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Home >> Jeep >> 1999 >> Grand Cherokee Laredo, 4.7 N >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes >> Self-Diagnostic Tests >> DTC P0171: 1/1 Fuel System Lean
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DTC P0171: 1/1 Fuel System Lean

NOTE: For connector terminal identification, see CONNECTOR IDENTIFICATION  . For circuit identification and wiring diagram, see GRAND CHEROKEE wiring diagram in WIRING DIAGRAMS article.
NOTE: DTC P0171: 1/1 FUEL SYSTEM LEAN is monitored with engine running in closed loop mode, ambient/battery temperature at more than 20°F (-6.7°C), and at altitude less than 8000 feet. DTC may be stored in Powertrain Control Module (PCM) when PCM senses fuel system is running too lean on 2 sequential trips. Possible causes are: defective Engine Coolant Temperature (ECT) sensor calibration, engine mechanical problem, fuel pressure out of specification, defective 1/1 Heated Oxygen Sensor (HO2S) heater or fuel system not switching from rich to lean.
NOTE: 1/1 HO2S is located in upstream position.
  1. Turn ignition on, with engine off. Using scan tool, read DTCs. If FUEL SYSTEM LEAN or FUEL SYSTEM RICH GOOD TRIP COUNTER is displayed and displayed count is "0", go to next step. If FUEL SYSTEM LEAN or FUEL SYSTEM RICH GOOD TRIP COUNTER is not displayed and/or displayed count is not "0", go to step 9).
    NOTE: Throttle linkage must be moved very slowly while looking for a smooth change in voltage in this step.
  2. Ensure ignition is on, with engine off. Slowly open and close throttle plate while monitoring TP sensor voltage. If voltage change is smooth, go to next step. If voltage change is erratic, replace TP sensor. Perform TEST VER-5A.
  3. Turn ignition on, with engine off. Ensure throttle linkage is against stop. Using scan tool, read TP sensor voltage. If voltage is .92 volt or less with throttle closed, go to next step. If voltage is more than .92 volt, replace TP sensor. Perform TEST VER-5A.
  4. Ensure ignition is on, with engine off. Using scan tool, read ECT sensor value. If engine coolant temperature is more than 180°F (82.2°C), allow engine coolant temperature to cool down to 150°F (65.6°C) before proceeding. If engine coolant temperature is 150°F (65.6°C) or less, start engine and monitor scan tool until engine coolant temperature reaches 180°F (82.2°C). If ECT sensor value increases to 180°F (82.2°C) or more, go to next step. If ECT sensor value does not increase to 180°F (82.2°C) or more, check for cooling system problems and replace ECT sensor. Perform TEST VER-5A.
  5. Turn ignition off. Using vacuum tee, connect a vacuum gauge to manifold vacuum source. Start engine and allow it to idle. If engine will not idle, maintain a constant RPM above idle. Using scan tool, read MAP sensor vacuum. If vacuum reading is within one in. Hg of teed-in vacuum gauge, go to next step. If vacuum reading is not within one in. Hg of teed-in vacuum, replace MAP sensor. Perform TEST VER-5A.
    WARNING: High fuel pressure may be present in fuel lines. Open fuel system with caution. See FUEL PRESSURE RELEASE  .
  6. Turn engine off. Release fuel pressure. Connect fuel pressure gauge to fuel rail. Turn ignition on, with engine off. Using scan tool, actuate ASD fuel system test. Allow fuel pressure gauge to stabilize. Stop ASD fuel system test. Fuel pressure should be 44.2-54.2 psi (3.23-3.96 kg/cm2 ). If fuel pressure is less than specified, go to next step. If fuel pressure is within specification, go to step 8). If fuel pressure is more than specified, check fuel return line for kinks or restrictions. If fuel line is okay, replace fuel filter/pressure regulator. Perform TEST VER-5A.
    WARNING: High fuel pressure may be present in fuel lines. Open fuel system with caution. See FUEL PRESSURE RELEASE  .
  7. Turn ignition off. Check and repair restricted fuel line between fuel rail and fuel pump module. Fuel pump module is located in fuel tank. If fuel line is okay, release fuel pressure. Raise and support vehicle. Disconnect fuel pressure line from fuel filter/pressure regulator. Connect Fuel Line Adapter (6539) between disconnected fuel line and fuel filter/pressure regulator. Connect fuel pressure gauge to fuel line adapter. Turn ignition on, with engine off. Using scan tool, actuate ASD fuel system test and note fuel pressure. Fuel pressure should be 44.2-54.2 psi (3.23-3.96 kg/cm2 ). If fuel pressure is within specification, stop ASD fuel system test, release fuel pressure and replace fuel filter/pressure regulator. Perform TEST VER-5A. If fuel pressure is not within specification, stop ASD fuel system test, release fuel pressure and replace fuel pump module. Perform TEST VER-5A.
  8. Check the following additional items as possible mechanical problems:
    • Engine Vacuum Must Be At Least 13" In Neutral
    • Engine Valve Timing Must Be Within Specification
    • Engine Compression Must Be Within Specification
    • Engine Exhaust System Must Be Free Of Restrictions
    • Engine PCV System Must Flow Freely
    • Engine Drive Sprockets Must Be Properly Positioned
    • Torque Converter Stall Speed Must Be Within Specification
    • Power Brake Booster Must Have No Internal Leaks
    • Fuel Must Be Free Of Contamination
    • Fuel Injectors Must Be Free Of Restrictions
    • Fuel Injector Control Wires Must Be Installed On Correct Injector

    Repair mechanical problems as necessary. Perform TEST VER-5A2. If no mechanical problems exist, go to next step.

    WARNING: High fuel pressure may be present in fuel lines. Open fuel system with caution. See FUEL PRESSURE RELEASE  . DO NOT operate fuel pump for more than 7 seconds. Reservoir may run empty and damage to fuel pump may occur.
  9. Turn ignition off. Release fuel pressure. Disconnect fuel supply line at fuel rail. Connect Fuel Line Adapter (6539) or Adapter (6631) to disconnected fuel supply line. Insert other end of adapter into approved fuel container. Turn ignition on, with engine off. Using scan tool, actuate ASD fuel system test for 7 seconds, then stop ASD fuel system test. Measure amount of fuel pumped into container. If fuel pump is okay, at least 1/4 gallon of fuel should be pumped into container. Test is complete. If less than 1/4 gallon of fuel is pumped into container, check and repair restricted fuel line between fuel tank and fuel rail. If fuel line is okay, replace fuel filter/pressure regulator. Perform TEST VER-5A.
  10. At this time, DTC P0171: 1/1 FUEL SYSTEM LEAN does not exist or is an intermittent problem. Using scan tool, read and record FREEZE FRAME data. Road test vehicle under conditions in FREEZE FRAME data, paying particular attention to fault setting conditions such as speed, temperature, load and MAP vacuum. Using scan tool, read DTCs. If DTC P0171: 1/1 FUEL SYSTEM LEAN returns, visually inspect related connectors and wiring harness for damage. Repair connectors and wiring harness as necessary. Perform TEST VER-5A. If DTC P0171: 1/1 FUEL SYSTEM LEAN does not return, DTC P0171: 1/1 FUEL SYSTEM LEAN no longer exists. Test is complete.
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