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.
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. 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.
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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.
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..
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, 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.
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