C 3500 Truck 2WD V8-454 7.4L (1990)
Refer to the labor time guide for applicable labor times for various models.
FUEL SYSTEM PRESSURE TEST ALL ENGINES
Circuit Description:
When the fuel pump is running, fuel is delivered to the injector(s) and then to the regulator where the system pressure is controlled to about 62 to 90 kPa
(9 to 13 psi). Excess fuel is then returned to the fuel tank.
Test Description:
Numbers below refer to circled numbers on the diagnostic chart.
1.
Pressure, but less than 62 kPa (9 psi) falls into two areas:
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Regulated pressure but less than 62 kpa (9 psi) - Amount of fuel to injector OK but pressure is too low. System will be lean running and may set
Code 44. Also, hard starting cold and poor overall performance.
^
Restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 62 kPa (9 psi) at idle will not be driveable. However,
if the pressure drop occurs only while driving, the engine could surge and stop when pressure is too low.
2.
Restricting the outlet side of the gage allows the pump to develop its maximum pressure (dead head pressure). With battery voltage applied to the
pump test terminal, fuel pressure should rise to 90
to 103 kPa (13 to 15 psi) as the pressure gage outlet hose is gradually pinched.
3.
This test determines if the high fuel pressure is due to a restricted fuel return line or a throttle body pressure regulator problem.
Diagnostic Aids:
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If the vehicle is equipped with a fuel module, the module must be disconnected before performing the fuel system pressure test. Refer to
SECTION 4 of the Fuel and Emissions Manual.
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Fuel system is under pressure. To avoid fuel spillage, refer to procedures in SECTION 4 for testing or making repairs requiring disassembly of
fuel lines or fittings.
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On V6 or V8 engines, the fuel pressure drops to almost zero psi after pump shuts "OFF".
