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cylinder misfire p303 and 304

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by awigator, Feb 1, 2023.

  1. awigator

    awigator New Member

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    I was driving the other day and my car bumped a curb with my rear wheel as I was leaving a parking lot. It wasn't bad, but right afterwards I noticed a shuddering and the check engine light eventually came on. It only shudders when idling and driving 20 mph and below. The check engine code is p303 and p304, so misfires in cylinder 3 and 4. I've looked up the general causes for such a misfire to see what it might be other than the spark plugs. But I havent checked the spark plugs since the engine is set so far back i need to remove some of the windshield wiper area to check them properly, which seems like a much more elaborate task. I figure the car needs new plugs soon anyway since I bought it 7 years ago, but I dont know if that will be sufficient to fix the problem.

    Anyhow, I'd love to save money by avoiding a mechanic, but Im not sure what to check other than the spark plugs. I dont see any obvious issue with the wiring. I am trying to figure out what might cause 2 cylinders to misfire at once and what cause might be potentially related to a jolt in the rear right wheel since that seems like the proximate cause, although it might be totally unrelated and just coincidental. I dont know if its worth replacing the plugs and then if it still doesnt work, checking other potential problems.

    The car has been having a couple other more minor problems. I think the 12volt needs to be changed too since the car has issues starting sometimes, but that's a much easier task than an engine misfire obviously.
     
  2. tankyuong

    tankyuong Senior Member

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    would start with replacing 12v battery
     
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  3. Tombukt2

    Tombukt2 Senior Member

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    You need to open up the spark plug holes and look at your coils see how much red city deposit you have on the coils that's generally engine coolant and see if it's the same and all four bores ? Usually the one with the most dust the red stuff in the hole is the one doing the most leaking generally speaking. If you pull the plugs out one by one look for small water droplets on the plug and the smell of coolant you know generally find your leaker.
     
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  4. PriusCamper

    PriusCamper Senior Member

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    Almost everyone I've worked for who gets an engine misfire codes doesn't know/doesn't want to learn how to check their oil level and once you get down to 2 quarts or less oil remaining the crankshaft bearing will vibrate enough from a lack of oil to trigger a misfire error code.

    Also sudden braking, or going around a sharp corner real fast, or even hitting a curb will cause the red triangle to flash from the oil pump not having enough oil to pump.

    Once you've confirmed your oil level is ok and your 12v battery is fully charged the next step is moving your ignition coils around to see if the problem travels to a different cylinder with the ignition coil or if it stays in the same place. If it travels, then you replace that bad coil, if it doesn't then next step is to do the same experiment with your fuel injectors. Though bench testing the injectors is more ideal if you can find someone who has one those.
     
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  5. awigator

    awigator New Member

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    thanks guys! Does a low 12 volt cause an engine misfire? The websites I checked didn't list that on the general causes. I might try getting an oil change and switch my battery before checking the spark plugs and the ignition coils.
     
  6. rjparker

    rjparker Tu Humilde Sirviente

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    This is typical of a head gasket leak. Usually a bad plug or coil will continue to create problems at higher speeds. However, most people want to change the plugs and the coils (which are not cheap) before they do a borescope inspection. I would find someone who has a quality borescope and experience using it. Often the leaky head gasket causes a steam cleaned piston.
     
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  7. rjparker

    rjparker Tu Humilde Sirviente

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    That is because a low 12v battery won't cause trouble when the hybrid system is in Ready. At that time the hv battery, motor generators, regen and the inverter takes the load.
     
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  8. awigator

    awigator New Member

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    update - the 12v in my car seems to be near dead now. going to replace it as soon as I can afford a new battery and I'll see if that fixes the misfire too.
     
  9. awigator

    awigator New Member

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    one more thing, i havent checked the condition of the spark plugs yet since its a bit of an ordeal to get at them since the windshield wipers are in the way, and was hoping to wait until I had the plugs ready ... even if they're not the cause, they are probably due to be replaced soon since I'm at about 120k miles
     
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