Q: Misfires on cylinder 3

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My check engine light started coming on last week. I pulled the code: P303 - Misfire on cylinder 3. The car has a little less than 150,000 miles on it. The spark plugs were replaced near 100,000. The valve cover gasket was leaking near cylinder 4 at the time. That got fixed under warranty. About 20,000 miles later, the same gasket was leaking, so it got replaced under warranty again. Now it's leaking again. I've tried the following to fix the misfires: Swapped the coils between cylinders 2 and 3. I reset the codes, but still got a new P303 reading. I tried swapping the spark plugs, and got the same code. I tried replacing my spark plugs with ones from a different manufacturer. Later, I swapped out my fuel injector, but STILL got the same code. I used grounded plugs with the coils on 2 and 3 last night with a friend, and the sparks look identical and clear. I tried driving 20 miles, and while the plugs were still warm, I did a compression test. (175, 165, 120, 180). Could my cylinder 4 coil be soaked from an oil leak, and would that cause a misfire? I never checked cylinder 4, and I'm currently driving a different car. Do I need to check out the 4th coil and maybe try swapping it out? Or might something else be going on?

Hello. Instead of only replacing the coils between cylinders 2 and 3, I think you should replace all of the coils with new coils, if you haven’t done so yet. The results of your compression test show that you have good compression on all of the cylinders, so it’s pretty safe to say that you don’t have an internal engine problems.

Once you change all of the coil packs, it would be a good idea to inspect the wiring going to the coil packs and fuel injectors. It would also be wise to inspect the wiring harness going from the injectors to the Powertrain Control Module (PCM), and wiring harness going from the coil packs to the Powertrain Control Module (PCM).

It is also possible that the Powertrain Control Module (PCM) may be going bad. This could be the answer for why the spark plugs aren’t firing like they are supposed to. Before considering whether or not the Powertrain Control Module (PCM) is the problem, your first step should be to have your coil packs replaced by a certified technician.

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