Syclone V6-262 4.3L Turbo (1991)
CIRCUIT DESCRIPTION
The fuel pump delivers fuel to the fuel rail and injectors, where the system pressure is controlled to 245 to 256 kPa (35 to 38 psi) by the pressure
regulator. Excess fuel is returned to the fuel tank. When the engine is stopped, the pump can be energized 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. (The pressure will not leak down after the fuel pump is stopped on a correctly functioning system.).
2. While the engine is idling, manifold pressure is low (vacuum). When applied to the fuel regulator diaphragm, the pressure will result in a lower
fuel pressure at about 190-200 kPa (25-30 psi).
3. The application of vacuum to the pressure regulator should result in a fuel pressure drop.
4. Pressure leak-down may be caused by one of the following:
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In-tank fuel pump check valve not holding.
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Pump coupling hose leaking.
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Fuel pressure regulator valve leaking.
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Injector sticking open.
5. Pressure less than 245 kPa (35 psi) may be caused by one of two problems.
-
The regulated fuel pressure is too low The system will be running lean and may set Code 44. Also, hard cold starting and overall poor
performance is possible.
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Restricted flow is causing a 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 surge and then stop as pressure begins to drop rapidly.
6. Restricting the fuel return line allows the fuel pump to build above regulated 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.
DIAGNOSTIC AIDS
Improper fuel system pressure may contribute to one or all of the following symptoms:
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Cranks but won't run.
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Code 44 or 45 .
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Cutting out (May feel like ignition problem).
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Hesitation, loss of power or poor fuel economy. Refer to "Symptoms." See: Computers and Control Systems/Testing and Inspection
