Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.
Engine misfires can catch you off guard. You are driving smoothly down the highway; next thing you know the engine starts to buck and shake.
An engine cylinder misfires when it is unable to efficiently burn the air/fuel mixture in the combustion chamber. This problem leads to all kinds of engine performance issues: The engine jerks and loses power, and fuel consumption and emissions go up.
How Easy Is It to Diagnose a Misfire?
That depends on the root cause of the misfire. A misfire may originate in one or more systems where components have failed or aged. Sometimes misfires are hard to diagnose even for experienced technicians, especially intermittent misfires.
The good news is that most misfires happen because of lack of proper maintenance of common and accessible parts. So you have a chance of diagnosing the problem at home, and even fixing it, if you know how to go about it.
The first thing you want to know, though, is that you should start diagnosing the misfire on your car as soon as possible. Not only because you are wasting fuel and contaminating the environment unnecessarily, but also because your repair costs can go up significantly as time goes by without a repair. For one, your catalytic converter may fail due to unburned fuel.
This guide will help you find those common misfire sources. Before, though, let's take a look at the problem in a little more detail so that you know what kind of information to look for during your diagnostic procedure.
I. What Can Cause a Misfire?
Symptoms to Watch Out for If You Suspect a Misfire
I. How to Save Diagnostic Time with a Scan Tool
II. Things to Do Before Starting an Engine Misfire Diagnosis
Misfire Related DTCs
III. Checking for Lack of Ignition Spark
A Trick To Diagnosing Misfires Quicker
IV. Checking Fuel Supply Problems
CEL or MIL Flashing - A Warning Sign
V. Checking for Vacuum Leaks
VI. What If My Ignition, Fuel and Vacuum Systems Turn Out OK?
I. What Can Cause a Misfire?
As mentioned above, a misfire is the failure of one or more cylinders to fire properly or at all. Misfiring means the combustion process—the igniting of the air-fuel mixture that enters the cylinder—has been upset in some way.
A misfire usually happens because of:
- a component failure in the ignition system (including abnormal ignition timing advance),
- problems in the fuel system,
- vacuum leaks,
- compression drop in one or more cylinders, or
- problems with key valves or sensors the car computer uses to calculate the correct air-fuel ratio.
Misfires due to the absence of spark—as opposed to the absence of fuel—are particularly worrisome because the unburned fuel may find its way into the exhaust system and catalytic converter.
If this happens, the raw fuel will gradually destroy the converter; hence the need to fix the problem as soon as possible. On most modern vehicles, the Check Engine Light (CEL) or Malfunction Indicator Light (MIL) will begin to flash if a misfire threatens to damage your catalytic converter.
Symptoms to Watch Out for If You Suspect a Misfire
|When an engine misfires, you may notice one or more of these symptoms:|
* Engine loses power
* Engine is hard to start
* Fuel consumption increases
* Emissions increase
* Engine produces popping sounds
* Intake or exhaust manifold backfires
* Engine vibrates, jerks or stumbles
* Engine stalls
I. How to Save Diagnostic Time With a Scan Tool
Modern vehicles use highly accurate crankshaft position sensors that can detect crankshaft angle position at any speed. This greatly helps the Powertrain Control Module (PCM--car computer) calculate crankshaft acceleration time. The PCM can detect when a cylinder decelerates (an indication of a misfire), store a Diagnostic Trouble Code (DTC) in memory, and turn on the Check Engine Light. Some car computers detect misfires by measuring electrical activity [ionization] at the spark plug electrodes.
You can retrieve stored Diagnostic Trouble Codes with a code reader (for OBD I systems on pre-1996 vehicle models) or a generic scan tool (for OBD II systems-1996 or newer models; some scan tools can read both OBD I and II).
Depending on your vehicle make and model—and the sophistication of your scan tool—you may get this and other data from your car computer when diagnosing a misfire:
- The cylinder or cylinders where the misfire or misfires were detected.
- Whether the affected cylinder has actually fired since the problem was detected.
- Number of misfires within a recent number of cycles.
- Engine RPM when the misfire was detected.
- Misfire history.
All these are important clues that car technicians used to locate the source of misfires.
If you don't own a scan tool, though, you can buy a relatively inexpensive, aftermarket scanner you can use for this diagnostic. A scan tool is a good investment because you can use it for other small repairs and maintenance tasks in the future.
Alternatively, if you don't want to buy a scan tool now, remember that some auto parts stores in your area may retrieve any stored computer codes in your vehicle without charge.
Not all misfires are directly translated into DTCs, especially intermittent ones. Yet, it's a good idea to scan your computer memory because other potential stored codes may help you in diagnosing and fixing your engine misfires (see the box below on Misfire-Related DTCs).
Whether or not you have access to a scan tool, this guide gives you important tips you can use to troubleshoot and fix the misfire in your vehicle.
II. Things to Do Before Starting an Engine Misfire Diagnosis
Before you start diagnosing the problem, try to gather as much information about the operating conditions in which the misfire occurs. The information that you gather gives you important evidence to help you pinpoint the cause behind the misfire. Think about the conditions in which the engine misfire occurs, for example:
- Only when the engine is cold
- Only when the engine warms up
- Only when accelerating
- It happens consistently
- Only when it rains
- Engine misfires randomly
Perhaps the misfire began after you, or someone else made a repair or performed some maintenance work (changed spark plugs). Maybe you or someone else left a hose loose or electrical connector loose or unplugged.
The next three sections—Ignition, Fuel, Vacuum—present the systems most likely to be behind the misfire. The section following those gives you a list and a brief explanation of other specific components that can also give you trouble. It is meant to help you dig a bit deeper when necessary.
The Powertrain Control Module (PCM-car computer) can be a valuable source of information for your diagnostic process. If you find one or more of the following Diagnostic Trouble Codes (DTCs), pair it with the information presented in this guide. Even if the code doesn't pinpoint the reason for the misfire, it can hint at the root of the problem.
Here are some common DTCs you may find in your car computer that can help diagnose a misfire:
- P0100-P0104: Mass air flow sensor related codes.
- P0171 or P0172: Lean or rich fuel conditions.
- P0200 code series: Fuel injector circuit malfunctions.
- P0220-P0229: Throttle or pedal position sensor related codes.
- P0300: Multiple, random misfires.
- P0301-P0312: The last two digits point to the cylinder number where a misfire happened.
- P0335-P0339 - Crankshaft position sensor related codes
- P0400: Some codes in this series indicate an EGR system malfunction.
If you have a pre-1996 vehicle, check the manual that comes with your code reader to see the definition of the codes you retrieve from your car computer.
III. Checking for Lack of Ignition Spark
Worn out or bad ignition system components are a common source of misfires. If you haven't inspected or replaced ignition components during the past 3 to 5 years, a worn or failed component in the system may be the cause.
If you retrieve a DTC pointing to a particular cylinder, concentrate on those components related to that cylinder. For example:
- Spark plug,
- spark plug wire,
- coil on plug (COP) component,
- fuel injector
On the other hand, if a DTC points to multiple cylinder misfires, check components that affect all cylinders:
- ignition coil
- distributor cap
- ignition control module
If you are not scanning the car computer but suspect a particular cylinder or multiple cylinders misfiring, you still can use the following tips.
First, verify that you have a spark at the affected cylinder or cylinders. You can use a cheap spark tester tool for this.
In addition, visually examine the spark plugs. Even if only one cylinder misfires, check each spark plug and compare its condition with the rest. Here's what you can do:
- Use the spark plug charts that come with popular aftermarket vehicle repair manuals to help you diagnose common spark plug problems. For example, a fouled plug that can't fire appropriately may cause a misfire; but the fouled plug may not be the root cause of the problem. Past fuel system problems may have fouled the plug, and you need to pay attention to them as well. Also, worn out piston rings, valve seals, or guides that are causing oil to leak into the combustion chamber may be at the root of the problem.
- Also, check that all spark plugs have the correct gap. Miles of service will wear the electrodes and upset the spark plug gap. Consult your car owner's manual or vehicle repair manual for your particular model for the correct gap. Use a wire feeler gauge to check and reset the spark plug gap.
- Check your service or repair manual for the spark plugs' service interval and replace them, if necessary.
- Check that you are using the correct spark plugs for your vehicle. Engines are designed to work with specific types of spark plugs and heat range, depending on the particular application.
- Nowadays, you can find iridium, copper, and platinum spark plugs. Some of these plugs may work fine for your vehicle; others may not. If you are not careful, using a type of spark plug different from the one recommended by your car manufacturer (or using one with a different heat range or electrode design) can introduce engine performance problems, including misfires. Check your vehicle repair manual for more information about the recommended spark plug for your engine.
- When installing new spark plugs tighten them to the torque listed in your repair or service manual. Leaving a spark plug loose (under-torquing) will lead to overheating, pre-ignition, or misfires. In addition, when installing spark plugs, avoid using anti-seize compound on the threads, or use only a dab on the upper half towards the wire end of the plug. If the compound reaches the insulator tip (around the center electrode), it may cause a misfire.
Spark Plug Wires
Spark plug wires wear out after miles of service, which will have an adverse effect on the spark.
- Examine the wires for cracks, burns, and other damage. If your misfire only occurs when it rains, start the engine and spray a little water on the wires and see if sparks appear. If so, you need to replace the wires.
- Spark plug wires can wear down internally too. So check each wire resistance. You can use a multimeter for this. Usually, the wire should not have more than 12,000 ohms for each foot of wire. Consult your repair manual. If you haven't replaced the spark plug wires at the recommended interval—see the repair manual—replace them even if they look good.
If the ignition coil wears out on engines with a single coil, it will cause multiple misfires; if it shorts out, it can cause an intermittent misfire. If your engine uses coil-on-plug (COP) coils, and you are troubleshooting a particular cylinder misfire, include the COP in your diagnostic.
A shorted ignition coil may be hard to diagnose. Usually, the misfire will appear when the engine reaches operating temperature. The higher temperature will cause the coil to expand, causing the short to reveal itself.
Ignition Control Module
A failing ignition control module can cause symptoms like those of a bad ignition coil, especially after the engine has warmed up. It'll cause the engine to miss or stall.
Your vehicle repair manual may have the procedure to check the ignition module, along with the voltage and resistance specifications for your particular module.
Check the distributor, if your engine has one. Examine the cap and rotor for cracks, corroded terminals, and carbon traces that can interfere with the spark. Replace the distributor cap and rotor if they're four years old or older.
Finally, make sure the ignition timing is correct. Excessive ignition timing advance will cause a misfire.
On modern vehicles, ignition timing is controlled by the car computer; sometimes you can still check base timing, though. Older vehicle models equipped with a distributor are more prone to misfires due to bad ignition timing; in these models, you can check and adjust timing. Consult your vehicle repair manual, if necessary.
IV. Checking Fuel Supply Problems
Just like an insufficient spark, an air/fuel mixture that is too lean or too rich air/fuel will affect proper combustion.
- Take, for example a clogged fuel filter. As your miles driven increase, so the amount of particles and impurities trapped inside the fuel filter. Eventually, these particles begin to have an adverse effect on fuel flow. They can restrict fuel flow at high speeds, enough to cause your engine to starve for fuel and misfire.
- Impurities and chemicals in the fuel are another common source of problems. These impurities stick to valves inside fuel injectors, causing loss of power, rough idle, hesitation, and misfires.
- A similar problem may come from a leaking fuel injector. If after leaving your car sitting overnight you get a misfire at startup that goes away soon after, this may indicate fuel dripping through a fuel injector valve.
- Fuel injectors can also suffer from internal electrical circuit problems, wiring harness or driver circuit in the PCM itself. Your repair manual will tell you how to do some of these fuel injector checks.
- A bad fuel pressure regulator can pass too much or too little fuel into the engine.
V. Checking for Vacuum Leaks
A vacuum leak can produce a misfire as well. Usually, a leak of this kind will cause one or more cylinders to misfire at idle or low speeds, but will disappear once you get to the highway. Still, you need to find the source of the problem. So here are some tips you can follow to find that leak.
- Make sure each vacuum hose is properly connected.
- Check hoses for cracks or tears.
- Trace your fingers along the length of each vacuum hose. If the hose feels too hard or too soft, replace it.
- Start the engine and engage the parking brake. Then, listen for hissing sounds coming from the engine compartment while the engine is idling. A hissing sound may indicate a loose hose, torn hose or leaky intake manifold gasket.
Damaged gaskets at the intake manifold, throttle body or carburetor will affect cylinder combustion as well. You can check vacuum hoses and gaskets using a length of rubber hose:
- With the engine at idle, put one end of the hose against your ear.
- Use the other end to follow the length of a vacuum hose or the edge of a gasket area.
- If your hose or gasket is leaking externally, you'll hear a clearing hissing sound.
Consult your vehicle service manual to locate vacuum hoses and further diagnostic information.
NOTE: Symptoms of a manifold gasket leaking internally resemble those of a vacuum leak except that coolant may be seeping through the ruptured gasket area. Thus, if you detect a misfire at start up that disappears at high speeds, and you notice that the coolant level goes down after a period of time, you may suspect an internally leaking head gasket.
VI. What If My Ignition, Fuel and Vacuum Systems Turn Out OK?
Besides components in the ignition, fuel, and vacuum systems, failures in other related components, and some changes in engine operating conditions, can lead to engine misfires as well. Under this category, here's a list of known components and operating conditions that are bound to give you trouble.
The Positive Crankcase Ventilation (PCV) valve is part of the maintenance schedule in many vehicles equipped with one. The main reason is the buildup of combustion byproducts that may clog the valve. You can check the valve in minutes and replace it if necessary. A stuck opened valve will cause your engine to miss at idle. Consult your repair manual to check the valve.
A contaminated or failing Mass Air Flow (MAF) sensor may interfere with your car computer and affect fuel supply to the engine. Check inside the sensor housing first for dirt and debris that may be interfering with the sensing element readings. Consult your repair manual to troubleshoot the sensor for potential problems.
Engine Compression Problems
Mechanical problems like wornout piston rings or cylinder walls, a stuck valve or damaged piston, and worn valve seats can lead to misfires. Do a compression test using a compression gauge to look for low or uneven compression among cylinders. Follow that with a leak-down test to locate the weak cylinder.
A leaky or blocked Exhaust Gas Recirculation (EGR) valve is another common source of misfires. The EGR system introduces controlled amounts of exhaust gases back into the combustion chamber to lower cylinder temperature and harmful emissions. So check for a bad EGR valve or carbon buildup under the valve.
Other sensors you want to check when you can't find the source of a misfire:
- Oxygen (O2) sensor
- Engine Coolant Temperature (ECT) sensor
- Throttle Position Sensor (TPS)
Maintenance Can Prevent Misfires
Troubleshooting an engine misfire can be difficult at times. But one of the best ways to prevent one from occurring in the first place is to perform adequate car maintenance. You can check and replace system components at the recommended manufacturer intervals.
The best part is that you can do many of these maintenance tasks and small repairs yourself. These components include changing the fuel filter, air filter, PCV valve, oil and oil filter, hoses, spark plugs, spark plug wires, distributor cap and rotor. Your car service manual, which you can buy at most local parts stores, will help you service, check and troubleshoot many of these systems and components.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: I had an engine misfire on C5 and replaced the plug and wire. I took apart the wire and noticed that it was burned up close to the spark plug. What does this indicate?
Answer: It's possible engine heat caused the damaged.
Question: My 2008 Ford Escape keeps having a cylinder misfire. It's always the same cylinder. The mechanics replaced the spark plugs, the spark plug wire, and the ignition coils before they finally replaced the computer. Yet the check engine light is still coming on! Does this sound like my mechanic is on the right track, or am I being ripped off?
Answer: One of the most common problems for a misfire on these models are faulty ignition coils. You need to make sure there's good spark and fuel (faulty injector?); sometimes the problem is in the circuit for the injector. Download the trouble code and get an opinion from a reputable car shop.
Question: What could be causing a misfire in my vehicle?
Answer: Check that fuel is actually reaching the cylinder. You may need to test the injector.
Question: My car misfires during start-up in the morning or when the engine is cold. What could be the cause?
Answer: There could be a leaking injector. The fuel that gathers in the cylinder vaporizes once the engine begins to warm up. The leak could be small enough to have the cylinder misfire at startup, but the injector might still hold some pressure during operation. Make sure the misfire is not still there during operation. Sometimes you can tell by listening to the exhaust. This could be an ignition system problem.
Question: If an engine misfire is always isolated to one cylinder does that rule causes such as a vacuum leak which I'm assuming would affect all cylinders?
Answer: A vacuum leak can affect one cylinder and usually upsets idle and fuel economy. The ECM tries to fix the problem for the leaned condition to that one cylinder, but it overcompensates in all cylinders.
Question: I have a Lincoln vehicle that sat for a month. I put in a new battery, started it up and got misfires. First, it said misfire 1 and 2. I cleared the codes. Then I got misfire 2 and 4; then the engine ran good, but I didn't drive it, just ran it for a while. The next day there was a misfire in cylinder 2, and a rotten egg smell. What could be the cause?
Answer: If the vehicle hasn’t been moved for a while, try driving on the highway for a few minutes (20 or more), and see if the misfire goes away. This could be carbon buildup in the cylinder(s).
Also, It is possible the misfire is happening in one cylinder and causing a “sympathetic” misfire in adjacent cylinders. If this is not a random misfire check the ignition system first. Swap spark plugs or wires or coil on plug (if applicable) to see if the misfire goes with the swapped component. Then check the injectors and fuel system, if necessary. The rotten egg smell can be something serious. Too much raw fuel may be burning in the catalytic converter. If it hasn’t yet seriously affected the car, fix the misfire as soon as possible.
Question: My truck has a misfire and won’t start at all. I give it full gas and it starts for a couple of seconds then cuts off. What could it be?
Answer: If the engine has just a distributor and coil, there could be a problem with either of those, or the rotor. There could be other possibilities. But your engine seems wanting to start. So I think there could be a clogging fuel filter, worn out fuel pump or bad fuel pressure regulator. Try spraying some starting fluid when cranking the engine. If it seems like it catches for a few seconds while spraying, this could be a fuel related issue.
Question: My 2011 Ford Escape started running rough. The mechanic changed the head gasket and suggested we change the plugs and coils. We did that, but the car completely stalls out after a few seconds of running now. We have replaced the fuel pump and throttle body as well. Code reader shows misfire on cylinder 1 and 3. What is a reasonable next step?
Answer: Make sure you have good spark and fuel pressure. Then check the fuel injectors, wiring and connectors for 1 and 3. Other possibilities include the mass airflow sensor and fuel pump relay.
Question: What is the cause of my spark scorching around the ceramic of my engine?
Answer: There could be several reasons for this, for example, abnormal combustion, spark plug tightened incorrectly, ignition timing too advanced, vacuum leaks, poor engine cooling, or lean fuel mixture.
Question: What can cause misfires on different cylinders when the gas gets down to about a quarter of a tank?
Answer: It's possible the tank is dirty and, when low in fuel, the pump is sucking impurities that restrict the pump strainer; you may have a worn or damaged pump; water in the tank can have the same effect. You may want to check fuel pressure and compare it to specs. This other post may help:
Question: My 2000 Range Rover misfires and I can't figure out why? When you start it up, and between 40-50 mph, it has a misfire on 1368. It is getting a good spark. I have tried injector cleaner and it is still missing.
Answer: The problem seems to be in one of the ignition coil circuits. It may have a short or a leak that is causing the spark to fail. I hope I could be more specific. The problem could be in cylinder 5.
Question: I have a 1975 Ford truck. It runs fine at idle, and fine even revving it up in neutral, but get up to about 25-35 mph it misfires, backfires, sputters and the engine dies! I've changed the distributor cap and rotor, points, plugs and wires! I've pressure tested coolant system and there are no leaks, no water vapor in the exhaust, and no water in the oil! Can you please give me your thoughts?
Answer: First, make sure you have replaced all fuel filters in the line and screen filter going into the carburetor. There could be a problem with fuel system pressure as well. If your carburetor has a diverter where the fuel lines attach to, check that. If the filters or screens are clogged, you may need to clean the fuel tank. Check for fuel flow as well.
Question: I have a 2002 Honda CR-V. The check engine light keeps coming on for multiple misfires. The mechanics replaced spark plugs and ignition coils. They said that the only other thing they think could be is a computer issue. My vehicle typically misfires when I start my engine. It seems to happen more often when the air is cooler. What do you think the problem could be?
Answer: This is a guess, but if the misfires are more noticeable with a cold engine or outside cold temperature, it's possible the fault is in the fuel system (lean condition) or a sensor failing to send the correct data to the computer. You may want to monitor fuel pressure when the engine starts cold. Also, check the computer for troubled codes pointing to a sensor. Take a look at section VI in the post.
Question: My Chrysler 300 only misfires on cylinder 1 when I start my car after I’ve been driving it. It’ll stay misfiring until I park it and let the engine cool all the way down again. After that, I can start it and have no problem with it. How can I troubleshoot my Chrysler's misfiring?
Answer: Usually, a bad ignition coil may cause a misfire when the engine warms up. Check the wiring and connector--loose wire, contamination, etc..
Question: I have a 2009 Buick Lacross. The car misses when warm with a P0300 code. We changed one coil that seemed to have no spark when warm, but the new coil did not change the misfire. New plugs and wires were installed 11 months ago. Could the problem be electrical to the coil?
Answer: Usually, faulty electrical components show up when the engine warms up. This may be ignition coils, ignition control module and even a bad fuel pump. Spark plug wires with a carbon string core tend to cause issues after operating for some miles under high temperatures. Other than that, there could be a number of issues. This other post may help:
Question: My car (Vitz new model) has a misfire and when I checked the water system, water was so hot and it just erupted, I checked on the spark plugs and there was no problem. So what problem could it be?
Answer: If you've been keeping with the service as scheduled by the manufacturer, the misfiring problem should be covered under your warranty. Overheating and misfire usually points to loose (under-torqued) spark plugs, leading to a faulty firing. However, it's better to bring it to your dealer and ask them to look into it. Hope this helps.
Question: What could be the reason why my car doesn’t start in the morning?
Answer: A common problem is a clogging fuel filter, but fuel pressure may be leaking. This could be a bad fuel pressure regulator or a worn out fuel pump (check valve) or even a leaking injector(s) (on some models). You need to check the fuel system to locate the fault or, possibly, clean the injectors.
Question: I am getting misfire code and reading on #1 and #2 cylinders. However, the engine is running very smoothly, no misfiring or knocks at all. What else could be the problem? We have changed spark plugs, ignition coil, Pistons, fuel regulator. Could it be the Ecm?
Answer: Check the compression on each cylinder and see how they compare - also, you can check the mechanical condition of the engine using a vacuum gauge:
Question: My misfire happens on cylinders 1 and 4 only when it rains, and once the engine warms up the problem goes away. Is there a way to waterproof coil 1-4 or should I replace it?
Answer: Sometimes you can find rubber covers for the ignition coil. Do a search online for your particular model.
Question: My V6 is missing on two of six cylinders. The spark plugs in both are completely clean (as they are brand new), while the working ones show signs of ignition. We’ve checked the continuity of the wires up to the computer, and they’re all fine. Could it be a computer problem?
Answer: Try changing the plug wires to different cylinders and see if there's any difference. Also, make sure the coils are good. If the coils are fired through an ignition module, make sure the module is commanding the coils -- or the ECM, depending on the model.
Question: My 99 Pontiac is misfiring even after changing the plugs and wires, what could be the problem?
Answer: Check for trouble codes. That could give you a clue about the problem. Faulty fuel injectors, a clogging fuel filter or a faulty sensor can also lead to misfires.
Question: I have a 2007 Toyota HiLux model. I was driving and it suddenly stalled. Then went again and it is now running rough. It came up with multiple cylinder misfire code, and I have found a wire that's been chewed by rats. Would this be the cause of the problem?
Answer: The bad wire may be affecting a sensor, or something in the ignition system. Have it fixed and take if from there.
Question: My Golf mk4 1.4 16v 1999model is misfiring on the first cylinder (all the way to the left cyl). Spark plugs, ignition coil, spark wires, and fuel injector connection is good on all 4 cylinders. I got a fault code saying (DTC) "00282 Throttle position actuator DTC status: open or short to plus." Is this guaranteed to be the Throttle position sensor causing only misfire on the first cylinder?
Answer: The fault may be in the actuator itself, the circuit or a mechanical fault preventing the actuator from working properly. This page may be of some help:
Question: My car only misfires after I get it up to operating temperature and turn it off and try to start it again. If I let it cool down, it runs smoothly again. Also continues to run smoothly so long as I don't shut down and try to restart. What would cause my car to misfire?
Answer: A faulty fuel injector can throw these symptoms. You may want to start there. Then check the fuel system (fuel pressure) and ignition system (for strong cylinder spark). If you haven't seen the check engine light come on, scan for trouble codes, regardless. Sometimes a pending code can guide you in your diagnostic.
Question: I have a misfire on three cylinders after several miles. I have replaced the spark plugs, which were oily, cleaned the tubes, replaced tube seals (which were cracked), and have not seen oil leaks since, but the misfires persist. The wires are fourteen months old. I do not see any damage and I checked them with a multimeter. The distributor was leaking oil and the points were corroded. I replaced them, but the misfires continue. What do I check next to stop my car's cylinders from misfiring?
Answer: Closely check the distributor cap. it can also cause problems when hard to notice cracks and traces of carbon mess up with the spark. Since the problem is concentrated on those three cylinders, you may want to check the valve train components for those three cylinders. Make sure the valves are operating as they should. If possible, download trouble codes, just in case there's a pending code.
Question: My 2011 Toyota Sienna is misfiring on cylinder one all the time. How can I identify the problem and fix it?
Answer: This could be a fuel or ignition related issue. These are the most common problems. You can check the injector first to make sure it's working. This post may help:
Then check that you got a good spark for that cylinder.
Question: Is it a misfire if I removed all the rubber cap off the back spark plug one by one to check where is the misfire, but the engine sound did not change at all?
Answer: You need to unplug the spark plug wire from the plug. If you notice no change, there's where the problem might be. You might want to use a spark plug tester. That way you can check for a weak spark that has no power for combustion. A fouled spark plug can also misfire. You might want to remove the spark plugs, one by one and check their condition.
Question: I have a 2008 Mazda 6. Car runs great at start-up and my 30-minute commute to work. When I shut the car off and restart within about a 4-5 hour span, car misfires cylinder 1 every time. The misfires only occur when the engine is warm. Any clue for a fix?
Answer: I think the ignition coil for that cylinder has a circuit fault. Try swapping coils with another cylinder and see if the misfire moves to that cylinder. If it does, you may need to replace the cylinder. If the same cylinder misfires, check the electrical connector and wires for a loose or bad connector or wire.
Question: I have a 2008 Subaru Legacy GT that just had the engine rebuilt. It misfires randomly in all cylinders when the engine is warm and stalls at idle. Would a tight valve clearance maybe cause random misfires?
Answer: There could be several reasons for this. Try downloading trouble codes, even if the check engine light is not on. These other posts may help as well. Start with the stall-at-idle issue. This may solve the random misfire problem.
Question: My 2004 Jeep Liberty Limited V6 is misfiring only within cylinder 6. We replaced the spark plug and ignition coil connected to it and the mechanic said that it doesn’t look to be the wires at first glance. The previous owner (I bought it a day and a half ago and then it misfired and stalled backing out of a parking space) said that it had a vacuum hose issue somewhere. There is also a code for oil pressure low. What do you suggest I do next? Compression/pressure test? Or check distributor?
Answer: Usually, ignition and fuel system related issues are the source of misfires. have the fuel injector for that cylinder checked. A vacuum leak can also lead to this problem. It's hard to tell whether the low oil pressure and misfire are related. Unless the cylinder is leaking oil and fouling the spark plug, leading to the misfire. If a bad head gasket is leaking oil into the cylinder this might be the cause. They need to find out whether there's actually low pressure.
© 2016 Dan Ferrell
Dan Ferrell (author) on August 28, 2020:
Check to see if the misfire happens only to one cylinder or randomly on any cylinder. That'll make your diagnostic easier. If only one cylinder is affected, check the plug wire, plug, coil (if applicable) for that cylinder, valve (including spring).
If it's affecting any cylinder make sure to check simple stuff like fuel filter, fuel pressure regulator and air filter, MAP sensor.
Misfires at idle are usually caused by vacuum leaks. This type of leak sometimes is hard to find.
Make sure to check for DTCs (diagnostic trouble codes). If you don't see the check engine light on, there could be a pending code. This can help you locate the system causing trouble.
Hope this helps.
Legit on August 28, 2020:
My Toyota Opa is misfiring at idling? I just had a mechanic address the problem of a ridge in the engine block and changging of pistons. The problem still persists, what could be the problem/
Dan Ferrell (author) on August 18, 2020:
You may find it in the lower left hand side, driver side. This video may help:
Steve west on August 17, 2020:
Does my 1998 peugeot boxer 2lt petrol have a port to plug in a code reader if so where is it i have taken all the lower dash off and i cant see it
Dan Ferrell (author) on December 19, 2019:
it seems the codes are pointing to the circuit for injectors 3 and 4; check the connectors and harness for those injectors. There could be a bad connection a loose or damaged wire. Hope this helps.
Gregvul on December 19, 2019:
Having an issue with Vauxhall Zafira 1.6 2008 petrol model. When the engine is hot I have random misfires, cutting out and failing to restart.
Stored Fault Codes: P0300-RANDOM MULTIPLE CYLINDER MISFIRE,P0303-CYLINDER 3 MISFIRE,P0203-INJECTOR CIRCUIT 3,P0204-INJECTOR CIRCUIT 4
ECU has been tested and passed test. Coil packs, cylinder 3 and injector has been replaced. Mechanic doesnt think it is related to air/fuel mixture because the car runs fine until it reaches certain temperature then Engine check light appears and engine cuts out. If it was air/fuel issue then he thinks he would get fault codes for all the cylinders.
any ideas please?
Dan Ferrell (author) on October 05, 2019:
There might be a problem in the wiring for the coil on that cylinder. As temperature increases, so does resistance. Check the connector for a loose wire, dire, oil, grease. You may need to trace the wire back to the computer. Your vehicle repair manual may help you trace the voltage, if necessary. You may want to check the spark once the engine warms up and you notice the misfire. Hope this helps.
Mark on October 05, 2019:
I have a Audi A5 2.0 tfsi. It misfires after warm up when driven for 45 mins and always on cylinder 2 and Ive changed all 4 coils and spark plugs and have done a compression check witch has come back good. Every time it misfires i remove the cyinder 2 spark plug and its always coated with carbon build up but im only getting a misfire code on that cyinder
Dan Ferrell (author) on September 12, 2019:
Probably the problem is caused by the fuel filter. Download the trouble codes. That'll help you diagnose the problem. This other post may help as well:https://axleaddict.com/auto-repair/P0300-Random-Mi...
Darren Elliot on September 11, 2019:
My 2007 Holden Astra twin top 2.2l petrol, Randomly miss fires, some times it will run fine for a while then it will miss it is worse on longer trips, the engine light comes on and stays on sometimes, Spark plugs are good, I cant find a fuel filter? any ideas on the problem
Dan Ferrell (author) on May 04, 2019:
This other post may help:
Letlhakeabra on May 04, 2019:
I'm having a problem with my VW Jetta 5 TDI 1.9 diesel I've changed glow plugs , and glow plugs relay as well as my injectors but still my car starts up and dies
Dan Ferrell (author) on April 30, 2019:
You might want to check the injectors. This post may help:
Dan Ferrell (author) on May 08, 2017:
If the car vibrates and misfires and overheats indicates there's a problem that needs fixing. It's strange that the scanner didn't retrieve any trouble codes. However, even if no TC showed up, get the car diagnosed, just the misfire and the overheating can give you a lot of problems.
Susan Martinez on May 07, 2017:
My 2004 Ford Focus engine light on but nothing shows up on diagnostic check. The car vibrates and misfired. Starts overheating sometimes while sitting still. The A arm just went last night. Should I fix it or is engine bad if nothing showed up on diagnostic check?