How to Check the Injectors on a 7.3L

by Skip Shelton
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There are eight fuel injectors on the 7.3-liter diesel engine; one corresponding to each cylinder. Placed under the fuel rails, the fuel injectors have a solenoid which opens a valve when engaged to allow pressurized fuel to spray into the combustion chamber. Fuel injector failure may result in lean or rich conditions in individual cylinders during combustion. Improper fuel distribution may result in loss of power, triggering of engine codes or mechanical failure. Identifying fuel injector failure early can prevent costly repairs associated with too rich or too lean combustion conditions.

Injector Valve

Step 1

Park the vehicle in an area away from traffic, set the emergency brake and start the engine.

Step 2

Touch a stethoscope, or the tip of long handled screwdriver, to the top of the fuel injector. Listen through the stethoscope or press the handle end of the screwdriver to your ear. Operational fuel injectors will produce a clicking sound as the solenoid and valve are engaged.

Test the wiring on a fuel injector that is not operational. If the wiring is functional, replace the fuel injector.

Test the Fuel Injector Wire Voltage

Step 1

Turn the ignition key to the "On" position. It is not necessary to start the engine. Disconnect the electrical plug from the fuel injector to be tested.

Step 2

Set the multimeter to "Volts." Insert the multimeter leads into each side of the injector plug. No specific polarity is required. The voltage output on the multimeter should be approximately 12 volts.

Replace the fuel injector if the Engine Control Module (ECM) is noting a failure in the part, but the lead is providing 12 volts. If the wire is not producing 12 volts, replace it.

Fuel Injector Resistance

Step 1

Set the multimeter to "Ohms." Touch the red and black multimeter leads onto each side of the fuel injector plug terminals. Note the results.

Step 2

Take a similar reading from the remaining seven fuel injectors. Compare all results. The readings should be very similar or the same for all fuel injectors. A failed injector will have a significantly different reading than the others, displaying either too much or too little resistance.

Replace any fuel injector whose ohmmeter reading differs vastly from that of the others.

Fuel Pressure Regulator

Step 1

Pull the vacuum line from the fuel pressure regulator.

Step 2

Inspect the fuel pressure regulator. Look into the hole from which the vacuum line was pulled. If fuel is present, the diaphragm has been ruptured.

Replace the fuel regulator if fuel is present. Replace the vacuum hose if the fuel pressure regulator is operational.

Troubleshooting Electrical Shorts

Step 1

Disconnect all eight fuel injector plugs from the engine.

Step 2

Set the multimeter to "Volts." Connect the black lead to the vehicle battery positive terminal. Touch the red lead to the fuel injector's wired plug.

Step 3

Have a helper attempt to start the engine. Monitor the voltage on the multimeter. The reading should alternate between zero and 12 volts during the engine start attempt.

Step 4

Use the same wire plug to test the other fuel injectors and repeat the test for each. It is not necessary to test other plugs while checking for a short. If a short is present, the test will fail when the plug wire is connected to a faulty injector. For the final test, disconnect a previously connected plug and verify the voltage to test the last fuel injector.

Replace any fuel injector which causes a short, which results in no alternation between zero and 12 volts during the test. Shorted fuel injectors will prevent the electrical current from being supplied to other injectors.

Fuel Injector Leaks

Step 1

Inspect the fuel injector connection to the fuel rail. If fuel is present, the fuel injector O-ring is likely damaged. Damaged or improperly seated O-rings will leak fuel under pressure. The fuel lines are pressurized during operation.

Step 2

Depressurize the fuel system and remove any bolt-on accessories, such as the air intake tube, vacuum hoses or electronics, which impede access to the fuel rail. Lift the fuel rail off the fuel injector.

Remove the injector from the engine by pulling the injector directly out of the hole. Replace the O-rings on the fuel injector. Apply a small amount of motor oil to the O-rings. Reinstall the fuel injector and fuel rail.

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