Engine Misfire

Engine Misfires can be very difficult to diagnose.  If the engine is misfiring, it can cause all sorts of problems and leave you guessing as to what is the root cause.  When the engine misfires, you will lose power, your gas mileage will drop and the vehicle can even fail emissions.  In fact, cylinder misfires are one of the most common causes of emission failures on cars nowadays.  A specific cylinder can misfire or multiple cylinders can misfire.  If the engine starts to misfire and the vehicle is newer than 1996, then the check engine light may come on or flash and the computer may store a diagnostic trouble code like P0300 or something along those lines.  If the vehicle is built prior to 1996, the computer may not pick up on the misfire and may not light the check engine light.   If you would like to get in contact with a Mechanic, I recommend using Just Answer as they have Mechanics online 24 hours a day and can answer any questions that you may have.  They can give you the most likely possible causes for any misfire condition based on the year, make and model of your vehicle.

What is an Engine Misfire?

To understand fully what an engine misfire is we need to understand what it means. If the engine is running perfect and all of the cylinders are “firing” you could say there is no misfire. Once the air-fuel ratio inside the combustion chamber in any of the cylinders gets interrupted (for any reason), the engine will misfire. One cylinder can misfire or more than one cylinder can Misfire depending on the situation.  The misfire can be caused by a number of different things.   While ignition components are very common to go bad causing the engine to misfire, there is still fuel delivery (low fuel pressure or faulty fuel injectors) or even base engine mechanical issues that can cause the engine to misfire.

Most common causes of engine misfires per Manufacturer

Ford, Lincoln and Mercury cars and trucks: The most common cause of a misfire condition on the newer Ford cars and trucks are faulty ignition system components.  Fuel injectors don’t usually go bad on Ford’s.

Chevy or any GM vehicles: Faulty spark plugs and ignition coil or coils depending on the ignition system.  The older GM vehicles with ignition wires and possibly distributor cap and distributor rotor as well.  However, some of the older V6 engines the fuel injectors would either short out or plug up causing the certain cylinders to misfire.  This is the case with the early 2.8 and 3.1 motors.

Dodge Jeep and Chrysler: Based on my experience, there are two common causes.   Some newer vehicles have issues with broken valves springs or burned valves. The fix is to remove the entire cylinder head or heads and send them out to a machine shop to get rebuilt.  However, the MOST likely cause would be the tune-up components  (which is very common).   Fuel injectors do not go bad very often on these vehicles (but still a possibility).

Honda: Misfires on Honda’s are usually caused by faulty spark plugs or other secondary ignition components.   Tight valves are also common on Honda’s if the valves are not adjusted regularly.  A simple valve adjustment will usually remedy a misfire condition caused by tight or incorrectly adjusted valves.

Toyota: Spark plugs, ignition coils or any other failed secondary ignition component can cause the engine to misfire. Rarely do I see a base engine mechanical problem or faulty fuel injectors cause a rough running or misfire condition on Toyota’s. (However, it is still a possibility). Also, if your engine uses a timing belt, don’t forget about that too as the engine may simply stop running if it breaks or jumps time due to a lack of maintenance.

Chevy 3.1

Here we have a P0301 cylinder misfire code on a Chevy 3.1 liter engine.  The check engine light was on and flashing at times and the computer had a pending P0301 cylinder one misfire code.  I pulled this code from the computer using my Actron Pocket Scanner.  It works great because it is extremely fast (just plug it in and hit the “read” button) and does not require any batteries because it gets powered up through pin 16 of the data link connector of the vehicle.

Update.  You can get a newer updated Autel Maxilink ML329 code reader that can read readiness monitors here.    The ability to read I/M Readiness Monitors is good to know as it will tell you if your vehicle is ready to pass emissions.  (BONUS:  The Maxilink ML329 also has the Auto VIN detection for quick DTC definition and can be updated for free via the Internet.)


When I took a look at the number one spark plug wire, this is what I found.  The number one spark plug wire was broken or got pulled off of the spark plug booth causing the engine to misfire.   We simply fixed the broken wire and reattached it and the engine ran like new
broken spark plug wireBelow is a video of the engine running rough because of the broken spark plug wire.

P0300 Random Cylinder Misfire

P0300 is a code generated by the Powertrain Control Module or Engine Control Module basically telling you that it has detected a random cylinder misfire.  It can be anywhere from two or more cylinders misfiring causing this P0300 code to appear in the computer.  There could be a number of things that can cause the engine to misfire.  Fuel delivery, spark delivery or engine mechanical related problems.  Usually, you will find that any Engine Misfire condition will fall into one or more of those three categories.  Here is a tip, you can go to alldata to get specific information related to the year, make and model of your vehicle.  A lot of times the Vehicle Manufacturer will release a Technical Service Bulletins (TSB’s) related to a rough running or Misfire condition specific to your vehicle.  If you find a match, it will explain the root cause as well as the fix for the Engine Misfire condition.  It will also have component locations, repair procedures, and factory wiring diagrams.  Here are some of my own tools I use when diagnosing misfire conditions.

How to check Ignition Components

One of the most common causes of an engine misfire condition is the tune up parts. Primary or secondary ignition components can go bad and prevent the spark from being delivered properly to the cylinder which would result in an engine or cylinder misfire.  An ignition misfire can be consistent (all of the time) or it can come and go (intermittently misfire).  I have found that an ignition misfire will usually happen more under a load than at idle.  This is because of the higher demand for spark.  Not only the Piston inside the Cylinder is moving faster when you step on the gas (under a load), but also because the throttle plate opens when accelerating causing ignition coil or coils to work harder to overcome the turbulence inside the cylinder.

Below are Denso spark plugs that were removed from a 2008 Toyota rav4.  While they were not causing the engine to misfire, we replaced them anyway as they were pretty carboned up as you can see.  This particular engine was burning a significant amount of engine oil.

Denso Spark plugs

There are a number of ways to check your ignition components.  Depending on your ignition system, you can use different tools or even try different strategies to see if the ignition system is working properly.  Sometimes a visual look at the tune-up parts is all that is needed.  You might see arching coming out of the ignition coil/coils or even the ignition wires when the engine is running which can cause a P0300 through P0308 (depending on the engine size).  However, sometimes they go bad and prevent the spark from reaching the cylinder and there will be no visible signs of arching.  In addition, you can check the tune up parts is with an Ohm Meter.  If you suspect the tune up parts are causing the engine to misfire, you can ohm out each individual spark plug wire, ignition coil (or distributor cap and rotor if equipped) and compare the readings to each other to see if the reading is much more than the others.  Just keep in mind, the longer the spark plug wire the more resistance it will have.  Also, don’t forget about the spark plug itself.  The spark plug can crack or just simply go bad causing an individual cylinder to misfire.  A quick test for this would be to switch the spark plug from the cylinder that is misfiring to another cylinder and see if the misfire moves with the spark plug.  If so, time to replace the plug.  Another way would be checking the ignition wires with a Lab Scope.  This test is very accurate and a Technician can see which cylinder or cylinders are misfiring as the misfire occurs.  However, this is a bit more involved so I would recommend to leave this up to a professional.

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Checking Fuel Delivery

Another common cause would be a fuel delivery issue causing the engine to misfire causing a P0300 code (or any misfire related code) to be stored in the computer.  It can be a lack of fuel or too much fuel.  On a vehicle that has one fuel injector per cylinder a plugged fuel injector or even a leaking fuel injector can cause a specific cylinder to misfire.  Some vehicles are more prone to bad fuel injectors.  For example, some older Nissan’s the Fuel Injectors would short out.  Some older Chevy and or GM vehicles, the tip of the fuel injector would plug up restricting fuel flow into the cylinder.  Depending on how the fuel injector goes bad will determine what needs to be done in order to get it fixed.  If the fuel injector is plugged up, sometimes putting fuel injector cleaner into the fuel tank or even doing a fuel injection cleaning service with a motor vac may be needed in order to clear the debris off of the tip of the Fuel Injector.  However, if the fuel injector goes bad electronically, the fuel injector will need to be replaced.  You can check for this by simply using an ohm meter. Just take the two leads of your ohm meter and touch the two prongs on top of the fuel injector and take a reading.  Be sure to look up the resistance specifications for the fuel injectors on your vehicle as each may vary.  If the Fuel Injector is plugged up, you might be able to get by with a Fuel Injector cleaning.

Checking Injector Pulse using a Noid light

Another way to check fuel delivery is to use a tool called a Noid Light.  This is checking the signal from the Computer to the Fuel Injector.  It simply plugs into the Injector connector after it is disconnected from the Injector and it should blink as the engine is cranking over or running.  You can check this on any gas Fuel Injector engine at each cylinder provided the engine has one Fuel Injector per Cylinder.  The Computer could go bad causing the Fuel Injector to stay open all of the time or prevent the Fuel Injector from opening up at all.  Both of which will cause that particular Cylinder to misfire.   I demonstrate how to check the Injector signal here.

Checking Fuel Pressure

In addition, fuel pressure is a major factor in this too.  Low fuel pressure can cause more than just one cylinder to misfire.  If the fuel pressure is too low, then all cylinders will run lean causing it to misfire.  If this is the case, your computer may generate a P0171 or a P0300.  Some manufacturers use a Fuel Pump Driver Module to control the speed of the Fuel Pump.  For example, a 2004 Ford F150 with a 5.4 liter uses a Fuel Pump Driver Module and these corrode and go bad all of the time which can cause no or low fuel pressure.  Other fuel pump circuits are a bit more basic and have one constant power and ground to operate the Fuel Pump.  If you think you have a Fuel delivery concern causing a misfire condition and want to check your fuel pressure, I would recommend that you go to alldata and get the exact fuel pressure specifications for your vehicle.  I have put together a page on how to check fuel pressure here.

Engine Mechanical

In addition, low compression (bad cylinder, piston rings, sticky or leaky valves or even valves out of adjustment) will affect the amount of air coming into the cylinder which will cause a cylinder to misfire and could set a P0300.  A simple Compression Test will be able to verify if a certain cylinder is low on Compression.  If you find that a Cylinder has low Compression and would like to know why using Cylinder Leakage Tester will be able to tell you how much air is leaking out of the cylinder and where it is leaking out from.  It could be the intake valve, exhaust valve, piston or piston rings, cylinder wall, cylinder head or even the cylinder head gasket.

One of the easiest ways to check for a base engine Mechanical problem would be to use a handheld Vacuum gauge.  Just hook it up to a hose or line that comes from the intake manifold or throttle body and start the engine.   Most engines nowadays pull in upwards of about 19 or 20 inches of vacuum and nice and steady at idle.  If your engine is running rough or if there is a cylinder misfire due to a mechanical issue, the needle on the vacuum gauge will bounce around indicating a mechanical failure with the engine itself that will need to be addressed.  If you have a major compression loss or if you just want to see inside of the cylinder/combustion chamber to inspect the piston or cylinder wall of a specific cylinder for nicks or scratches without removing the cylinder head, you could use a tool called a Borescope.  This neat little device will do just that.   Just remove the spark plug and insert the small end of this tool into the cylinder and have a look for any mechanical damage inside the combustion chamber.   For advanced engine mechanical testing procedures, I recommend using an In-Cylinder Pressure Transducer which can tell you a lot of the Characteristics of the combustion chamber.   Here is a good training program I recommend for advanced individuals that would like to learn more about this diagnostic approach.

Other possible causes

There are also some less likely or not so common things that may cause the engine misfire which could result in a P0300 code stored in the computer. Blown head gasket (coolant inside the combustion chamber), cylinder temperature too high resulting in engine pinging (lean air-fuel ratio or even plugged exhaust), Vacuum leak or even EGR issues like the EGR valve sticking closed or open causing too much EGR flow in one or more cylinders will obviously cause the engine to misfire and run rough.  However, if there is an EGR system failure, the computer will usually flag a P0401 Insufficient EGR Flow or even a P0402 Excessive EGR flow code but this is not always the case.  For example, some Honda’s and Ford’s that a specific EGR port in the intake manifold will plug up restricting Exhaust flow to a specific cylinder.   When this happens, excessive exhaust gases will flow into the other cylinders which will affect the air-fuel ratio and cause-specific cylinders to misfire.  Another possible cause would be plugged exhaust (Catalytic converters and mufflers) can cause the engine to misfire or bog down under a load while trying to accelerate and may not flag a misfire code in the computer.   On vehicles newer than 1996, the computer system (known as OBD2) will likely flag a P0420 catalyst efficiency code in the computer if the converter is plugged up.

Scanning for Misfire Codes

Below is a picture of my Actron Pocket Scanner checking for engine misfire or other diagnostic trouble codes in the PCM on my vehicle.  Since my vehicle is running good and there are no codes stored in the computer, this pocket scanner displays “no codes”.  If there were any misfire codes, this would display a P0300 through P0306 (since it is only a 6 cylinder engine) indicating which cylinder is misfiring.   This is on a 1998 Oldsmobile Intrigue

Checking for Engine Misfire codes
Checking for engine misfire codes


There are several things that can cause an engine misfire which may result in the check engine light and a rough running engine.  Remember, there are three categories that any misfire will fall into.  They are; Fuel delivery system, Ignition system (primary or secondary ignition) or base engine mechanical components.  If you suspect that your engine is running rough due to the misfire condition, please feel free to use the information that I have provided on this page to help you diagnose the root cause.  If you would like to speak with an Expert that specializes on your vehicle, click here and type in your question.

Ford misfire TSB’s

TSB 06-14-10

Chevy Misfire TSB’s



Pontiac Misfire TSB’s