Fuel Pressure: Testing and Inspection
FUEL SYSTEM PRESSURE TEST 1 of 2
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. Excess fuel pressure is bypassed back 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.Improper fuel system pressure may contribute to one or all of the following symptoms:
- Cranks but won't run
- Code 44 or 45
CAUTION: To reduce the risk of fire and personal injury, it is necessary to relieve the fuel system pressure before servicing fuel system components After relieving system pressure, a small amount of fuel may be released when servicing fuel lines or connections In order to reduce the chance of personal injury, cover fuel line fittings with a shop towel before disconnecting, to catch any fuel that may leak out Place the towel in an approved container when disconnect is completed.
- Disconnect negative battery terminal to avoid possible fuel discharge if an accidental attempt is made to start the engine.
- Loosen fuel filler cap to relieve tank vapor pressure.
- Connect gage J 34730-1 to fuel pressure connection. Wrap a shop towel around fitting while connecting gage to avoid spillage.
- Install bleed hose into an approved container and open valve to bleed system pressure. Fuel connections are now safe for servicing.
- Drain any fuel remaining in gage into an approved container.
Test Description: Number(s) refer to circled number(s) on the diagnostic chart.
1. Install pressure gage J 34730-1 to fuel pressure tap
- Start engine. With ignition "ON", pump pressure is controlled by spring pressure and throttle body vacuum within the pressure regulator assembly.
- Ignition "OFF" for 10 seconds. Pressure should not leak down after the fuel pump is shut "OFF."
2. When the engine is idling, the throttle body vacuum is high and is applied to the fuel regulator diaphragm. This will offset the spring and result in a lower fuel pressure.
3. The application of 12-14 inches of vacuum to the pressure regulator should result in a fuel pressure less than step 1.
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
FUEL SYSTEM PRESSURE TEST 2 of 2
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. Excess fuel pressure is bypassed back 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.
Improper fuel system pressure may contribute to one or all of the following symptoms:
- Cranks but won't run
- Code 44 or 45
- Cuts out, may feel like ignition problem
- Hesitation, loss of power, or poor fuel economy
Test Description: Number(s) refer to circled number(s) on the diagnostic chart.
5. Pressure but less than specifications falls into two areas:
- Regulated pressure but less than specifications; the amount of fuel to injectors is OK, but pressure is too low. The system will be lean running and may set Code 44. Also, possible hard starting cold and overall poor performance.
- Restricted flow causing pressure drop; Normally, a vehicle with a fuel pressure of less than 165 kPa (24 psi) 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 517 kPa (75 psi).
7. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.