Chart A-7, Fuel System Diagnosis: Notes

The ECM will turn on the electric fuel pump when the ignition is turned on. The fuel pump is capable of producing fuel pressure as high as 65 psi (4.6 kg/cm² ). Normal pump pressure, with the vacuum hose disconnected from the pressure regulator, is 35-38 psi (2.5-2.7 kg/cm² ). Excess fuel passes through the pressure regulator and is returned to the tank via a fuel return line.

1. This test checks for adequate fuel delivery pressure. Pressure is controlled by spring pressure within the regulator assembly. Pressure should not leak down after the fuel pump is shut off.
2. Manifold vacuum is applied to fuel pressure regulator to control fuel pressure. When engine is idling, manifold pressure is low (high vacuum) and this will lower fuel line pressure of approximately 25-30 psi (1.8-2.1 kg/cm² ). This pressure drop at idle indicates proper regulator control.
3. The application of vacuum to the pressure regulator should result in a fuel pressure drop.
4. Pressure that continues to fall is caused by one of the following:
   • In-tank fuel pump check valve not holding.
   • Pump coupling hose leaking.
   • Fuel pressure regulator valve leaking.
   • Injector sticking open.
5. Pressure less than 35 psi (2.5 kg/cm² ) can be of 2 types:
   • Regulated pressure is low when the amount of fuel to injector is okay, but pressure is too low. System may run lean and set Code 44. In addition, engine may be hard starting and have poor overall performance.
   • Restricted fuel flow can cause a pressure drop. Normally, a vehicle with a system pressure of less than 9 psi (.6 kg/cm² ) at idle will not be driveable. If the pressure drop occurs only while driving, the engine will begin to surge and then stop as pressure drops rapidly.
6. Restricting fuel return line allows pump to develop maximum pressure. Pressure should exceed 65 psi (4.6 kg/cm² ).
7. This test determines if high pressure is due to a restricted fuel return line or a pressure regulator problem.