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The long return to 48 MPG and a random misfire

Discussion in 'Gen 2 Prius Technical Discussion' started by evallded, Dec 27, 2017.

  1. evallded

    evallded Junior Member

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    I am looking for some advice on a random misfire I am getting. At this point it is not throwing a code but my Mini VCI is picking it up anyway. The health check is coming back with engine and ECT in yellow with an empty DTC bar.

    The live data comes back with no codes again, but with a misfire counted 14 times total the first time and similar the second. mostly on cylinders 1 and 2 but also a couple on 3 and 4.

    The back story is I recently swapped out this engine for a used engine and trans with lower miles. After a few issues (a plug not all the way connected) we got her back on the road and running fine....we thought. But getting home we pulled up to 48 MPG on a 100 mile drive, for the later half we only pulled 37 MPG. With the car being higher mileage (222000) we decided it was time to do a few other replacements and did the cat packs, air filter, correct spark plugs, fuel injectors, brakes and struts. I have also cleaned the MAF sensor and done a few other things as well.

    I know the 12 volt battery is good, I am suspecting the coil packs but am doubtful considering it is happening on more than 1 cylinder. You can feel the misfire at an idle, especially when cold. Other than that though it seems to run alright but just not with the MPG I would expect.

    I am not sure what the nect step should be, with new plugs installed at the engine swap they were checked when home and were covered in black dry soot. Any suggestions?
     
    #1 evallded, Dec 27, 2017
  2. edthefox5

    edthefox5 Senior Member

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    Be aware misfires that move around are sometime caused by bottom end bearing failure. Usually from a bad lack of oil event or an overheat somewhere in its life. There is alot of sub harmonic vibration and it disturbs the crank/cam readings so the engine throws a misfire as crank/cam revolutions are not as expected.

    Put the car in Inspection mode and listen under the front of the car to the bottom end of the motor and see if you can hear suspicious sounds going on under there.

    Go on youtube and search Prius Inspection mode.
     
    #2 edthefox5, Dec 28, 2017
  3. padroo

    padroo Senior Member

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    Have you checked for vacuum leaks? Check to make sure you have no leaks around where the MAF sensor is also.
     
    #3 padroo, Dec 28, 2017
  4. Dxta

    Dxta Senior Member

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    Asides the misfires, do you feel some kind of power loss sort of?

    Y in asking this is because of the top cylinder head gasket has a little crack in between those cylinders you complained of, no matter what you do, the problem would still be there.

    Try a compression test on the cylinders and see what you get on the readings.

    Another thing I could think of it might be some sort of a partial harness shorts on the paths of the ECU to the fuel injectors/coil harnesses. Have those looked at.
     
    #4 Dxta, Dec 28, 2017
  5. evallded

    evallded Junior Member

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    I am not sure how to go about this. There really arent a lot of places to get an air leak on these as they have few connections really. I did inspect around the MAF before installing it but everything looked ok.

    There is no real loss in power, though it has been so long since I first got the car that I dont really remember what power it had so there could be some drop off, certainly nothing significant though. I too believe a compression test is in order.

    I have a stethoscope I can pull out to hear the lower end and see if there is a knocking or rumbling. I will certainly check that out.

    Thanks everyone for the replies, I have a few things I can look into now, though I do wonder since it is so slight if these could be the case. I think a fracture in the head gasket would only cause a problem between those 2 cylinders. I did replace the TB gasket when I did the engine as I kept my old TB. And as I mentioned there really arent a lot of places for an air leak to be otherwise, well not that I know of anyway.
     
    #5 evallded, Dec 28, 2017
    Raytheeagle likes this.
  6. Dxta

    Dxta Senior Member

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    Check the wire harnesses on those particular cylinders for the injectors and coils. There might be some kind of short somewhere, since it's an intermittent stuff.

    Lastly, you might believe, but the engine ECU module, or harnesses, could be a culprit.
     
    #6 Dxta, Dec 28, 2017
  7. chronon

    chronon Active Member

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    do you use Mobil 1 full synthetic oil ? k&n air filter ? tire pressure 40 or more psi ?
    Having these 3 things should give you over 50 mpg !!!
     
    #7 chronon, Jun 9, 2020
    Last edited by a moderator: Jun 9, 2020
  8. dolj

    dolj Senior Member

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    You realize the poster to whom you're replying posted that 2½ years ago and was last on here a year and a half ago?

    That said I don't think the things you mentioned would address his issues.

    Just a reminder to check the post date and take that into consideration before writing a reply.
     
    #8 dolj, Jun 9, 2020
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