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Diagnostic Aids

Perform this test when directed by QUICK TEST or if directed by other test procedures. This test is used to diagnose:

WARNING: Fuel system remains under high pressure even when engine is not running. To avoid injury, release fuel pressure before disconnecting any fuel system hose or component. See FUEL SYSTEM PRESSURE RELEASE procedure under FUEL SYSTEM in REMOVAL, OVERHAUL & INSTALLATION article.

  1. 1) Check System Integrity

    Turn ignition switch to ON position for 5 seconds. Turn ignition switch to OFF position. Inspect fuel system for leaks, damaged or kinked hoses. Inspect wiring harness for damage or loose connectors. Ensure the following conditions are present:
    • Inertia Fuel Shutoff (IFS) Is Set
    • Battery Is Fully Charged
    • Fuel System Fuses Are Okay
    • Fuel Level In Tank Is Okay

    Repair problems as necessary. If no faults are found, go to next step.

  2. 2) Check Voltage At Fuel Pump Connector

    Connect battery charger to battery. Ensure IFS is reset. Connect scan tool to Data Link Connector (DLC). Disconnect fuel pump harness connector. Turn ignition switch to ON position. Connect DVOM between fuel pump harness connector fuel pump PWR circuit terminal and GND terminals. See WIRING DIAGRAMS article. Using scan tool, access OUTPUT TEST MODE. See OUTPUT TEST MODE  under ADDITIONAL SYSTEM FUNCTIONS. Command fuel pump ON and monitor DVOM voltage measurement.

    If voltage measurement is more than 12.5 volts, turn ignition switch to OFF position and exit OUTPUT TEST MODE. Reconnect fuel pump harness connector and go to next step. If voltage measurement is 12.5 volts or less, go to TEST KB, step  70) for all models with Electronic Returnless Fuel System (ERFS). See FUEL SYSTEM IDENTIFICATION  table. On all other vehicles, check for open or short circuits in fuel pump PWR or GND circuits.

  3. 3) Check For Proper Fuel Filter Maintenance

    Check last time fuel filter was replaced. If fuel filter has been serviced regularly (within 30,000 miles), go to next step. If fuel filter has not been serviced regularly, replace fuel filter and go to next step.
    4. NOTE: When energized by scan tool, fuel pump will operate for only 8 seconds.

  4. 4) Check Fuel Pressure
    Turn ignition switch to OFF position. Release fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE procedure under FUEL SYSTEM, in REMOVAL, OVERHAUL & INSTALLATION article. Install fuel pressure gauge supplied in Kit (134-00114) to Schrader valve located on fuel rail. Turn ignition switch to ON position. Using scan tool, access OUTPUT TEST MODE. See OUTPUT TEST MODE  under ADDITIONAL SYSTEM FUNCTIONS. Command fuel pump ON. Record fuel pressure. Compare fuel pressure to specification. See FUEL PRESSURE SPECIFICATIONS  table. If fuel pressure is within specification on vehicles with returnable fuel system, go to next step. If fuel pressure is within specification on vehicles with ERFS, go to step  6). If fuel pressure is within specification on vehicles with MRFS, go to step  12). See FUEL SYSTEM IDENTIFICATION  table. If fuel pressure is not within specification, proceed as follows:
    • If fuel pressure is more than specified on vehicles with MRFS, replace fuel pressure regulator in fuel tank. Recheck fuel pressure.
    • If fuel pressure is less than specified on vehicles with MRFS, go to step  6).
    • If fuel pressure is less than specified on vehicles with ERFS, less than specified on vehicles with returnable fuel system, go to step  13).
    • If fuel pressure is more than specified on vehicles with ERFS, repeat QUICK TEST . Other DTCs should be present if fuel pressure problem exists.
    • If fuel pressure is more than specified on vehicles with returnable fuel system, go to step  10).

  5. 5) Check Fuel Pressure Leakdown

    With fuel pressure gauge supplied in Kit (134-00114) connected to Schrader valve located on fuel rail, turn ignition switch to ON position, engine off. Using scan tool, access OUTPUT TEST MODE. See OUTPUT TEST MODE  under ADDITIONAL SYSTEM FUNCTIONS. Command fuel pump ON. Record maximum fuel pressure. Turn ignition switch to OFF position. If fuel pressure remains within 5 psi (34 kPa) of maximum fuel pressure for 60 seconds after fuel pump turns off, go to step  7). If fuel pressure does not remain within 5 psi (34 kPa) of maximum fuel pressure for 60 seconds after fuel pump turns off, go to next step.

  6. 6) Check Pressure Regulator & Pulse Damper Diaphragm

    With fuel pressure gauge supplied in Kit (134-00114) connected to Schrader valve located on fuel rail, start engine and allow to idle for 10 seconds. Turn ignition switch to OFF position and wait 10 seconds. Start engine again and allow to idle for 10 seconds. Turn ignition switch to OFF position. Disconnect and inspect vacuum hose from fuel pressure regulator or pulse damper. If hose is wet with fuel, replace fuel pressure regulator or pulse damper. If hose is dry, go to step  12) for vehicles with ERFS or returnable systems, or go to step  13) for vehicles with MRFS.

  7. 7) Check Fuel Pressure While Test Driving Vehicle

    Ensure fuel pressure gauge supplied in Kit (134-00114) is connected to Schrader valve located on fuel rail. Disconnect and plug fuel pressure regulator vacuum hose. Ensure fuel pressure gauge can be seen by vehicle operator. Drive vehicle under heavy acceleration while monitoring fuel pressure gauge reading. If fuel pressure reading stays within 3 psi (21 kPa) of original fuel pressure reading, go to next step. If fuel pressure reading does not stay within 3 psi (21 kPa) of original fuel pressure reading, go to step  13).

  8. 8) Check Fuel Pressure Regulator Response

    With fuel pressure gauge supplied in Kit (134-00114) installed, reconnect fuel pressure regulator vacuum hose. Install vacuum gauge to intake manifold. Ensure fuel pressure and vacuum gauges can be seen by vehicle operator. Drive vehicle, accelerating and decelerating engine speed while monitoring fuel pressure and vacuum gauge readings. Fuel pressure gauge reading should increase as vacuum gauge reading decreases and decrease as vacuum gauge reading increases. If gauge readings do not operate as specified, go to next step. If gauge readings operate as specified, fuel system is functioning properly. Diagnose by symptom. See SYMPTOMS in TROUBLE SHOOTING - NO CODES article.

  9. 9) Check Vacuum Supply

    Turn ignition switch to OFF position. Ensure fuel pressure gauge supplied in Kit (134-00114) is connected to Schrader valve located on fuel rail. Disconnect and plug fuel pressure regulator vacuum hose. Install vacuum pump to fuel pressure regulator. Start engine and operate at idle. Observe fuel pressure gauge while applying vacuum to fuel pressure regulator. If fuel pressure reading changes as applied vacuum amount changes, repair vacuum source to fuel pressure regulator. If fuel pressure reading does not change as applied vacuum amount changes, replace fuel pressure regulator.

  10. 10) Check For Restricted Fuel Return Line

    Ensure fuel pressure gauge supplied in Kit (134-00114) is connected to Schrader valve located on fuel rail. Turn ignition switch to OFF position. Leave scan tool connected to DLC. Release fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE procedure under FUEL SYSTEM, in REMOVAL, OVERHAUL & INSTALLATION article. Disconnect fuel return line at fuel rail. Connect a separate hose to fuel rail and place opposite end of hose in clean, one quart container. Turn ignition switch to ON position, engine off.

    Using scan tool, access OUTPUT TEST MODE. See OUTPUT TEST MODE  under ADDITIONAL SYSTEM FUNCTIONS. Command fuel pump ON. Record maximum fuel pressure and ensure fuel flows into one quart container from hose connected to fuel rail. One command of fuel pump ON is sufficient. Compare fuel pressure to specification. See FUEL PRESSURE SPECIFICATIONS  table. If fuel pressure is within specification and fuel flows into one quart container, go to next step. If fuel pressure is not within specification and/or fuel does not flow into one quart container, replace fuel pressure regulator.

  11. 11) Check Fuel Return System

    Ensure fuel return line is disconnected at fuel rail. Disconnect fuel return line at fuel pump. Check return line for restrictions. Apply 3-5 psi (21-34 kPa) compressed air to return line. If air flows freely through fuel return line, replace fuel pump. If air does not flow freely through fuel return line, repair or replace fuel return line.

  12. 12) Check Fuel Injector Flow & Leakage

    Turn ignition switch to OFF position. Use Rotunda fuel injector tester supplied in Kit (134-00114) to flow test fuel injectors. Follow injector tester instructions. If flow rate for any fuel injector is not within specification, replace faulty fuel injector. If flow rate for each fuel injector is within specification on vehicles with EFRS or MRFS, check for additional symptoms and repair as necessary. See SYMPTOMS in TROUBLE SHOOTING - NO CODES article. If flow rate for each fuel injector is within specification on vehicles with a returnable fuel system, check for leaks in fuel system. Repair as necessary. If no fuel system leaks exist, replace fuel pump.

  13. 13) Check Fuel Supply Line For Restriction

    Turn ignition switch to OFF position. Release fuel system pressure. Disconnect fuel supply line at fuel rail and at fuel pump. Check supply line for restrictions. Apply 3-5 psi (21-34 kPa) compressed air to fuel supply line. If air flows freely through fuel supply line, replace fuel pump. If air does not flow freely through fuel supply line, repair or replace fuel supply linee.
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