Chart A-7 - Fuel System Pressure Test: Notes

Fuel pressure control valve controls fuel pressure at 41-46 psi (2.8-3.2 kg/cm² ). Excess fuel is returned to fuel tank. When engine is stopped, fuel pump can be energized by jumping fuel pump relay connector with fused jumper.

1. Connect Fuel Pressure Gauge (J-34730-1) or equivalent. Wrap shop towel around fuel pressure tap to absorb spilled fuel when installing gauge. On a correctly functioning fuel system, pressure will not leak down after fuel pump is stopped.
2. While engine is idling, manifold pressure is low (high vacuum). When vacuum is applied to fuel regulator diaphragm, vacuum will result in lower fuel pressure, at approximately 25-30 psi (1.7-2.1 kg/cm² ).
3. Application of high vacuum to fuel pressure control valve should result in fuel pressure drop.
4. Fuel pressure leak down may be caused by one of the following:
   • Fuel Pump Check Valve Not Holding
   • Pump Coupling Hose Leaking
   • Fuel Pressure Control Valve Leaking
   • Injector Stuck Open Or Leaking
5. Pressure less than 41 psi (1.7 kg/cm² ) may be caused by one of the following:
   • Regulated fuel pressure is too low. System will be running lean and may set diagnostic Code 44. Also, hard cold starting and overall poor performance may occur.
   • Restricted flow is causing a pressure drop. Normally a vehicle with a fuel pressure loss at idle will not be driveable. However, if pressure drop occurs only while driving, engine will surge and then stop as pressure begins to drop rapidly.
6. Restricting fuel return line allows fuel pump pressure to be greater than regulated pressure. When battery voltage is applied to fuel pump relay, pressure should be greater than 65 psi (4.5 kg/cm² ).
7. This test determines if high fuel pressure is due to a restricted fuel return line or a fuel pressure control valve problem.