An engine misfire means one or more cylinders fail to ignite the air-fuel mixture correctly, resulting in reduced engine performance and a rough, uneven ride.
Common causes include:
- Ignition system problems: Worn spark plugs or spark plug wires, or a failing ignition coil or coil pack, can prevent the fuel from igniting properly.
- Fuel-related issues: Clogged fuel injectors, dirty fuel filters, failing fuel pumps, low-quality fuel, problems in the fuel line, or faulty fuel pressure regulators can disrupt the air-fuel mix.
- Air intake and vacuum problems: Vacuum leaks, clogged air filters, or malfunctioning mass airflow (MAF) sensors can throw off the fuel mixture.
- Mechanical faults: Low cylinder compression, bent pushrods, timing chain or belt problems, or carbon buildup in valves or combustion chambers can stop cylinders from firing correctly.
- Faulty exhaust system: Malfunctioning oxygen sensors, clogged catalytic converters, or faulty EGR valves can interfere with the combustion process.
- Electrical or computer issues: A faulty engine control unit (ECU) can mismanage fuel delivery or ignition timing, causing cylinders to misfire.
Signs of an engine misfire:
- Uneven or shaky idling at a stop
- Hesitation or power loss during acceleration
- Engine or vehicle vibrations
- Unusual sounds such as popping, banging, or chuffing
- Reduced fuel economy
- Check engine light, either flashing or steady
What to do if your engine is misfiring:
- If the check engine light flashes, stop driving immediately and call a mobile mechanic or tow the car to a repair shop.
- If the light is steady, you can drive carefully to a shop, but get it checked as soon as possible. In the long run, leaving the issue unchecked can lead to severe engine damage.
- Don’t delay consulting a qualified mechanic to diagnose the exact cause and perform necessary repairs.
Want to know more?
In this guide, we’ll dig deeper into the common causes of engine misfires, show you how to spot them, and explain what you can do to avoid expensive repairs.
This Article Contains:
- What Is an Engine Misfire?
- What Causes Engine Misfires?
- What Happens to Your Car When the Engine Misfires?
- Is it Safe to Drive with a Misfiring Engine?
- How Do You Diagnose an Engine Misfire?
- Is an Engine Misfire Expensive to Fix?
- What Can You Do to Prevent Misfires?
What Is an Engine Misfire?
Proper combustion requires fuel, oxygen, and a spark to come together in the correct order at the right time.
A misfire is when combustion doesn’t take place at the right time, meaning something has thrown the firing order out of sync in the combustion chamber. Your engine then loses power.
Engine misfires can involve more than one cylinder. For example, if you have a 4-cylinder engine (as in most cars) and one misfires, you lose 25% power at that moment.
Let’s take a closer look at what can trigger these issues.
What Causes Engine Misfires?
The potential cause of an engine misfire can stem from several directions. We’ll break them down into six groups:
1. Ignition System Problems
When it comes to engine misfiring, these three ignition components are a common cause:
- Spark plug: A bad spark plug is your best initial suspect. A faulty spark plug that is black with carbon deposits or wet from unburned fuel is a sign of incomplete combustion. It can mean that the spark plug isn’t firing at maximum strength.
- Spark plug wire: A faulty spark plug wire means a spark plug might not receive enough current for a full spark.
- Ignition coil or coil pack: A faulty ignition coil or coil pack won’t deliver enough current to ignite the needed spark for combustion.
2. Fuel Issues
Misfires can occur when the fuel delivery system fails to supply the correct amount or quality of fuel for proper combustion.
Here are some common causes:
- Clogged fuel injectors: Block fuel flow to an engine cylinder, causing uneven combustion.
- Clogged fuel filters: Reduce fuel flow in the lines, preventing cylinders from getting enough fuel for proper combustion.
- Damaged fuel pumps: Reduce fuel pressure, making it harder for cylinders to fire efficiently.
- Contaminated or low-quality fuel: Prevent the air-fuel mixture from burning completely, resulting in incomplete combustion.
- Faulty fuel pressure regulators: Throw off the air-fuel ratio, resulting in incomplete combustion.
Unlike other misfires, a misfire from fuel system issues tends to happen when you’re idling instead of driving.
3. Air Intake and Vacuum Problems
Restricted airflow or faulty sensors can upset the balance of the air-fuel mixture, causing cylinders to misfire. These problems often include:
- Vacuum leak: Creates an imbalance in the air-fuel mixture. The leak may also distort sensor readings delivered to the engine control unit (ECU), causing the ECU to miscalculate the air-fuel mixture ratio.
- Faulty mass airflow (MAF) sensor: Sends inaccurate data to the ECU, resulting in an improper air-fuel mixture that causes misfires.
- Clogged air filter: Restricts airflow to the engine, creating a rich fuel mixture that causes the cylinders to misfire.
4. Mechanical Faults
Wear or damage inside the engine can prevent cylinders from compressing or burning fuel properly, leading to misfires. Common mechanical issues are:
- Low compression in one or more cylinders: Worn piston rings, cracked or warped cylinder heads, or bent valves let the air-fuel mixture escape, lowering pressure and causing misfires.
- Bent pushrods or collapsed lifters: Prevent valves from opening or closing fully, disrupting airflow and causing cylinders to misfire.
- Timing chain or belt problems: Make the valves open or close at the wrong time, causing the engine to misfire.
- Carbon buildup on valves or in combustion chambers: Blocks airflow and fuel burn, preventing the cylinder from firing properly.
5. Faulty Exhaust System
The following problems with the exhaust system can disrupt combustion and cause misfires:
- Malfunctioning oxygen (O2) sensors: Send wrong signals to the engine computer, making the air-fuel mixture too rich or too lean, which causes misfires.
- Clogged catalytic converter: Restricts exhaust flow, creating backpressure that disrupts combustion and leads to misfires.
- Malfunctioning EGR (exhaust gas recirculation) valve: A stuck-open valve or blocked passages alters the amount of exhaust gas returned to the cylinders, resulting in rough idling and misfires.
6. Malfunctioning Engine Control Unit (ECU)
The engine control unit (ECU) manages fuel delivery and ignition timing. If it experiences software glitches, misreads sensor data, suffers internal component failure, or receives unstable power, it can send incorrect signals that cause cylinders to misfire.
No matter what causes it, a misfire tends to feel the same — because combustion isn’t happening the way it should.
What Happens to Your Car When the Engine Misfires?
If your engine develops a misfire, here are some symptoms you’ll experience:
- Hesitation during acceleration: Your car may have trouble accelerating — when it does, it shudders and shakes.
- Intermittent power loss: Your engine may experience stuttering or loss of power intermittently, which can affect engine performance.
- Rough idle: At idle, your car may vibrate more than usual, and you may notice an uneven RPM.
- Odd sounds: The engine may produce popping or coughing sounds.
- Black smoke: Emissions may increase as unburned fuel exits the engine, showing as black smoke. The uncombusted gasoline could damage your catalytic converter.
- Reduced fuel economy: As a misfire implies incomplete combustion, some fuel will go unburnt, affecting fuel economy.
- Lit check engine light: A misfire usually triggers the check engine light on your dashboard. A flashing check engine light is more severe.
A severe misfire will have more prominent symptoms, so you’ll be hard-pressed to ignore them. However, milder misfires could be subtle and intermittent. For instance, the only indicator might be an illuminated check engine light — which is why you should never ignore this light, tempting as it may be.
While a single-cylinder misfire may not create significant mechanical issues, if left unresolved, it could affect more cylinders, ultimately causing your engine to cease functioning.
Is it Safe to Drive with a Misfiring Engine?
The short answer is no.
To elaborate, an active misfire, where the check engine light flashes continuously, is bad. It can damage your (costly) catalytic converter, among other things.
A solidly lit check engine light for a previous or intermittent misfire is less severe but still requires checking. Misfires reduce engine power, which is dangerous if it happens when you’re passing another car or trying to avoid an object. You can’t brush off or ignore the problem, as it increases the risk of engine damage in the long run.
So, let’s go over the misfire diagnosis next.
How Do You Diagnose an Engine Misfire?
Diagnosing involves a process of elimination. However, engine misfires can be frustrating to diagnose and fix due to the numerous potential causes.
NOTE: If you’re unfamiliar with engine parts, it’s best to let an auto repair professional deal with an engine misfire.
Root causes typically lie in the ignition system or a clogged fuel injector, so we’ll address the general steps to diagnose these:
- Check for trouble codes: Extract the OBD trouble codes stored in the ECU using a scan tool. This will help identify which cylinder(s) misfired and other possible issues that may have contributed. For instance, trouble code P0302 indicates a misfire in cylinder #2. Typical codes encountered with a misfire:
- P0300: Random misfire
- P0301-P0312: For misfires in cylinders 1 to 12
- P0200: Fuel injector issues
- P0100-P0104: Mass airflow sensor issues
- P0171–P0172: For lean/rich air-fuel mixture
- Check spark plugs: If there are no OBD codes to refer to, checking the condition of each spark plug is a good place to start. If OBD codes are available, a trouble code should indicate the misfiring cylinder. Swap the spark plug from this cylinder with another, and see if the cylinder misfire moves — the error code should follow a bad spark plug. If there are no faulty spark plugs, proceed to the next step.
- Check spark plug wires: If the vehicle uses spark plug wires, test the wire resistance. Check for any cracks in the spark plug wire sheath and test for spark jumps due to electrical leakage. If the spark plug wires are sound, check the ignition coils.
- Check ignition coils: With a multimeter, test the resistance of the primary and secondary coil circuits. The required values are in the owner’s manual. Swap coils to see if the misfire returns in the same engine cylinder. If the ignition coils are fine, the next step is to check the fuel injectors.
- Check fuel injectors: Fuel rails usually hold each fuel injector in place, so be careful when pulling them off to avoid damaging the injectors or the O-rings. Test the resistance of the fuel injectors (refer to the owner’s manual for required values) and check for physical damage or clogging that can affect fuel delivery to the misfiring cylinder.
- Perform a compression test: If the ignition system is fine and the fuel injectors are sound, do a compression test. Compression issues could mean costly repairs, such as a head gasket replacement. At this point, it’s advisable to get professional help for a more advanced diagnosis.
After identifying the problem, the next step is to consider engine repair.
Is an Engine Misfire Expensive to Fix?
The cost of an engine repair for misfires depends on the cause. Your vehicle’s make and model will also affect the cost. While components can be easy to replace, getting to the repair location may be difficult, which drives up labor costs.
Here are example replacement costs, including labor:
- Spark plugs: $50 and $750
- Spark plug wires: $190 and $229
- Ignition coil: $85 and $470
- Fuel injector: $335 to $2,000
- Vacuum leak: $150 – $1,500
What Can You Do to Prevent Misfires?
Here are some tips to help prevent engine misfires:
- Check and replace worn spark plugs, plug wires, and ignition coils at the manufacturer’s recommended intervals.
- Use high-quality fuel. Lower-quality fuels can leave deposits that lead to clogged injectors.
- Do periodic fuel system cleaning. This helps to keep the fuel injectors clean for proper functioning.
- Keep up with engine maintenance, including regular oil changes, air and fuel filter replacements, and ensure the cooling system works properly.
Resolve Engine Misfires Quickly and Efficiently
Engine misfires can result from problems in the ignition, fuel system, air intake, mechanical components, or electrical faults. They often show signs such as rough idling, sluggish acceleration, engine vibrations, or a flashing check engine light.
Ignoring a misfire can cause critical engine damage and costly repairs. However, diagnosing misfires can be complicated.
If you need help, AutoNation Mobile Service is just a few clicks away.
We’re a mobile auto repair and maintenance solution available seven days a week. We offer a 12-month, 12,000-mile warranty on all vehicle maintenance and repairs, and the best part is that we can fix your misfire right from your driveway.
Contact us, and we’ll send our expert mechanics to lend you a hand.