The Short Answer
The main symptoms of bad fuel injectors are misfires under acceleration, rough idling or stalling, fuel or black residue around the injectors, poor fuel economy, hard starting, black exhaust smoke and the engine management light. Confirm the fault by reading the codes with an OBD2 scanner, and try a fuel injector cleaner before replacing parts.
A failing fuel injector rarely keeps itself to itself. Once one starts to stick, clog or leak, the engine runs on the wrong mixture and the knock-on effects show up in the way the car idles, pulls and starts, as well as in your fuel bills.
Injectors fail in three main ways. They break mechanically, they clog with dirt and deposits, or they leak either internally or around their seals. The symptoms below are the ones we look for first when an engine starts behaving oddly.
The Main Symptoms Of Bad Fuel Injectors
- Misfires under acceleration
- Rough idling or stalling
- Fuel or black residue around the injectors
- Higher fuel consumption
- Difficult starting, especially when hot
- Black smoke from the exhaust
- A failed MOT emissions test
- The engine management light
Engine Misfires
When an injector can't deliver the fuel a cylinder asks for, the mixture in that cylinder goes lean and it misfires. You feel it as a stutter or shudder through the car, and it gets worse the harder you accelerate, as that's when the engine needs the most fuel.
Rough Idling And Stalling
The same wrong mixture makes the engine hunt and shake at idle. If it stalls as well, too little fuel is reaching the combustion chamber, and we wouldn't drive the car far until it's been checked.
Leaking Fuel Or Black Residue
This is the one symptom you can spot with the engine off. Look along the fuel rail and around the base of each injector for wet fuel or a dark crust. A failed O-ring is the commonest cause on a petrol engine, though the injector body or nozzle can crack and leak too.
On a diesel, a leaking sealing washer lets combustion gas and fuel escape around the injector, and the mess quickly bakes into the thick tar you can see in our photo. An engine degreaser breaks it down far more easily than scraping.
A leak doesn't always mean the injector itself is finished. A new seal or washer can cure it, but have the injector tested before you refit it, so you're not putting a faulty one back in.
Poor Fuel Economy
When an injector under-delivers, the ECU tries to compensate by asking for more fuel, and a leaking injector simply wastes it. Either way, you'll see the miles per gallon fall and the car feel flatter than it should.
Difficult Starting
Faulty injectors can make the engine slow to fire, particularly when it's hot, and in bad cases it won't start at all. If it does catch after a long crank, expect misfires and a lumpy idle to follow.
Black Smoke From The Exhaust
Black smoke means the mixture is too rich, and a leaking or dribbling injector is a common source. It's more obvious on a diesel, where a puff on hard acceleration turns into a cloud when an injector is faulty.
Failed MOT Emissions Test
The MOT includes an exhaust emissions check, and the DVSA's in-service exhaust emission standards set out how testers carry it out. An engine running on clogged or leaking injectors burns fuel badly, and that shows up in the readings. Our guide on reducing car emissions for an MOT covers the other causes worth ruling out.
Engine Management Light
An injector fault will normally bring on the engine management light, though plenty of other faults trigger it too. Plug in an OBD2 scanner and read the stored codes, as injector circuit faults and cylinder-specific misfire codes point you to the culprit.
How To Clean Fuel Injectors
Keeping injectors clean is the cheapest way to avoid all of the above. A fuel injector cleaner goes into the tank before or after you fill up, depending on the product, and mixes with the fuel as it passes through the injectors and into the engine.
We've tested a range of these additives on our own cars. Used regularly, they're a sensible bit of maintenance for very little outlay.
Should You Replace One Injector Or All Of Them?
It depends on the age and mileage of the car. On an older engine where the first injector has just failed, the rest have done the same work and are likely to follow, so replacing the set saves you repeat labour. On a younger engine, one bad injector can simply be bad luck, and swapping just that one is the cheaper fix.
What Does It Cost To Replace A Fuel Injector?
The bill depends on the engine, whether you choose a new or reconditioned injector and how much labour is involved in getting to it. Reconditioned injectors cost less than new ones, and on many diesels the injectors need coding to the ECU after fitting, which adds to the job.
Injectors on some engines also seize in the head, like the one we pulled out below, which can turn a simple swap into a long day. Get a few quotes and ask whether coding and new seals are included.
Our Advice On Bad Fuel Injectors
Injector problems are common on most engines and the symptoms are easy to read once you know them. Before you pay for new parts, try a course of injector cleaner and read the fault codes, as a dirty injector can come back to life where a broken one won't.
Watch Us Do It
Testing Fuel Injector Cleaners
Fuel Injector Questions We Get Asked
How Do I Know Which Fuel Injector Is Bad?
Read the fault codes with an OBD2 scanner first, as a misfire code names the cylinder that is struggling. On many diesels, live data shows injector correction values, and the one furthest from the rest is your suspect. Swapping a suspect injector with its neighbour and seeing whether the misfire moves is the classic home test.
Can You Drive With A Bad Fuel Injector?
You can for a short distance if the car only idles a little rough, but it is not something to put off. A misfiring cylinder can damage the catalytic converter, and a leaking injector is a fire risk if fuel is dripping onto a hot engine. If the car is stalling or you can smell fuel, stop driving it.
Does Fuel Injector Cleaner Actually Work?
Injector cleaner works on deposits, so it helps a clogged or dirty injector and does nothing for one that is broken or leaking at the seals. Used regularly, an additive keeps the nozzles clean and the spray pattern correct. If a course of cleaner makes no difference, the injector needs testing or replacing.
What Causes Fuel Injectors To Fail?
Deposits from fuel are the most common cause, slowly blocking the fine nozzle holes. Contaminated or poor-quality fuel, water in diesel, worn internal parts and failed seals account for most of the rest. Short journeys make deposits worse, as the engine rarely gets hot enough to burn them off.
Will A Bad Injector Fail An MOT?
It can do. The MOT includes an exhaust emissions test, and a clogged or leaking injector affects how cleanly the engine burns its fuel, which raises the readings. Clean the injectors or fix the fault before the test date rather than hoping the readings scrape through, as a fail means a retest and more time off the road.
Is A Leaking Injector Seal Easy To Replace?
On many petrol engines, replacing an injector O-ring is a straightforward job once the fuel rail is off. Diesel injector sealing washers are harder, as the injector can seize in the cylinder head and need specialist pulling tools. Either way, fit new seals every time an injector comes out and torque the clamps correctly.
Dan founded YourCar and has owned well over 50 cars since passing his driving test. His current fleet runs from German classics he has restored from barn-find condition to a load lugger, a daily driver, an Audi R8 and a 911 GT3, and every product on YourCar gets used on cars like these first.