Engine Misfiring Code Secrets

For engine misfiring code to appear in your dash panel, it has be a random misfire called P0300 or specific cylinder misfire. The last one could be P0301, P0302, P0303 depending on the numbers of cylinders you have. In this example, P0301 is for cylinder one and P0302 is for cylinder number 2 and so forth. So basically, what is the difference between a random misfire and a specific engine cylinder misfire?

Random misfire occurs if the misfiring happens on any cylinder at any time. There is usually a common cause for this problem: if you have a vacuum leak, each cylinders will have lean fuel mixture. So when the engine runs, it could misfire on cylinder 1 after a second, cylinder 3 or 4 for the next second. These is no permanent misfiring from any cylinder but it keeps jumping from one cylinder to another. Another example of random misfire is caused by weak ignition coil or module. When these parts don’t work properly, it could make the engine misfire at any cylinder. The same condition will happen if one of the wirings to these parts are shorted or open like wiring loose connections.

P0301 tester You will notice that random engine misfire affects your engine combustion. For your engine to run, you need spark, air and ignition at the right time. If one of these components is weak or deteriorating, then you have random engine misfiring code. Like the weak weak ignition parts above, the whole engine is affected. The same goes for vacuum leak due to a cracked vacuum hoses or cracked intake manifold gaskets. This is also why if your engine computer is not providing the proper engine timing, you will have random misfire too. For random misfire to occur, these are the major players:

1.) engine electronic components such as computer, sensors (crank, cam amd mass sensor), output solenoids like egr including their wiring.

2.) Other main engine component part includes alternator, fuel regulator or any part that supply air, fuel, timing and ignition to your engine except the ones for the specific cylinders.

3.) You could have a mechanical defect too to have random misfire: worn fuel pump, jumped timing belt or chain, bad fuel or fuel filter, bad carburetor or throttle body, restricted exhaust muffler to name a few.

The engine misfiring code by individual cylinder is a different story. Here, the misfire occurs on the specific cylinder only. This means the problem is confined to one or two cylinders and it stays there all the time. if found on one cylinder, it means that particular cylinder has a defect present to that cylinder. Checking is easy because if you transfer the defective part of that cylinder or compare its performance, you will mostly find the problem. You can try this: switch the spark plug, plug wire, coil plug (for engines without plug wires), injector to another cylinder. If the problem moved to the cylinder where those parts were transferred, then those parts are the culprit. Just make sure to switch and test the misfire code one at a time so you catch the exact part causing the misfiring.

Specific misfire could happen in group of 2 cylinders also. If there is a cylinder head gasket leak between cylinders 2 and 3, then you will have an engine misfiring code occurring like P0302 and P0303. It can also trigger if 2 cylinders are running mate or opposite side of combustion flow and there is a common intake manifold runner between them. Example for this is cylinder 1 and 6 for six cylinder engines. If there is a cracked(vacuum leak) on the intake manifold between them then you will get P0301 and P0306. Ignition coils which supply spark for 2 cylinders could do it too and switching the ignition coils is the best test for this problem.

Tips on testing engine misfiring code:

Always reset the engine computer and take the vehicle for a road test to confirm if you got either a random or specific engine misfiring code. Remember: random misfire code has a common engine culprit whereas specific misfire is one or two sources from a particular cylinder.

For random misfiring code, find the common supply or power components: vacuum? spark? timing? These can be verified manually by visual inspection or basic engine testing. Try answering also these obvious questions: how good is your vacuum supply? what is the condition of the distributor? ignition coil? How good are the major sensors? tried tapping the computer? If you have an engine code reader with live data display for electronic parts, it can monitor each sensors while you are driving. This is the reason if you buy a more expensive engine code reader with troubleshooting features, it will pay for itself later and save you big bucks later.

For specific codes, if switching components does not help, verify the compression reading of the cylinder (actually always do this first). Don’t rule out that the cylinder is out of adjustment like loose valve adjustment or any mechanical defect. I have also seen this P0301 problem where a broken wire to the injector was discovered although the injector itself was found good.

Sample of Difficult Engine Misfiring Problems:

2004 Tahoe Chevy 5.3L misfiring P0301 code

Symptoms: Missing on number one cylinder. Changed all the #1 cylinder parts like ignition and injectors including the computer. What am I missing here? I got the compression tested and it was above 140 psi. Problem is intermittent.

Fix: Although you took a compression test, it is not enough. Try using a cylinder air leak down tester because it is intermittent. This tester will show if the valve train parts are good: cam lobe, push rod, valve seat, valve spring and rockers. Borderline compression problem is difficult to find with compression tester alone.

2005 Mazda Tribute 3.0L engine with P0301 code

Symptoms: Poor idle and missing on #1 cylinder when cold. Replaced just about all the ignition parts for #1 cylinder and still misfiring.

Fix: If this problem occurs when it’s cold, remove the spark plug and check for signs of coolant in the tip. You can even smell it for coolant leak. If possible do the same for the next cylinder beside it. Most likely, this is a head gasket problem. A cylinder leak down tester can verify this.

2005 Toyota Matrix 1.8L engine with P0301 code

Symptoms: Missing on #1 cylinder even after replacing most ignition parts. The leak down test showed normal. Suggestions?

Fix: Have you checked the #1 fuel injector? You can have it cleaned too by your local garage.

Before you touch anything and start working on your engine misfiring code, make sure you have the specific information like manual or online mechanic like ATS (we use common fixes to guide you to your specific engine model) to help you. This will ensure you will get the exact fix for the misfiring problem and avoid wasting your time.




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