Mechanics

Fuel Injector Test

Test Fuel Injector - When a fuel injector malfunctions it can allow excessive or a minimal amount of fuel into the engine. The fuel injector is manufactured with a pintle valve that allows fuel to flow at prescribed amounts. If this valve sticks open it can flood the cylinder with fuel. If the valve sticks closed no fuel is allowed into the engine cylinder. Both excess and the absence of fuel can cause black smoke from the exhaust pipe. When an injector is stuck closed the absence of fuel will cause the cylinder to run lean, effectively fooling the oxygen sensor and causing the system to add too much fuel into the engine through the remaining fuel injectors. (A common misconception about a cylinder misfire causes the engine to run rich, in reality the cylinder runs lean which causes the aforementioned lean condition.) Performing a few basic tests you can determine if one or more fuel injectors have failed causing an engine run-ability problem.

Difficulty Scale: 3 of 10

This test pertains to all cars that utilize an electronic fuel injector system.

Tools and Supplies Needed

  • Test light or voltmeter
  • Small standard screw driver
  • Wrench set
  • Socket set
  • Protective eyewear and gloves
  • Shop towels

Begin by parking the vehicle on level ground, engine off with the parking brake set.

Step 1 - While an engine is running you should be able to hear each injector clicking as the electronic valve opens and closes. To aid in this inspection a stethoscope can be used to touch it against each injector. If no audible sound is heard as compared to the remaining injectors proceed to next step.

Step 2 - This test will tell you if the computer has proper system voltage and injector trigger signal. Remove the injector electrical connector, turn the key to the "ON" position without starting the engine. Remove an electrical connector from the fuel injector that needs to be tested. Probe both sides of the wiring connector with a grounded test light or voltmeter. (Set the voltmeter to DC voltage.) Have a helper crank the engine over while observing the test light or voltmeter. The test light/voltmeter should illuminate/read one side of the connector. (The voltmeter should read about 12 volts.) If no voltage is observed check the system fuses and inspect the wiring harness for damage. If tests okay proceed to next step.

Step 3 - This test will test the trigger circuit (pulse.) Switch the test light or voltmeter lead (black) to the positive side of the battery. Probe the side of the connector that did not light up in the previous in test 2, have a helper crank the engine over and observe the test light, it should blink on and off, or the voltmeter should bounce from 0 to 12 volts. (Note: If no injector pulse is present try disconnecting the remainder of injectors and re-test, if a fuel injector is shorted it can shut down the injector driver causing no injector pulse. If injector pulse returns plug the injectors in one at a time until the pulse fails. Replace the shorted injector. If this test is positive and the injector still will not operate the injector has failed and replacement is required.) If this test revealed that there was no pulse but system has power, the PCM is not generating a fuel injector trigger. If there is no trigger to the fuel injector it will not allow fuel to enter into the engine. Some of the most common reasons that can cause this condition include a shorted fuel injector, injector wiring damage or shorted PCM.

Step 4 - While the injector trigger wire is off, test the fuel injector windings by setting the voltmeter to ohms and attach the leads to either side of the injector. Your readings should be between within factory specifications (by testing a known good injector you can acquire the ohms for the remaining injectors.) Test all injectors; if one injector reading is considerably different from the others replace that injector.

Helpful Information

Fuel injector pulse and supply voltage output (test is used for most cars). This test will tell you if the computer system has operating voltage and injector trigger signal. Remove an electrical connector from a fuel injector (it doesn't matter which injector) probe both sides of the connector with a grounded test light (there are only two terminals). Have a helper turn the key to the "on" position without cranking the engine and observe the test light. The test light should illuminate one side of the connector only.

Next, switch the test light lead to the positive side of the battery to test the system ground injector trigger, probe the side of the connector that did not light up, have a helper crank the engine over and observe the test light, it should blink on and off. If this test checks ok continue to next step. (Note: if no injector pulse is present try disconnecting the remainder of injectors and re-test, if a fuel injector is shorted it can shut down the injector driver causing no injector pulse. If injector pulse returns plug injectors electrical connectors in one at a time until the pulse fails and replace that injector)

If this test revealed that there was no pulse but system has power the PCM is not generating a fuel injector trigger. If there is no trigger to the fuel injector it will not allow fuel to enter into the engine. Some of the most popular reasons that can cause this condition include a shorted crankshaft angle sensor, shorted camshaft position sensor or shorted PCM. (When a system trouble code scan is performed it does not always catch a crankshaft angle sensor, camshaft position sensor failure).

Tip: try disconnecting all non-essential sensors, example: oxygen sensor, coolant sensor, throttle position sensor, air intake temperature sensor, mass air flow or map sensor and EGR valve pressure differential sensor. Crank the engine over, if the injector pulse returns, one of the sensors is shorted causing the system to not operate. Plug the sensors in one at a time until the injector pulse fails then replace that sensor and reassemble.

If the test reveals that the connector has no power on either side at any time the system power has been disrupted. Some of the most common reasons for this is condition are the main PCM fuse, main PCM power relay and main PCM power feed wire failure. (Some vehicle PCM feed wires are located near the battery and corrosion can stop the voltage feed). If all power sources check out the system ground needs to be checked, this is done by reversing the test light lead and installing it on the positive side of the battery.

Now the test light will illuminate when grounded. Use the test light to check main system grounds to the PCM, most system ground wires are black but to be sure you will need an online auto repair manual. If repairs have recently been made a system ground lead could have been left off of the engine causing the system not to power up, so double check all engine wiring harness grounds.

Common Problems

  • A fuel injector is subject to high temperatures that can cause the injector to short circuit. Each injector has specific ohms of resistance reading that can be tested. This ohms reading can be found in a car repair manual. Temperature will cause readings to vary slightly. Injectors should be tested hot and cold. This is necessary because the heat or cold can create a condition that can short circuit the injector.

  • Moisture can cause the injector connection to short circuit. Always inspect and clean all electrical connections at the fuel injector.

Best Practices

  • When replacing fuel injectors always use top quality replacement parts.
  • Fuel is present when working with injectors so use caution.

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-12)