How to Tell if You Have a Bad Fuel Injector

Alex Palmeri
March 16, 2018

Fuel injectors can fail either mechanically or electronically, and in some cases you’ll see warning signs that something is wrong. 

1. Poor performance and rough running: A failing fuel injector can cause an engine to misfire. Because fuel injection systems use one injector per cylinder, your engine may not completely shut down if only one injector isn’t working. Instead, you will notice a lack of performance, hesitation on acceleration, and shaking or vibration felt through the steering wheel and floorboards. 

2. Your check engine light is on: Misfires are monitored by the ECU, and in most cases, the check engine lightwill illuminate to warn you of an issue. If the check engine light is flashing, pull over to a safe spot and shut the engine off, as running the car this way can cause damage to the catalytic converter. In most states, an illuminated check engine light results in an automatic failure of the emissions inspection.

3. Hard starting and poor idle: In some cases, fuel injectors can lose the ability to seal when the internal valve is closed. When this happens, fuel can leak into the cylinder and make for a rich condition — where there is too much fuel in the combustion chamber — when you turn the key. This is most noticeable after the car has sat overnight. The first start of the day can take longer, and the engine can run rough for the first few seconds. This should be taken seriously. In extreme conditions, fuel can fill a cylinder and prevent compression, ruining the engine.

4. Poor fuel economy: When a fuel injector nozzle becomes dirty or clogged, it can no longer fully atomize the fuel. Without a fine mist of fuel, the combustion process becomes inefficient and fuel is wasted. This will drastically reduce fuel economy and cost you more at the pump. 

5. Black smoke and a foul odor from the tailpipe: Because the ECU is designed to calculate fuel requirements based on a fully functioning injector, a faulty injector may cause your engine to run rich. A rich condition can cause black smoke to come out of your tailpipe. It may also smell like rotten eggs or sulfur.

6. The smell of raw fuel: If an injector body breaks or an injector O-ring fails, raw fuel can leak into the engine compartment. If you suspect a fuel leak, don’t attempt to start your car, as the fuel could ignite on the hot engine or exhaust. Instead, call a towing service to take your car to a local repair shop. 

Get it diagnosed by a professional

How a fuel injector works

Long gone are the days of temperamental and inefficient carburetors delivering fuel to run your engine. Cars now use sophisticated fuel injection systems that drastically lower emissions and increase fuel economy. 

The heart of any fuel injection system is the fuel injector, and while they are simple and reliable devices, they aren’t immune to failure. 

A fuel injector is an electronically controlled valve that delivers fuel to the engine in the form of a fine mist. The injector is supplied with pressurized fuel from the fuel pump. It’s controlled by the engine control unit (ECU) — part of the car's computer system — and is capable of opening and closing many times per second. 

Most current fuel injection systems have one injector per cylinder, and each can be controlled separately. There are two main types of fuel injection systems: port fuel injection and direct fuel injection. Port fuel injectors are installed on the intake manifold and require less fuel pressure. Direct fuel injectors are installed into the cylinder head and spray fuel directly into the cylinder. They require much higher fuel pressure. Direct injectors make for a more efficient combustion process that helps with lowering emissions and increasing fuel economy.

» MORE: Get an estimate for your fuel injector replacement

Fuel injector repair advice

Although fuel injectors have evolved and become very reliable, it’s still possible for one to fail. A fuel injector is not a maintenance item, so there is no specific interval for inspection or replacement. If you suspect a faulty fuel injector, it’s best to have a complete diagnosis performed at a repair shop. In some cases, a faulty sensor or an issue with the ignition system can mimic a bad fuel injector. 

A technician will perform tests to determine if a fuel injector has failed. This will include electrical testing at the fuel injector harness, a complete scan of the engine’s computer and in some cases a flow or leak-down test. If one fuel injector is found to be faulty, some shops will recommend you replace the entire set, depending on the mileage and condition of your vehicle. 

The cost of replacing fuel injectors can vary greatly, depending on the make and model of your vehicle. In most cases, the replacement procedure is very straightforward and not usually labor intensive, but there are exceptions when dealing with high-performance or complex engines. Direct injection systems can be very expensive to repair due to high labor and parts costs.

Should I have my fuel injectors cleaned?

Many shops offer fuel injector cleaning services that do not require the removal of the fuel injectors. This is typically sold as maintenance, but will rarely be sufficient to repair an issue with a fuel injector. It’s worth noting that most automakers do not include this service in their normally scheduled maintenance, and whether it even works is debatable. 

You might also see fuel injection cleaners being sold at gas stations. But if you use gas branded as Top Tier, these additives won’t be necessary or likely even helpful.

If your fuel injectors are hard to find or expensive, there are companies that offer rebuilding and cleaning services. Although this requires the removal and delivery of the injectors to the company — a potentially time-consuming process — results can be on par with replacing the injectors with new parts.

If you do it yourself

Fuel injector replacement should be left to a professional in almost all cases. The risk of fire is very real, and this repair can turn complicated. A good amount of labor goes into removing the injectors, intake manifold and other components, and attempting it without the proper tools or knowledge could lead to injury or vehicle damage.

Standard port fuel injectors are easier to replace than direct ones. Proper diagnosis is key. If you have a faulty injector, pay special attention to the work instructions, use high-quality, manufacturer-recommended parts, and make sure to replace the rubber O-ring seals at both ends of the injectors.

The replacement of direct fuel injectors should be left to the professionals as the parts are under a great deal of pressure and can be dangerous to work on.


Alex Palmeri

About the Author

Alex Palmeri worked nine years as a master technician at Mercedes-Benz of Chicago and is currently the foreman at a large fleet garage. He writes about automotive news, maintenance and racing, and runs a YouTube channel called Legit Street Cars.

1 User Comment

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By , September 28, 2010
Fuel pump is located INSIDE the fuel tank. It's accessed through the top of the fuel tank. The fuel tank must be removed to access the fuel pump.

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