Chart A-7 Fuel System Pressure Test

Fuel System Pressure Test (Part 1 of 2):

Diagram for Fuel System Diagnosis:

CHART A-7 FUEL SYSTEM PRESSURE TEST



Circuit Description:


The fuel pump will deliver fuel to the fuel rail and injectors, then to the pressure regulator, where the system pressure is controlled to 245 to 256 kPa (35 to 38 psi). Excess fuel pressure is returned to the fuel tank. When the engine is stopped, the pump can be turned "ON" by applying battery voltage to the test terminal located in the engine compartment.


Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Use pressure gage J-34730-1. Wrap a shop towel around the fuel pressure tap to absorb any small amount of fuel leakage that may occur when installing the gage. Pressure should not leak down after the fuel pump is shut "OFF."
2. When the engine is idling, the vacuum is high and, if applied to the fuel regulator diaphragm, will offset the spring and result in a lower fuel pressure, 190-200 kPa (25-30 psi).
3. The application of vacuum to the pressure regulator should result in a fuel pressure drop.
4. Pressure that leaks down may be 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.


Diagnostic Aids:

Improper fuel pressure may contribute to one or all of the following symptoms:
Cranks but won't run
May set Code 44 or 45
Cuts out, may feel like an ignition problem
Hesitation, loss of power or poor fuel economy

Fuel System Pressure Test (Part 2 of 2):

Test Description: Numbers below refer to circled numbers on the diagnostic chart.
5. Pressure but less than 245 kPa (35 psi) falls into two areas:
-a. Regulated pressure; The amount of fuel to injectors is OK but pressure is too low. The system will be running lean and may set Code 44. Also, possible hard starting cold and overall poor performance.
-b. Restricted flow causing pressure drop; Normally, a vehicle with a fuel pressure loss at idle will not be driveable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop as pressure begins to drop rapidly.
6. Restricting the fuel return line allows the fuel pump to develop its maximum pressure (dead head pressure). When battery voltage is applied to the pump test terminal, pressure should be above 450 kPa (65 psi).
7. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem