1990 Chevrolet Forward Control 7.4 N, Automatic, 3L80-HD/M40FACTORY OEM SERVICE PROCEDURE

Fuel System Pressure Test: Notes

CAUTION: Begin fuel system trouble shooting and diagnosis with fuel system pressure check. Relieve fuel system pressure before disconnecting any components or installing fuel pressure gauge.
  1. Turn engine off and relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF. Remove air cleaner and plug air cleaner (THERMAC) vacuum port (if equipped). Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658A) and Adapter (J-2968A-85) between throttle body and in-line fuel filter. Start engine and observe fuel pressure reading.
  2. Fuel pressure should be 9-13 psi (.6-.9 kg/cm2 ). Go to step 3) if fuel pressure is present but less than specification. If fuel pressure is greater than specification, go to step 4). If no fuel pressure is evident, check for power to fuel pump relay. If power is available to fuel pump relay, test fuel pump relay. See SYSTEM/COMP TESTS article.
  3. If fuel pressure is present during cranking or with fuel pump relay by-passed, but is less than 9-13 psi (.6-.9 kg/cm2 ), gradually pinch off fuel return line to fuel tank. If fuel pressure remains low, check for a plugged fuel filter or restriction in fuel delivery line. If filter is okay and no restrictions are present, replace fuel pump. If pressure increases to within specification when return line is pinched off, replace fuel pressure regulator.
    CAUTION: DO NOT  pinch off fuel return line completely. DO NOT  allow fuel pressure build-up to exceed specification, as damage to fuel pressure regulator may occur.
  4. If fuel pressure is present, but is greater than 9-13 psi (.6-.9 kg/cm2 ), check for restriction in return line to fuel tank. If restriction is not present, replace fuel pressure regulator.
  5. After testing, relieve fuel pressure. Remove fuel pressure gauge and connect fuel line to fuel filter. Start vehicle and check for leaks. Remove plug from throttle body vacuum port (if equipped). Install air cleaner.
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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.

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