SELF-DIAGNOSTICS - 6.5L)DIAGNOSTIC TESTSDTC P0251: INJECTION PUMP CAM SYSTEMDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, 2002 Chevrolet RV Cutaway, and 2002 Chevrolet Cutaway. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check-Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Is DTC P0370 set also? If yes, see DTC P0370: TIMING REFERENCE HIGH RESOLUTION  . If no, go to next step.
  3. Start and idle the engine. With the throttle closed, observe the CAM Ref Missed display on the scan tool. Does scan tool display 8? If yes, go to step  5 . If no, go to next step.
  4. The DTC is intermittent. If no additional DTCs are stored, see DIAGNOSTIC AIDS  . If additional DTCs are stored, see DIAGNOSTIC TROUBLE CODE DEFINITIONS  .
  5. Turn OFF the ignition. Disconnect the optical/fuel temperature sensor electrical connector. Turn ON the ignition, with the engine OFF. Use a Digital Multimeter (DMM) in order to measure the voltage between the optical sensor 5-volt reference circuit at the harness connector and chassis ground. Does the voltage measure 4.8-5.2 V? If yes, go to next step. If no, go to step  8 .
  6. Probe the sensor ground circuit with an Unpowered Test Light (J 34142-B) connected to battery voltage at the harness connector. Does the test lamp illuminate? If yes, go to next step. If no, go to step  9 .
  7. Reconnect the optical/fuel temperature sensor electrical connector. Start and idle the engine. With a scan tool, command the engine to 900 RPM. With a DMM set on the Hertz (Hz) scale, backprobe the pump CAM signal circuit at the PCM. Is the Hz reading 57-63 Hz? If yes, go to step  15 . If no, go to step  11 .
  8. Turn OFF the ignition. Disconnect the PCM and check the optical sensor 5-volt reference circuit for the following conditions:
    • An open.
    • A short to ground.
    • A short to the sensor ground circuit.

    If the optical sensor 5-volt reference circuit is open or shorted to ground, repair as necessary. Did you perform a repair? If yes, go to step  21 . If no, go to step  10 .

  9. Check for an open in the sensor ground circuit. If you find a problem, repair the problem as necessary. Did you perform a repair? If yes, go to step  21 . If no, go to step  15 .
  10. Check the optical/fuel temperature 5-volt reference circuit for a faulty connection at the PCM and replace the terminal if necessary. Did you perform a repair? If yes, go to step  21 . If no, go to step  20 .
  11. Turn ON the ignition, with the engine OFF. Disconnect the optical/fuel temperature sensor electrical connector. With a DMM connected to a ground, probe the CAM signal circuit. Does the voltage measure 4.8-5.2 V? If yes, go to step  16 . If no, go to next step.
  12. Does the voltage measure more than 5.2 V? If yes, go to next step. If no, go to step  14 .
  13. Repair the short to battery/ignition voltage on the CAM signal circuit. After repairs, go to step  21 .
  14. Turn OFF the ignition. Check the CAM signal circuit for an open or short to ground. Repair the circuit as necessary. Did you perform a repair? If yes, go to step  21 . If no, go to next step.
  15. Check for a proper connection at the PCM harness terminal and replace the terminal if necessary. Did you perform a repair? If yes, go to step  21 . If no, go to step  20 .
  16. With a DMM set to the mA scale, measure the current between the CAM signal circuit and the ground circuit at the optical/fuel temperature sensor electrical connector. Does the current measure less than 35 mA? If yes, go to step  18 . If no, go to next step.
  17. Repair the short to reference voltage on the CAM signal circuit. After repairs, go to step  21 .
  18. Check for a faulty connection at the injection pump. If you find a problem, repair the problem as necessary. Did you perform a repair? If yes, go to step  21 . If no, go to next step.
    19. NOTE: The new injection pump must be timed. See appropriate ON-VEHICLE ADJUSTMENTS article.
  19. Replace the injection pump. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. After repairs, go to step  21 .
  20. Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  21. Using the scan tool, clear the DTCs. Start engine and idle the engine at the normal operating temperature. Select DTC and the Specific DTC function. Enter the DTC number that was set. Operate the vehicle within the Conditions for Setting DTC until the scan tool indicates the diagnostic ran. Does the scan tool indicate the diagnostic passed? If yes, go to next step. If no, go to step  2 .
  22. Observe the stored information, Capture Info with a scan tool. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no, system is okay.
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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.