Idle issue after Gen3->Gen4 engine swap

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by ilkehylke, Oct 22, 2025 at 5:46 AM.

  1. ilkehylke

    ilkehylke Junior Member

    Joined:
    Jul 7, 2023
    5
    1
    0
    Location:
    Finland
    Vehicle:
    2011 Prius
    Model:
    N/A
    Hello,

    I recently did the engine swap procedure on my 2011 Prius. The car runs and drives, but when idling or pressing the gas pedal when in P the idle is unstable (it sounds like you're lightly tapping the gas pedal repeatedly).
    When driving, I didn't notice this behaviour flooring the gas or under normal acceleration, but the engine is pretty quiet when normally accelerating anyway.

    The engine was taken from a 2016 XW50 and the engine code on it is X2ZR-Y22. The mechanic also had to replace the thermostat housing with a 16331-37100, because the one on the new engine didn't fit.

    As far as I know, they reused everything from the old engine, including the EGR. Or at least I pretty clearly told them to.

    What would be the cause of this issue? How to fix it? Is driving the car like this a stupid idea, or just a minor annoyance?
     
  2. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    113,532
    51,654
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    i thought using the gen4 egr or something was a problem?
     
    #2 bisco, Oct 22, 2025 at 1:47 PM
    Last edited: Oct 22, 2025 at 1:54 PM
  3. ilkehylke

    ilkehylke Junior Member

    Joined:
    Jul 7, 2023
    5
    1
    0
    Location:
    Finland
    Vehicle:
    2011 Prius
    Model:
    N/A
    Would that be the cause? As far as I know, they modified the Gen3 EGR and installed it.

    I know that using the Gen4 EGR can cause problems, but I didn't find anything actually detailing what kind beyond the vague claim of "driveability issues".
     
    bisco likes this.
  4. Mendel Leisk

    Mendel Leisk MMX GEN III

    Joined:
    Oct 17, 2010
    59,772
    41,091
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    He's saying it's still got 3rd gen EGR, I think.
     
    bisco likes this.
  5. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    113,532
    51,654
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    i'm basically saying that there have been problems, but maybe they have been sorted, idk.
    everything i can find talks about cooling, not rpm's.
     
    Mendel Leisk likes this.
  6. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    113,532
    51,654
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    looking around, there seem to be a lot of causes of fluctuating rpm's. maybe start with a hybrid battery health test.
    but not knowing the history of the gen 4 engine, it could be any of the likely suspects.
    have you checked all the vacuum hose connections? idk what would throw a dtc, and what wouldn't.

    strange-engine-behaviour-like-revving-while-stopped-at-light.194305
     
  7. ilkehylke

    ilkehylke Junior Member

    Joined:
    Jul 7, 2023
    5
    1
    0
    Location:
    Finland
    Vehicle:
    2011 Prius
    Model:
    N/A
    Funny that you mentioned this, because there were a multitude of issues with the shop, and the whole process took around three months because of that.

    I should probably mention that there was a transmission issue (DTC C2303) which another mechanic managed to fix by straightening some pins to my understanding.

    Anyways, for whatever reason, the hybrid battery actually got discharged during the engine swap and the car didn't start even after fixing the transmission. They charged the cells and it starts now, this being the only problem that remains.

    I'm interested if the car is drivable like this without causing permanent damage, while I try to sort out this problem because in the long run it probably won'g be good, even if it was a minor issue.