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 loose power, your gas mileage will drop and you may 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.

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.

How to check tune up parts

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 obviously 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 there of a 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.  However sometimes they go bad and prevent the spark from reaching the cylinder and see no sign 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.

Checking Fuel Delivery

Another common cause would be a fuel delivery issue causing the engine to misfire. 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.

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 alldata.com 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.  A simple compression test will be able to verify that the cylinder is sealing properly.  If you find that a cylinder has low compression, you can do a cylinder leak down test with a cylinder leak down tester.  This well 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 pump/tester.  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.

Other possible causes

There are also some not so common things that can cause the engine misfire too. 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 an 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

Conclusion

You see, there are several things that can cause an engine misfire. Ignition, fuel or even a mechanical issue can all cause your engine to misfire.  If you suspect that you have a misfire condition on your vehicle, please feel free to use the information that I have provided on this page to help you diagnose your rough running or misfire condition.  I like to hear success stories so please leave a comment if this page has helped you out!

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