The Schrader valve is located at the top of the fuel charging main body. This valve provides a convenient point to monitor fuel pressure, bleed down the system pressure prior to maintenance, and to bleed out air which may become trapped in the system during filter replacement. A pressure gauge with a adapter is required to perform pressure tests.
SYSTEM PRESSURE TEST
Testing fuel pressure requires the use of a special pressure gauge (T80L-9974-A or equivalent) that attaches to the pressure tap on the fuel charging assembly. Depressurize the fuel system before disconnecting any lines.
- Disconnect the fuel return line at the throttle body and position the hose into a 1 quart (448 g) calibrated container. Connect a pressure gauge.
- Unfasten the electrical connector at the fuel pump. The connector is located ahead of fuel tank (in-tank high pressure pump) or just forward of pump outlet (in-line high pressure pump). Connect an auxiliary wiring harness to the connector of the fuel pump. Energize the pump for 10 seconds by applying 12 volts to the auxiliary harness connector. Allow the fuel to drain into the calibrated container. Note the fuel volume and pressure gauge reading.
- Correct fuel pressure should be 35-45 psi (241-310 kPa). Fuel volume should be 10 oz. (280 g) in 10 seconds (minimum) and fuel pressure should maintain a minimum of 30 psi (206 kPa) immediately after pump cut-off.
If the pressure condition is met, but the fuel flow is below specification, check for blocked filter(s) and fuel supply lines. After correcting the problem, repeat the test procedure. If the fuel flow is still inadequate, replace the high pressure pump. If the flow specification is met but the pressure is not, check for a worn or damaged pressure regulator valve on the throttle body. If both the pressure and fuel flow specifications are met, but the pressure drops excessively after de-energizing, check for a leaking injector valve(s) and/or pressure regulator valve. If the injector valves and pressure regulator valve are okay, replace the high pressure pump. If no pressure or flow is seen in the fuel system, check for blocked filters and fuel lines. If no trouble is found, replace the inline fuel pump, in-tank fuel pump and the fuel filter inside the tank.
FUEL INJECTOR PRESSURE TEST
- Connect pressure gauge T80L-9974-A, or equivalent, to the fuel pressure test fitting. Unplug the coil connector from the ignition coil. Unfasten the electrical lead from one injector and pressurize the fuel system. Disable the fuel pump by disconnecting the inertia switch or the fuel pump relay and observe the pressure gauge reading.
- Crank the engine for 2 seconds. Turn the ignition OFF and wait 5 seconds, then observe the pressure drop. If the pressure drop is 2-16 psi (14-110 kPa), the injector is operating properly. Reconnect the injector, activate the fuel pump, then repeat the procedure for other injector.
- If the pressure drop is less than 2 psi (14 kPa) or more than 16 psi (110 kPa), switch the electrical connectors on the injectors and repeat the test. If the pressure drop is still incorrect, replace the suspected injector with one of the same color code, then reconnect both injectors properly and repeat the test.