Step by step guide on how to test an automotive engine fuel injector, this information
pertains to electronic fuel injected vehicles, not direct injection.
A fuel injector has two main sections, the electrical portion which activates
the valve assembly and allows fuel to enter the engine. This guide will check both
aspects along with the trigger and power circuits supplied by the computer.
Difficulty Scale: 4 of 10
Begin with the vehicle on level ground with the engine off and the parking brake
set. Fuel maybe present during these tests, usual fire precautions are necessary.
Use protective gloves and eye wear for safety.
- Start the engine and allow to idle, using
a long metal rod like a screwdriver touch the end of the rod to the injector, gently
lay an ear to the opposite end of the rod or handle to observe an audible clicking
sound, repeat this procedure across all injectors, this can help pinpoint the failing
injector. (Note: This sometimes can be difficult because of internal engine noise.)
Audible Fuel Injector
Step 2 - With the ignition key turned to the "ON"
position without starting the engine, connect a test light to the negative side
of the battery and probe the injector wires, one of the wires should illuminate
the test light. If a volt meter is used the multi-meter (voltmeter) should bounce
from 0 to 12 volts. If system power has been disrupted suspect an injector or computer
fuse, broken or shorted wire or bad computer, (Note: Seal test points with a small
dab of silicone rubber once testing is complete) this test can also be performed
with the connector removed as well. Some vehicle computer feed wires are located
near the battery, corrosion can stop the voltage feed. If all power sources check
out the computer system ground needs to be checked, this is done by reversing the
test light lead by installing it on the positive side of the battery which will
cause test light to illuminate when grounded, most system ground wires are black
but to be sure an online auto repair manual is needed. 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, double check all engine wiring harness grounds.
Step 3 - While the test light clamp is on the positive
side of the battery, move the test light probe to the opposite wire in the injector
connector, have a helper start or crank the engine while observing the light, when
cranking or under load the light will flash brightly, when idling or easy throttle
the light will flash more dimly. This trigger circuit for the injector is supplied
by the computer and follows the throttle and engine demand. If these test revealed
that there was no pulse and the computer has power and not generating a fuel injector
trigger signal suspect a crank angle sensor, (engine not running) or shorted computer
or sensor. A shorted injector can hinder the injector driver operation for additional
injectors, unplug all injectors and re-test pulse trigger, if pulse returns, plug
the injectors back in one at a time until the pulse fails, replace the shorted injector.
If tests check okay, proceed to next step.
Test Light Probe Injector Wiring
Step 4 - A voltmeter is used in this test, adjust
the meter to the ohms setting, these next steps can be performed with the injector
installed or uninstalled.
Test Light Flash
Step 5 - Identify the injector to be tested and
inspect for defects or bad connections.
Multi Volt Meter Set To Ohms
Step 6 - Connect the meter to either terminal of
the injector (non specific) to obtain a reading. This test gives a baseline ohm
reading of all injectors to be tested, a service manual also has this information.
(Note: Injectors should be tested cold unless otherwise specified, temperature
will change readings.) If the injector tests produce high resistance or an open
circuit, the injector needs replacement. Most injector readings will range
between 11 and 24 ohms.
Step 7 - To test the fuel injector valve while
installed, a fuel gauge is connected to the fuel system via a fuel rail port or
fuel filter. Turn the ignition switch to the "ON" position without starting the
engine, this will pressurize the fuel system, then pinch off the inlet and outlet
rubber hoses to hold pressure in the fuel rail. (Note: Some fuel systems are designed
with an inlet hose only.)
Probe Injector Terminals
Step 8 - Then, remove the electrical connector
from the injector in question for inspection, clean corrosion and rust.
Fuel Pressure Gauge
Step 9 - Removing the injector connector will expose
the terminals, carefully attach a 12 volt power (power and ground) source to the
injector. An audible click from the valve opening while simultaneously observing
an immediate drop of fuel pressure at the gauge confirms proper operation of the
Injector Connector Removed
Step 10 - For a final check, remove the injector
and attach pressured air to the injector inlet, then use a 12 volt power and ground
source to energize the injector, compressed air should be released from the outlet
valve. (Note: Traces of fuel will be expelled as well, use typical fire precautions.)
When a fuel injector malfunctions it can allow an excess or minimal amount of
fuel into the engine causing engine run-ability problems. The fuel injector is designed
with a pintle valve that allows fuel to flow as a spray pattern while the computer
provides opening signals in prescribed amounts regulating volume.
A common misconception about cylinder misfires is they cause the engine to run
rich, in the absence of combustion the cylinder contains 14 parts
air to 1 part fuel (14:1) which causes a lean condition.
When a system trouble code scan is performed detection of a sensor failure is
not guaranteed, a crankshaft position sensor for example, is difficult for the computer
If injector pulse is not detected, and computer system is powered up with all
obvious sensors and wiring checked or replaced, 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 and see if the injector pulse returns, one of these sensors could be shorted
causing the system to not operate.
Tools and Supplies Needed
- Test light or voltmeter
- Small standard screw driver
- Wrench set
- Socket set
- Protective eye wear and gloves
- Shop towels
A fuel injector is subject to high temperatures that can cause failure. Each
injector has specific ohms resistance reading that is used for testing.
Moisture can cause the injector connection to short circuit.
Article first published 2016-02-04