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How to Diagnose an Engine Misfire

Posted by Michael Manning on

Check Engine Light is on with Code P0300

A misfire is not usually a subtle symptom. The less working cylinders your vehicle has the more obvious the issue usually is. For example: An 8 cylinder engine with 7 good cylinders will still have symptoms of a misfire but in a 4 cylinder with only 3 good cylinders the symptoms will much more obvious and even scary when it starts. You will feel anything from a rough or pulsating idle to a large and almost violent stutter in the engine. This indicates a potential misfire. When this occurs, it is best to get off the road as quickly as possible. Driving with a misfire can damage your engine and mounts due to how the uneven torque can twist and turn the engine.

Your check engine light on the dash is standard warning of something wrong under the hood but with an Bluetooth OBDII scanner you can read the error code. For a misfire you will likely get codes in the P030X group. If you a code of P0301, that means that you are experiencing misfires on cylinder 1. A Code of P0302 identifies misfires on cylinder 2 and so on. If your OBDII scanner reads P0300, it means that 2 or more of your cylinders are misfiring.

Code P0300: How to Fix

Misfire Symptoms: What to Look For

  • A rough running engineor a sporadic idle.
  • The engine may sputter or stutter when accelerating.
  • A check engine light and OBDII readings of P0300, P0301, P0302, P0303, etc.

Diagnose Code P0301

Through the years the technology in our ignition systems has changed so depending on when your vehicle was manufactured you may have a distributor with cap and rotor and sparkplugs, ignition coils that are controlled by the computer, or you have the coil-on-plug technology where each spark plug has its own ignition coil that’s controlled by the computer. The coil-on-plug ignition coils are the most recent of the designs and started in the 2000s.

Coil-On-Plug Ignition Coil: How to Diagnose Code P0300

  • In what order does your engine fire?

Knowing the firing order is important because this tells which cylinder is number one, which is number two and so forth. Once you get the engine error code you will know which cylinder is reading as bad.

  • Remove the Ignition Coil.

You now know which cylinder is bad so let’s remove the ignition coil from that cylinder. Remove the mounting bolt for the coil, disconnect the electrical connector, and remove the ignition coil from the bad cylinder.

  • Look at the Ignition Coil. Is it bad?

What kind of shape is the coil in? Is it scorched? Are there white deposits? Do you see corrosion, cracks, degrading in the plastic? Any of this is a sign that the coil should be replaced.

  • Look at the Spark Plug. Did it go bad?

Do you see black soot, white deposits, or any other signs of damage, wear and tear, or scorching? If so, the sparkplug needs to be replaced.

  • Testing the Ignition System
  1. Plug in your OBDII and fire up the car. This handy scan tool will tell you if any other scannable issues exist with your vehicle’s engine.
  2. If you don’t have an advanced scan tool, you can test the firing process manually. Test the ignition system by swapping the plug and coil that tested bad. Remove the ignition coil and spark plug from the bad cylinder and swap it with one from another cylinder that tested good. Now start it up. If the cylinder that tested bad is now fine but the other cylinder is acting up, simply replacing the bad sparkplug and coil should resolve your misfire.

Mechanic Tip: The process to diagnose and fix these is pretty easy. Just make sure that you reseat the spark plug properly so it tightens down on the correct threads and doesn’t cross-thread. DO NOT over tighten.

When it comes to DIY Mechanics, Partsology has all the engine parts you need to fix a misfire or rebuild an engine, from cylinder parts to engine rebuild kits. If your car or truck starts to misfire and you need quality parts to fix it without breaking the bank, shop now or contact us at (844) 800-6866 today.


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