How to Test a Fuel Injector

how to test fuel injector

A fuel injector is responsible for allowing fuel into the engine and is controlled by the PCM as part of the fuel injection system. Fuel pressure supplied by the fuel pump is metered by the duty cycle of the computer which varies by engine load. This guide will show you how to check the trigger (ground) signal from the PCM, power circuit and the operation of the injector itself. To test the injector signal from the computer a test light works best. You will need a voltmeter to check resistance through the injector. Fuel maybe present during testing so the usual fire precautions are necessary. Use protective gloves and eye wear for safety. Test for FI systems only.

Fuel injector Testing

  • Observe audible clicking confirming injector operation
  • Check trigger signal output from the PCM
  • Confirm the injector has power
  • Test the coil windings resistance using a volt meter
  • Confirm injector valve operation
  • Inspect the injector casing for leaks
  • Check flow and spray pattern

Let's Get Started

Simple Test

Start the engine and allow it to idle. Using a long metal rod like a screwdriver touch the end of the screwdriver to the injector. Gently lay an ear to the opposite end of the rod or handle to observe an audible clicking sound to confirm the injector is working.
fuel injector listen

Injector Circuit Power Test

With the key in the on position use a test light or voltmeter connected to the negative side of the battery. Gently probe both sides of the injector wiring connector, one of the wires should react by registering around 12 volts on the meter or illuminating the test light. If neither wire reacts test the fuel injector fuse in the PDC. If the fuse is okay a fuel injection wiring diagram is needed to help trace the wire and repair the connection. Seal test points with a small dab of silicone rubber once testing is complete.
probe injector wire

Injector Ground Trigger Test

The PCM closes the injector circuit to trigger the injector operation. For this a test light works best to observe the pulse produced by the PCM. Attach the test light lead to the positive side of the battery and have a helper start or crank the engine. Probe the opposite side of the injector connector from the power circuit, you should observe the test light flicking which will respond to engine RPM/Load. If the engine is running and no pulse is observed suspect a bad wiring connection or a failed PCM injector driver which will warrant a PCM replacement. A shorted injector can hinder the injector driver operation for additional injectors, unplug all injectors and re-test the signal. If pulse returns plug the injectors back in one at a time until the pulse fails then replace the shorted injector. If the engine is not running check the crankshaft angle sensor which the computer uses to open the injectors (Note: A failed crankshaft angle sensor will not set a trouble code in most cases).

test light flash

Fuel Injector Winding Test (key off)

Using a voltmeter, adjust the setting to ohms. This test can be performed with the injector installed or uninstalled.

voltmeter ohms

Remove the injector electrical connector
injector connector

This will expose the injector electrical terminals.
injector terminals

Connect the voltmeter leads to the terminals, the polarity of the leads is irrelevant. This test gives a baseline ohm reading of all injectors, a service manual also has this information. Most injector readings should range between 11 and 14 ohms. Fuel injectors should be tested cold unless otherwise specified, temperature variances will change the readings. If the test shows high resistance or an open circuit the injector needs replacement.

probe injector terminals
Injector Spray Pattern Test

For this test the injector must be removed. Inspect the injector housing for leaks and electrical connectors for corrosion.
fuel injector

The valve and spray pattern is the most important part of testing a fuel injector. Valve testing can be done with the injector still installed using a fuel pressure gauge while hot wiring the fuel pump to stay on. Carefully attach a 12 volt power (power and ground) source to the injector, you should be able to see the gauge fluctuate as you connect and disconnect the circuit if the injector valve is working and not plugged.

To check the injector spray pattern the injector must be removed. Attach pressurized air to the injector inlet. Attach a 12 volt power and ground source to energize the injector. Compressed air should be released from the outlet valve with traces of fuel still remaining from the injector (use fire precautions). Observe the pattern which should be robust, if the pattern is muted the injector should be replaced.
injector discharge


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