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 now a days.  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 on call, I recommend this site as there are mechanics on line 24/7 to answer any question you may have.  They can give you the most likely possible causes for any specific misfire condition based on the year, make and model of your vehicle or they can help you diagnose the misfire condition if your a do-it-yourselfer.   If so, click here to view some of my personal tools that I use when diagnosing msifire conditions and would recommend that you use as well.

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, the engine will misfire. One cylinder can misfire or more than one cylinder can Misfire depending on the situation.  Here is a hint.  A lot of times the Vehicle Manufacture will release a Technical Service bulletin (TSB) related to a rough running or Engine Misfire condition specific to your vehicle.  You can go to ALLDATAdiy to gain access to all of the Technical Service Bulletins related to your year, make and model vehicle for a small fee.  If you find a match, it will explain the root cause as well as the fix for the Engine Misfire condition.

P0301 Cylinder one misfire

Here we have a P0301 cylinder misfire code on a Chevy 3100 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.


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 re attached 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.

How to check Ignition Components

One of the most common causes of engine misfires are the tune up parts. Primary or secondary ignition components can go bad and prevent spark from being delivered properly to the cylinder which would result 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.

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 can 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.  In addition, lets not forget about the spark plug itself.  The spark plug can crack or just simple 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.  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.

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.

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 the engine will run lean causing it to misfire.  If this is the case, your computer may generate a P0171 or a P0300.  If you think you have a fuel delivery concern and want to check your fuel pressure, you can go to ALLDATAdiy and get the exact fuel pressure specifications for your vehicle.

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 Tester 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 good fast trick I like to use is checking engine vacuum with a hand held Vacuum gauge.  Most engines now a days 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 an 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 itself or even the cylinder wall for nicks or scratches without removing the cylinder head or you could use a tool called a Borescope.  This neat little device will do just that.  It can be a bit expensive but may save you more money down the road in repairs.

Other possible causes

There are also some 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, usually if there is an EGR system failure, the computer will flag a P0401 or even a P0402 Excessive EGR flow code.   In rare cases, I have seen plugged exhaust (catalytic converter or even plugged mufflers) cause the engine to misfire as well and does not always trigger a Diagnostic trouble code Catalyst Efficiency code P0420.

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.

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, Ignition or Engine Mechanical.  If you suspect that your engine is running rough due to the engine misfiring, please feel free to use the information that I have provided on this page to help you diagnose the root cause.  In addition, please feel free to contact me with any questions, comments or concerns you may have about this page or my site by using the “contact us” tab on the lower right hand side of the screen.  If your interested in learning how to do advance diagnosing, Misfire Diagnostics is a great video on how to diagnose engine misfire conditions using simple tips and tricks known to Automotive Professionals in the field.

Misfire codes

P0300 Random Misfire Detected

P0301 Cylinder 1 Misfire Detected

P0302 Cylinder 2 Misfire Detected

P0303 Cylinder 3 Misfire Detected

P0304 Cylinder 4 Misfire Detected

P0305 Cylinder 5 Misfire Detected

P0306 Cylinder 6 Misfire Detected

P0307 Cylinder 7 Misfire Detected

P0308 Cylinder 8 Misfire Detected

P0309 Cylinder 9 Misfire Detected

P0310 Cylinder 10 misfire Detected