Exhaust Gas Analysis
An easy way to verify that a fuel injector is leaking on port-injected engines is to use an exhaust gas analyzer. this test does not locate the bad injector, but does verify that one or more are leaking.
- With the engine warmed up, but not running, remove the air duct from the airflow sensor.
- Insert the gas analyzer's probe into the intake plenum area.
- Be careful not to damage the airflow sensor or throttle plates while doing this.
- Look at the HC (hydrocarbon) readings on the analyzer.
- They should be low and drop as time passes.
- If an injector is leaking, the HC reading will be high and will not drop.
Checking Voltage Signals
When an injector is suspected as the cause of a lean problem, the first step is to determine if the injector is receiving a signal (from the PCM (powertrain control module)) to fire. Fortunately, determining if the injector is receiving a voltage signal is easy and requires simple test equipment. Unfortunately, the location of the injector's electrical connector can make this simple voltage check somewhat difficult.
For example, on some Chevrolet 2.8-liter V6 engines, the cover must be removed from the cast aluminum plenum chamber that is mounted over the top of the engine before the injector can be accessed.
Once the injector's electrical connector has been removed, check for voltage at the injector using a convenient noid light that plugs into the connector.
Typical location of fuel injector connectors. Courtesy of DaimlerChrysler Corporation.
After making the test connections, crank the engine.
- The noid light flashes if the computer is cycling the injector on and off.
- If the light is not flashing, the computer or connecting wires are defective.
If sufficient voltage is present after checking each injector, check the electrical integrity of the injectors themselves.
When performing this test, make sure to keep off the accelerator pedal. On some models, fully depressing the accelerator pedal activates the clear flood mode, in which the voltage signal to the injectors is automatically cut off. People unaware of this waste time tracing a phantom problem.
An ohmmeter can be used to test the electrical soundness of an injector. connect the ohmmeter across the injector terminals after the wires to the injector have been disconnected.
Checking an injector with an ohmmeter. Reprinted with permission.
- If the meter reading is infinite, the injector winding is open.
- If the meter shows more resistance than the specifications call for, there is high resistance in the winding.
- A reading that is lower than the specifications indicates that the winding is shorted.
If the injector is even a little bit out of specifications, it must be replaced.
Injector Balance Testing
If the injectors are electrically sound, perform an injector pressure balance test. this test will help isolate a clogged or dirty injector. an electronic injector pulse tester is used for this test. the tester is designed to safely pulse each injector for a controlled length of time.
- As each injector is energized, a fuel pressure gauge is observed to monitor the drop in fuel pressure.
- The tester is connected to one injector at a time.
- To prevent oil dilution, the electrical connectors to the other injectors are removed.
- The ignition is turned on until a maximum reading is on the pressure gauge.
- That reading is recorded and the ignition turned off.
- With the tester, activate the injector and record the pressure reading, after the needle has stopped pulsing.
- This same test is performed on each injector.
The difference between the maximum and minimum reading is the pressure drop.
- Ideally, each injector should drop the same amount when opened.
- A variation of 1.5 to 2 psi (10 kPa) or more is cause for concern.
- If there is no pressure drop or a low pressure drop, suspect a restricted injector orifice or tip.
- A higher than average pressure drop indicates a rich condition.
- When an injector plunger is sticking in the open position, the fuel pressure drop is excessive.
- If there are inconsistent readings, the nonconforming injectors either have to be cleaned or replaced.
To perform this test:
- Connect the fuel pressure gauge to the schrader valve on the fuel rail, and then relieve the pressure in the system.
- Disconnect the number 1 injector and connect the injector pulse tester to the injector's terminals.
- Connect the injector pulse tester's power supply leads to the battery.
- Cycle the ignition switch several times until the system pressure is at the specified level.
- Push the injector pulse tester switch and record the pressure on the pressure gauge.
- Subtract this reading from the measured system pressure.
- The answer is the pressure drop across that injector.
- Move the injector tester to the number 2 injector and cycle the ignition switch several times to restore system fuel pressure.
- Depress the injector pulse tester's switch and observe the fuel pressure.
- Again the difference between the system pressure and the pressure when an injector is activated is the pressure drop across the injector.
- Move the injector tester's leads to the number 3 injector and cycle the ignition switch to restore system pressure.
- Depress the switch on the tester to activate that injector and record the pressure drop.
- Continue the procedure for all injectors, then compare the results of each to specifications and to each other.
Injector Sound Test
If the injector's electrical leads are difficult to access, an injector power balance test is hard to perform. as an alternative, start the engine and use a technician's stethoscope to listen for correct injector operation.
Using a stethoscope (sound scope) to check injector action. Reprinted with permission.
A good injector makes a rhythmic clicking sound as the solenoid is energized and de-energized several times each second. If a clunk-clunk instead of a steady click-click is heard, chances are the problem injector has been found. Cleaning or replacement is in order.
- If an injector does not produce any clicking noise, the injector, connecting wires, or PCM may be defective.
- When the injector clicking noise is erratic, the injector plunger may be sticking.
- If there is no injector clicking noise, proceed with the injector resistance test and noid light test to locate the cause of the problem.
If a stethoscope is not handy, use a thin steel rod, wooden dowel, or fingers to feel for a steady on/off pulsing of the injector solenoid.
Injector Flow Testing
Some vehicle manufacturers recommend an injector flow test rather than the balance test.
To conduct this test:
- Remove the injectors and fuel rail from the engine and place the tip of the injector to be tested in a calibrated container.
- Leave all of the injectors in the fuel rail.
- Connect a jumper wire across the specified terminals in the DLC for fuel pump testing.
- Turn on the ignition switch and connect a jumper wire from the terminals of the injector to the battery terminals.
Setup for testing the flow of an injector. Reprinted with permission.
- Disconnect the jumper wire from the negative battery cable after 15 seconds.
- Record the amount of fuel in the calibrated container.
- Repeat the procedure on each injector.
- If the volume of fuel discharged from any injector varies more than 0.3 cu in (5 cc) from the specifications or the others, the injector should be replaced.
- When you have completed your testing, reconnect the negative battery cable and disconnect the 12 volt power supply.
- Connect the fuel pressure gauge to the fuel system.
- While the fuel system is pressurized with the injectors removed from the fuel rail after the flow test, observe each injector for leakage from the injector tip.
Each injector should be checked for leakage when it is turned off and there is pressure in the system. Reprinted with permission.
- Injector leakage must not exceed the manufacturer's specifications.
- If the injectors leak into the intake ports on a hot engine, the air/fuel ratio may be too rich when a restart is attempted a short time after the engine is shut off.
- When the injectors leak, they drain all the fuel out of the rail after the engine is shut off for several hours. This may result in slow starting after the engine has been shut off for a longer period of time.
- While checking leakage at the injector tips, observe the fuel pressure in the pressure gauge.
- If the fuel pressure drops off and the injectors are not leaking, the fuel may be leaking back through the check valve in the fuel pump.
- Repeat the test with the fuel line plugged.
- If the fuel pressure no longer drops, the fuel pump check valve is leaking.
- If the fuel pressure drops off and the injectors are not leaking, the fuel pressure may be leaking through the pressure regulator and the return fuel line.
- Repeat the test with the return line plugged.
- If the fuel pressure no longer drops off, the pressure regulator valve is leaking.