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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 220 (ENGINE CONTROLS SELF-DIAGNOSTICS - 2.2L BI-FUEL)DIAGNOSTIC TESTSDTC P0171: FUEL TRIM SYSTEM LEANDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2003 Chevrolet Cavalier. 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 (Alternative Fuels). If yes, go to next step. If no, go toDIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS (ALTERNATIVE FUELS)  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: If any DTCs are set, except P0171, diagnose those DTCs before proceeding with this diagnostic procedure. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  2. Install a scan tool. Review the Freeze Frame/Failure Records and record the displayed data for this DTC. Select FUEL TRIM DATA parameter. Start the engine and observe the FUEL TRIM INDEX. Does the scan tool indicate that the Fuel Trim Index is more than 20 percent? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .
  3. Operate the engine at idle. Observe the O2S parameters using a scan tool. Does the scan tool indicate that the values are 200-800 mV and fluctuating? If yes, go to next step. If no, go to step  5.
  4. Turn ignition switch to OFF position. Visually and physically inspect the following items:
    • Vacuum hoses for splits, kinks, and proper connections.
    • If fuel pressure is too low, or the flow is insufficient, this DTC may set. See FUEL PRESSURE TEST under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 2.2L ALERO, CAVALIER, GRAND AM & SUNFIRE article.
    • Lean engine condition such as incorrect CNG fuel pressure, vacuum leaks, PCV system malfunction or exhaust system leaks between engine and oxygen sensor.

    Did you find and correct the condition? If yes, go to step  8. If no, go to step  7.

  5. Turn ignition switch to OFF position. Turn ignition switch to RUN position, engine OFF. Observe the Manifold Absolute Pressure (MAP) sensor parameter using a scan tool. The MAP sensor pressure should be within the range specified for your altitude. See ALTITUDE VS. BAROMETRIC PRESSURE  table. Does the MAP sensor indicate the correct barometric pressure? If yes, go to next step. If no, go to DTC P0105: MANIFOLD ABSOLUTE PRESSURE SENSOR PERFORMANCE RANGE under DIAGNOSTIC TESTS in SELF-DIAGNOSTIC - 2.2L ALERO, CAVALIER, GRAND AM & SUNFIRE - GASOLINE article.
  6. Turn ignition switch to OFF position. Inspect for the following conditions:
    • The O2S for proper installation.
    • Ensure electrical connectors and wires are secured and not contacting the exhaust system.
    • A short between the signal circuit and low reference circuit.

    Did you find and correct the condition? If yes, go to step  8. If no, see FUEL SYSTEMS (CNG) in SYSTEM & COMPONENT TESTS - 2.2L ALERO, CAVALIER, GRAND AM & SUNFIRE article.

  7. Operate the engine at idle. Check for the following conditions:
    • Missing, loose, or leaking exhaust components. See EXHAUST SYSTEM CHECK under EMISSION SYSTEMS & SUBSYSTEMS in SYSTEM & COMPONENT TESTS - 2.2L ALERO, CAVALIER, GRAND AM & SUNFIRE article.
    • Vacuum leaks at the intake manifold, throttle body, and injector "O" rings.
    • The air induction system and air intake ducts for leaks.
    • The crankcase ventilation system for leaks.

    Did you find and correct the condition? If yes, go to next step. If no, diagnose base engine components. See appropriate article in ENGINES.

    8. NOTE: After repair, use the scan tool FUEL TRIM RESET function to reset the Long Term Fuel Trim.
  8. Clear the DTCs using a scan tool. Turn ignition switch to OFF position for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  9. Observe the CAPTURE INFO using a scan tool. Are there any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
(1) ALTITUDE VS. BAROMETRIC PRESSURE

Altitude - Meters Altitude - Feet Barometric Pressure - kPa
4267 14000 56-64
3962 13000 58-66
3658 12000 61-69
3353 11000 64-72
3048 10000 66-74
2743 9000 69-77
2438 8000 71-79
2134 7000 74-82
1829 6000 77-85
1524 5000 80-88
1219 4000 83-91
914 3000 87-95
610 2000 90-98
305 1000 94-102
0 0 (Sea Level) 96-104
-305 -1000 101-105
(1) Determine your altitude by contacting a local weather station or by using another reference source.
RENDER: 1.0x

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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.