i have a 09 impala 3.9 i changed the spark plug wires and spark plugs. i’m getting the code p0300, not to sure what to do. i’ve checked the back spark plugs one was cracked so replaced that and you can tell it’s still misfiring. please help the car ran perfectly fine before with no codes. someone told me the coil could be bad. how is that even possible?
First just double check your work and make sure you have the right wires going to the right cylinders.
How the coil could even be possible is because you found a spark plug with a cracked insulator so it's likely that plug was shorting out. When that happens it can damage the coil for that cylinder (which is also shared with another cylinder so that's why the P0300 dtc)
You have a wasted spark type coil pack. 3 coils for 6 cylinders.
Your coil pack looks like this and it's labeled. The coil on the left is for cylinders 6&3. The coil in the center is for cylinders 5&2. The coil on the right is for cylinders 4&1.
So again, if one coil in the coil pack is damaged it causes misfires on multiple cylinders (P0300)
so was that spark plug cracked by me or from it possibly shorting out? thank you for you info! i checked the spark plug wires and they are right. the coil probably is bad. thank you again
also how would one of the spark plugs short out? would it short it out and damaged the coil from it being cracked or when it did short out it cracked the spark plug?
I can't say how the spark plug's insulator got cracked but a faulty ignition coil won't crack the insulator on a spark plug
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A faulty ignition coil will cause either No Spark, Weak Spark, or Intermittent No/Weak Spark
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However a faulty spark plug can damage an ignition coil and the spark plug doesn't even have to be physically damaged (like a cracked insulator)
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Sometimes all it takes is what we used to call "spark plug flashover"
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That's when the electricity from the ignition coil travels on the outside of a spark plug to ground instead of jumping the gap between the spark plug's electrodes . Electricity is lazy. It always chooses the path of least resistance to ground
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That's why spark plugs need insulators. We're trying to force the electricity from the ignition coil to jump the spark plug gap between the spark plug's positive and negative electrodes and create a spark in the cylinder to ignite the air/fuel mixture
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And like I said the insulator doesn't even have to be broken for this to happen
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Sometimes dirt, oil, or even moisture on the spark plug's insulator can cause this to happen and when it starts "carbon tracking" occurs and creates a direct path to ground on the outside of a spark plug
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An extreme case looks like this where both the plug wire boot and the spark plug shows carbon tracking:
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A more subtle example looks like this so you always have to closely examine the outside of spark plugs and the spark plug wires and boots on both the spark plug end and the ignition coil end when chasing down a misfire:
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@mikia “It ran perfectly fine before”
Is it possible that while changing the plugs a vacuum hose was disconnected or damaged with a resultant vacuum leak, thereby affecting the fuel/air ratio?
i really don’t think so but i guess there’s always the possibility. i will check it when i get home. but i’m pretty sure it was the coil that’s bad
Yes, @jack62 provided a very knowledgeable and informative response. Way above my novice level.
Always check your new spark plugs for continuity through the positive electrode before installing. In the past year or so, I have gotten three sets where at least one of the plugs was bad, namely, NGK brand.
