Gen 4 engine swap worth it?

Discussion in 'Gen 3 Prius Technical Discussion' started by butchbs1985, Oct 25, 2025.

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  1. Mendel Leisk

    Mendel Leisk MMX GEN III

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    When you enter a zip code, the main Toyota Parts website will display dealerships near you. The link (somewhat subtle) I posted in #13 is to a US dealership website most convenient to me (just over the border in Washington State):: they're price, for the brand-new short block (which WILL have the revised pistons/rings) is $2129~ USD.

    upload_2025-10-25_14-16-10.png

    Short Block Assembly #11400-37140 | Autoparts.toyota.com
     
    #21 Mendel Leisk, Oct 25, 2025
  2. butchbs1985

    butchbs1985 Taking things apart is fun!

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    Yeah. I figured if I'm going that route I may as well do the head assembly too so added that to my cost. This is my point of comparison. Hoping for a call back Monday from Hybridpit too
     
    #22 butchbs1985, Oct 25, 2025
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  3. butchbs1985

    butchbs1985 Taking things apart is fun!

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    Well I didn't even bother borescoping it. Started it to move into the garage and get sweet white some so... There's for sure an issue as suspected.
     
    #23 butchbs1985, Oct 26, 2025
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  4. Mendel Leisk

    Mendel Leisk MMX GEN III

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    As Dylan said, it don' take a weather man, to know which way the wind blows.
     
    #24 Mendel Leisk, Oct 26, 2025
  5. George Jung

    George Jung Junior Member

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    Ive done gen 4 swap, I wouldnt say i had "no Issues", but after alot of tears everything is working good. I dont have issues with over heating. If i did it again i think i know now how to do it easily. This was the first time I ever attempted a repair like this, In fact this prius is the first car ive ever worked on, and that was just a year ago when i cleaned my egr and air intake. I think i paid like $1300 for it from lkq. I think its a good option, much simpler than buying a new block and switching over the head. Plus my engine was burning a lot of oil and i didnt know if it was the piston rings or the valves causing it.
     
    #25 George Jung, Oct 29, 2025
  6. butchbs1985

    butchbs1985 Taking things apart is fun!

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    It's been a while but I'm back and I'm seeking help.
    I did end up buying a reman engine from Hybrid Pit and that experience was great!

    Finally got time and warmer weather this weekend so I did the swap which was physically not a terrible process though galvanic corrosion is certainly a thing.

    Today I had help from my brother to get the engine back in and he stuck around and helped reassemble. We got it back together with a couple challenges but eventually fired it up in test mode and.... It started but it wasn't happy. Ran OK with throttle applied.
    Checked codes: code 1 indicated MAF sensor high input. OH Crap. Forgot the connector on the air box.
    Fixed that and fired it up again and it was similarly rough but ran.
    Checked codes: code 2 indicated Manifold pressure high input. Pulled the air box and that sensor wasn't fully clipped.
    Reseated that and test mode again... Now. It turns over then gets slower and quits. Code P3191
    Disconnected 12V just to reset everything and got dinner.
    Tried again after dinner with the same result but didn't check the codes.

    So I'm well aware that two critical systems weren't properly functioning and that's mostly on me. Not to make excuses but the power went out during final assembly and we finished wiht one shop light on the generator. I'm looking for constructive stuff not 'you're a bonehead' because I know that already.

    Other possibly relevant information:
    Before the work, the 12V was dead after sitting for a couple months. It's a nearly brand new Interstate battery but I know my kids left the rear hatch open for quite some time. I noticed that and at the time, the locks at least were still responding. I had a charger on it all day saturday. I have no placed it on a charger again and it's drawing 6A using the under hood terminals.

    Power's back on now (just as I finished wiring up the generator for 12 hour predicted outage...) so here I am trying to unwind.

    Thanks for any direction. My next thoughts are to disconnect the manifold pressure sensor and see if it starts again?
     
    #26 butchbs1985, Dec 29, 2025 at 1:52 AM
  7. ASRDogman

    ASRDogman Senior Member

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    You are not a bonehead, you had difficult situations when working.
    It is easy to miss something even under ideal conditions. Everyone has done that.

    Go back over EVERYTHING, slowly. Make sure everything is connected. Check the pvc hose,
    that's easy to forget to connect.
    Check for bent pins on the connectors, loose hoses, connections are connected to the correct spot.
    Check that the injectors are fully seated.
    If everything was functioning before the engine swop, it's likely something is not connected correctly.

    When you are certain everything is correct, You can check that the timing is correct for the engine.
    Put the crank at TDC, remove the valve cover gasket and check the timing. If the marks are not on their
    marks, you may have to rotate the engine another turn. The shiny mark likely won't be in there spots,
    butt the cams should.

    I know they do a good job, but they are not perfect, and could have possibly missed it.
     
    #27 ASRDogman, Dec 29, 2025 at 7:43 AM
  8. Brian1954

    Brian1954 Senior Member

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    As ASRDogman already mentioned, something did not get reconnected properly on the installation of the engine. My first guess would be vacuum hoses. Remove the air box again and check that the 4 to 5 vacuum hoses are all connected. Those hoses seem to be the number one reason why the engine will not start.
     
    #28 Brian1954, Dec 29, 2025 at 12:05 PM
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