(Updated on January 21, 2021)
Modern internal combustion engines operate with a precisely tuned balance of air and fuel. Fuel is delivered into the combustion chamber in very specific quantities through components known as fuel injectors.
What is a Fuel Injector?
A fuel injector is an electronically controlled device which sprays a fine mist of fuel under high pressure. The process of turning liquid fuel into this fine mist is called atomization.
Fuel injectors work a bit like tiny, electronically controlled squirt bottles. Fuel injectors spray fuel at a pressure anywhere from 30 psi (2 bar) up to thousands of psi (hundreds of bar), depending on the engine and injection type.
Fuel injectors may be located either at the throttle body (throttle body injection), in the intake manifold (port injection), or inside the combustion chamber itself (direct injection).
Some new vehicles have fuel injectors both at the intake port and inside the combustion chamber. This is called dual injection. Toyota is one manufacturer that uses dual injected engines.
7 Symptoms of a Bad Fuel Injector
An injector can fail in several ways. Different failures will yield different symptoms on this list, with a bit of possible overlap.
If your injector is clogged, it may have a hard time atomizing fuel. A clogged injector could lead to a lower flow rate, liquid fuel dripping into the combustion chamber, or the injector failing to inject fuel entirely.
An injector may also leak, which would cause liquid fuel to enter the combustion chamber when the injector is supposed to be closed.
An electrical issue could prevent the injector from firing altogether.
In all of these cases, you are likely to experience one or more of these symptoms.
Related: Best Fuel Injector Cleaners
1) Engine Misfires/Rough Idle
An engine misfire could be caused by many failing components, but one of those components is a bad fuel injector.
Misfires may occur when an injector is clogged, leaking, or not firing. If one injector has failed, it would feel as if one cylinder was not making any power. This would likely manifest as a rough or uneven idle.
If you suspect an injector isn’t working, there is an old trick that may help you determine if all injectors are operating normally.
Assuming you can reach your injectors, grab a flat head screwdriver. Put the handle to your ear, and the tip of the screwdriver on top of one injector. While the engine is running, you should hear a rhythmic click being transferred to the screwdriver. This click means the injector is probably firing.
If you notice that only one cylinder is missing that click, chances are good that you have a bad injector. You will probably want to test the electrical connection at this injector as well.
The factory service manual has specifications on expected voltages for each pin, so you can verify that it really is the injector and not the wiring that has a problem.
Another quick test you can do is to swap the suspected bad injector with one you heard clicking. If the silence follows the bad injector, you know the root cause is the injector itself. If the original cylinder remains silent, start troubleshooting the electrical system before replacing the injectors.
2) Check Engine Light
Engine misfires are often accompanied with check engine lights, but you may see a check engine light for any number of reasons.
Before you get too deep into troubleshooting or asking for help, scan the code to see what the ECU thinks the problem is. This will narrow down the problem considerably. Codes can often be scanned for free at your local auto parts store.
If you see a misfire code, swap your spark plugs and spark plug wires (or ignition coils) to different cylinders. When doing any sort of diagnostics or troubleshooting, it’s a good idea to start with the cheapest and simplest steps first; spark plugs are easier to swap than fuel injectors on most cars.
If you get a new check engine light for a misfire on a different cylinder, change the spark plugs. Repeat this process again with the fuel injectors. You may have to drive the car for a while before a new check engine light pops up.
3) Trouble Starting
If you’re having trouble starting your car, a leaky fuel injector may be flooding one cylinder. One or more failed fuel injectors may also make it more difficult to start your vehicle because those cylinders won’t be making any power.
Although this is a less common issue, it is still possible to have difficulties starting your car when you have a bad fuel injector.
4) Poor Fuel Economy
A leaky fuel injector may be dripping fuel into the engine even on the wrong stroke in the combustion cycle. This is wasted fuel, as it isn’t contributing to making power in any sort of way.
You are likely to notice a significant reduction in fuel economy and will probably see another symptom on this list if a fuel injector is leaking.
5) Sluggish Performance
If one or more cylinders are not achieving their target air fuel ratio, chances are good that your engine isn’t going to make as much power as it did from the factory.
If you experience sluggish performance, pay attention to other symptoms on this list, such as poor fuel economy and a rough idle. These may indicate an issue with the injectors.
6) Engine Surges
Intermittent failures are the hardest types of failures to diagnose. If a fuel injector is partially blocked, you may notice surges in engine power as one injector works properly, then cuts out.
Fuel injectors can be cleaned and also tested using special equipment. If you’re interested in doing this, you can do a quick search online to see what companies offer an injector cleaning or testing service.
7) Fuel Leak
If you notice a fuel smell in your engine bay or find a wet spot around your fuel rail, chances are you have a leaky injector. Port injectors have o-rings that seal them against the intake manifold. As these o-rings wear out, fuel may leak out of the injector externally, without making it into the engine.