If your engine is running rough, there may be an issue with the vehicle’s fuel delivery system. Having a faulty injector can cause one of your cylinders to not ignite fully, which will throw the balance of the engine off at all speeds. This ruins fuel economy since all of the fuel may not be combusting, and you have to press harder on the gas pedal to get the car to move.
Fuel injectors are a particular type of solenoid that are made to actuate their pistons very quickly. This allows the injector to deliver precise amounts of fuel into the cylinder, even while the engine is spinning at higher RPMs. Over the life of the car, the injectors fire millions of times and eventually can wear out or clog up, preventing the engine from operating correctly.
This guide will focus on making sure the injectors are receiving the correct amount of power and that the injector itself doesn’t have too much resistance. It is possible for the injector to cause issues even if it is receiving the correct voltage. They can clog up, which will decrease the spray inside the cylinder. This, in turn, causes the fuel to not burn completely and will create a misfire.
Part 1 of 2: Testing injector resistance
Digital Volt-Ohmmeter (DVOM) or multimeter with ohms setting
Note: Some engines have plastic panels that need to be removed before you can access the injectors. They are typically secured by bolts and can be removed with a basic socket set including an extension.
Step 1: Make sure the key is off. You don’t need power for this test.
Step 2: Remove injector wiring harness. There may be a slide lock that you need to move before you can press down on the tabs to remove the wiring harness.
Step 3: Set DVOM to measure ohms. Set the multimeter to measure ohms. Set it to the lowest range if the meter doesn’t autorange.
Step 4: Test resistance with DVOM. Place the meter leads on the prongs inside the connector, making sure that they are not touching each other.
High impedance injectors are the most common found on cars these days. They will range from 12 to 17 ohms.
Low impedance injectors are found on high performance and larger injectors. They have much lower resistance, typically around 2-5 ohms.
Step 5: Repeat with all injectors. They should all have a resistance within half of an ohm from each other.
Any major difference and that injector should be inspected to make sure that it is firing correctly.
Tip: You can find the correct resistance for your injectors by searching online or in your vehicle’s repair guide.
Part 2 of 2: Testing injector wiring
Step 1: Turn the car on. Turn the key to the second (ON) position for this test. You want battery power flowing, but don’t want the engine running.
Step 2: Set up DVOM to measure DC voltage. Use the lowest range possible if the meter doesn’t auto range.
Step 3: Touch the negative lead of the DVOM to a ground source. The car’s frame is connected to ground, so look for an unpainted piece of the frame under the hood.
- Tip: Some DVOMs have alligator clips so that you don’t have to hold the lead. This frees up your hands to focus on getting the positive lead in the correct spot.
Step 4: Place positive lead on the wiring harness terminal. The wiring harness will have two terminals that the prongs on the injector insert into.
One will be connected to ground and will read 0 volts. The other should read around 12 volts.
Step 5: Repeat with all injector wiring harnesses. Leave the ground lead in position and test all of your injector wiring harnesses.
They should all be around 12 volts. A lower reading means there is excess resistance in the wire somewhere.
Hopefully, these tests have allowed you to find the issue with your fuel injectors; but, as mentioned before, it may not be an electrical problem that is causing the injector to malfunction. The next step if the injector resistance is normal would be to remove the injector and test the spray pattern it produces on an injector tester. If you are experiencing any difficulty with testing your injectors, our certified technicians here at YourMechanic would be able to assist you with diagnosing the problem and replacing any bad fuel injectors.