Does your car struggle and shake when you accelerate?
And you notice your check engine light pops on?
That’s a sign something’s off with combustion, and you could have an engine misfire.
Engine misfiring is a serious problem. It creates a mechanical strain on your engine parts, and if you don’t remedy it quickly, the problem can grow to much larger (and expensive) proportions.
Let’s dive deeper into engine misfires and what you can do about them.
This Article Contains
- What Is an Engine Misfire?
- How Does a Misfire Affect Your Car?
- What Causes 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?
For proper combustion to occur, there should be fuel to burn, oxygen to sustain the burning, and a spark to light it all up — all timed correctly.
A misfire is when combustion doesn’t take place correctly, meaning something has caused those elements to become out of sync in the combustion chamber. Your engine then loses power.
Engine misfires can involve more than one cylinder. To put that into perspective, if you have a 4-cylinder engine (as in most cars) and one misfires, you lose 25% power at that moment.
Let’s take this further and see what a misfire does to your car.
How Does a Misfire Affect Your Car?
If your engine develops a misfire, here are some symptoms you’ll experience:
- Sluggish acceleration: Your car may have trouble accelerating; when it does, it shudders and shakes.
- Intermittent power loss: Your engine may stutter or lose power intermittently, affecting engine performance.
- Rough idle: At idle, your car might vibrate more than usual and you might notice an uneven RPM.
- Odd sounds: There may be popping or coughing sounds from the engine.
- Black smoke: Emissions may increase as unburned fuel exits the engine, showing as black smoke. The uncombusted gasoline could potentially damage your catalytic converter.
- Reduced fuel efficiency: As a misfire implies incomplete combustion, some fuel will go unburnt, affecting fuel efficiency.
- 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, and 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, more cylinders could get affected until your engine ceases to function.
Now that you know the signs of a misfire, let’s explore the likely root causes.
What Causes Engine Misfires?
The root cause of an engine misfire can stem from several directions. We’ll break them down into three groups:
1. Ignition System Issues
When it comes to engine misfiring, these are the three ignition components likely to cause issues:
- 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 are both signs 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: A faulty ignition coil won’t deliver enough current to ignite the needed spark for combustion.
2. Fuel Delivery System Issues
Misfires can occur if not enough fuel reaches the combustion chamber.
Here’s how this can happen:
- A clogged fuel injector may block fuel flow to an engine cylinder
- A clogged fuel filter reduces fuel flow in fuel lines
- A damaged fuel pump reduces fuel pressure to the cylinders
Unlike other misfires, a misfire from fuel system issues tends to happen when you’re idling instead of driving.
3. Mechanical Issues
Misfires caused by mechanical issues are the hardest to troubleshoot and potentially cause the most damage if unresolved. Here are some examples:
- Vacuum leak: A vacuum leak may cause an imbalance in the air-fuel mixture. The leak may distort sensor readings delivered to the engine control unit (ECU), causing the ECU to not calculate the correct air-fuel mixture ratio.
- Emission system issues: A clogged catalytic converter, bad oxygen sensor, damaged EGR valve, or faulty PCV valve can indirectly affect the ratio of the air-fuel mixture.
- Other mechanical problems: Broken piston rings, worn valve guides, timing belt slips, and gasket leaks, can all contribute to a misfire by changing the conditions that would have otherwise created proper combustion. For example, a timing belt slip will impact ignition timing in the cylinders.
Considering all this, you may wonder if it’s wise to keep driving with a misfire.
Is it Safe to Drive with a Misfiring Engine?
The short answer is no.
To expand on this a little more — an active misfire, where the check engine light flashes continuously, is bad. It can damage your (very expensive) 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. They can’t be brushed off as they won’t go away, and the longer you ignore the problem, the likelier you risk severe engine damage.
So, let’s go over 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 because of the many 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:
1. 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 example, 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
2. 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.
3. 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.
4. 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.
5. 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 (check the owner’s manual for required values) and check for physical damage or clogging that can affect fuel delivery to the misfiring cylinder.
6. 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 like a head gasket replacement. At this point, it’s advisable to get professional help for a more advanced diagnosis.
Once the problem has been identified, 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 (not including labor):
- Spark plugs: $2-$100 for individual plugs, depending on the type (standard, platinum, iridium, etc) and engine cylinder count
- Spark plug wires: $30 to $300 or more per set
- Ignition coil: Around $15 to $300 onwards for an individual coil
- Fuel injector: $14 to well over $400 for an individual injector
- Vacuum leak: $150 – $1000, depending on where the leak occurs
Repair costs for an engine misfire vary widely. So, it’s better to avoid it from happening in the first place.
What Can You Do to Prevent Misfires?
Here are some tips to help prevent engine misfires:
- Check and replace worn ignition components. This includes checking the spark plugs, plug wires, and ignition coils, and replacing them 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, like a regular oil change, air and fuel filter replacement, and ensure the cooling system works properly.
Resolve Engine Misfires Quickly and Efficiently
Ignoring misfires can lead to severe engine damage and steep repair costs. Addressing misfires promptly is vital to ensuring optimum engine performance and maintaining longevity.
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 we can fix your misfire right from your driveway.
Contact us, and we’ll send our expert mechanics to lend you a hand.