Gen 2 2007 Prius: p0300 codes, misfiring only after driving for a little while.

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Paul Edward, Nov 5, 2025 at 5:15 PM.

  1. Paul Edward

    Paul Edward New Member

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    I've been stumped on this issue for a little while. My gen 2 prius will start up just fine, and drive for however long I need to (just the other day I drove it over an hour straight with no problem). Then, after stopping the car and starting it again, it misfires horribly on random cylinders and barely limps down the road. After sitting for an hour or two it will then return to normal. It's worth noting that the amount I drive makes no difference, driving five minutes or an hour will both result in the same outcome. However, if I drive on a steep incline then after a couple minutes it will start misfiring as well even if I don't turn off the car.

    I've replaced spark plugs, cleaned coils, swapped coils around, cleaned fuel injectors and connections as well. Also changed valve cover gasket and changed oil (thought maybe an oil leak was causing it). I've also tested my 12v battery.

    I'm thinking maybe it has to do with fuel pressure but I'm not getting any codes relating to that. Any insight would be greatly appreciated.
     
    #1 Paul Edward, Nov 5, 2025 at 5:15 PM
  2. PriusCamper

    PriusCamper Senior Member

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    Sure would be nice if the misfires could create some codes for specific cylinders. If it were my job I'd probably replace the coils and if that didn't work I'd probably clean or maybe replace injectors because aftermarkets ones are cheap.

    Also if you have access to Toyota techstream you could probably view fuel pressure and associated readings in real time as you drive and hopefully are able to recreate the symptoms.

    But in general this sounds so glitchy I'd put my money on it being electrical not mechanical.
     
    #2 PriusCamper, Nov 5, 2025 at 8:22 PM
  3. pasadena_commut

    pasadena_commut Senior Member

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    Fuel pump? When it restarts and runs poorly will it do better if starting fluid is squirted into the throttle? Going up a hill is going to require more fuel than just driving around normally, and if the pump can't keep up, the car is going to run rough. Or worse. I can't think of an electrical problem that would care that the car is aimed up hill, unless the HV battery is so bad that it hasn't got enough power to keep the 12V system at a high enough voltage. Easy to rule that out by plugging a voltmeter into the power port under the glove box.

    A better idea would be to use Techstream or something similar and check the short term fuel trim on the restart when it is not working right. That will also tell you what voltage "12V" is.
     
    #3 pasadena_commut, Nov 5, 2025 at 8:55 PM
  4. PriusCamper

    PriusCamper Senior Member

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    That would make sense... Of course if you have a fuel pump problem in a Gen2 you're going to have to replace the entire fuel tank assembly with a used one because there's no serviceable access to the fuel pump and there's no affordable way to replace with brand new.
     
    #4 PriusCamper, Nov 5, 2025 at 9:03 PM
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