2003 Prius - Multiple Misfire - Just needed new spark plugs

Discussion in 'Generation 1 Prius Discussion' started by KitsuneVoss, Jun 3, 2025 at 2:06 PM.

  1. KitsuneVoss

    KitsuneVoss Member

    Joined:
    Nov 13, 2022
    288
    44
    0
    Location:
    Virginia
    Vehicle:
    2006 Prius
    Model:
    N/A
    This is a problem seemingly already solved but wanted to post it for others

    My 2007 Prius is down because the engine is effectively gone.

    I took my 2003 to work as a result Saturday night.
    Seemed okay although seemed a little lacking in power.
    On my way from work in the morning was misfiring like crazy.
    Strange how it went from okay to misfiring like crazy.

    Decided to start small and just replace the spark plugs.
    They honestly seemed okay which made me wonder.
    Still, I replaced them and seemed to now run fine and even slightly improved MPG.
    Driven three times since then and a total of 35 miles with speeds up to 59 mph and seems fine.

    Anyone understand how this could cause not just one misfire but multiple misfires?
     
    #1 KitsuneVoss, Jun 3, 2025 at 2:06 PM
  2. Trombone

    Trombone Active Member

    Joined:
    Oct 26, 2019
    286
    100
    0
    Location:
    Allentown, Pennsylvania
    Vehicle:
    2002 Prius
    Model:
    N/A
    How many miles on the old plugs? These iridium-tipped Denso plugs are rated for 90K miles, IIRC. I'm on my third set with my '02, just changed them at 170K miles, a little ahead of schedule but car was in the shop already for coolant change so I had the plugs done also. Something else, unrelated to the plugs, may have caused the misfire. How often do you drive the '03? Does it sit unused for long periods? Could be related to fuel flow, dirty MAF/engine air filter, although these don't usually self-correct. Were you able to pull any trouble codes? If DTCs were set, they may still be in the computer. If you don't have a code reader, you can take it to an auto store and they will read them and give you a print-out.
     
    #2 Trombone, Jun 3, 2025 at 2:22 PM
  3. KitsuneVoss

    KitsuneVoss Member

    Joined:
    Nov 13, 2022
    288
    44
    0
    Location:
    Virginia
    Vehicle:
    2006 Prius
    Model:
    N/A
    Codes I believe were multiple misfires, misfire cylinder two, misfire cylinder two, misfire cylinder four.
    No misfire cylinder three I believe but I believe two with cylinder two.

    As far as driving, not a ton. It is now, or was, more of a backup car.
     
    #3 KitsuneVoss, Jun 3, 2025 at 3:24 PM
  4. Trombone

    Trombone Active Member

    Joined:
    Oct 26, 2019
    286
    100
    0
    Location:
    Allentown, Pennsylvania
    Vehicle:
    2002 Prius
    Model:
    N/A
    This sounds, to my amateur ear, like a fuel issue. Check the gas cap for tight seal and fill up with fresh gas. If the car doesn't get run much, it probably means you have old gas in the tank. Maybe even some moisture gets in, which could cause the engine to stumble.
     
    #4 Trombone, Jun 3, 2025 at 3:31 PM
    Brian1954 likes this.
  5. KitsuneVoss

    KitsuneVoss Member

    Joined:
    Nov 13, 2022
    288
    44
    0
    Location:
    Virginia
    Vehicle:
    2006 Prius
    Model:
    N/A
    I am going to add a fuel injector cleaner when I next refuel the car as well
    Until I can get another engine in my other car, likely to be my primary car.
     
    #5 KitsuneVoss, Jun 3, 2025 at 3:52 PM
  6. Trombone

    Trombone Active Member

    Joined:
    Oct 26, 2019
    286
    100
    0
    Location:
    Allentown, Pennsylvania
    Vehicle:
    2002 Prius
    Model:
    N/A
    Go for it! They're great cars. I don't see many of our model in the wild around here, but a couple of days ago I ran into (figuratively speaking) one, an '03, parked right next to me! The young woman owner had acquired it from relatives and driven it all the way to Pennsylvania from Oregon! So you just never know....

    Keep us posted on your progress---good luck!
     
    #6 Trombone, Jun 3, 2025 at 5:43 PM
    Jakob likes this.
  7. KitsuneVoss

    KitsuneVoss Member

    Joined:
    Nov 13, 2022
    288
    44
    0
    Location:
    Virginia
    Vehicle:
    2006 Prius
    Model:
    N/A
    The one I have is a California model, blazoned on the side rear lower panel. Means it came all the way from California to Virginia.

    Also has an interesting feature where whenever you turn on the car, the headlights come on. When I replaced the original lights after I repaired it after a deer collision, roommate driving her in that case, I replaced the original bulbs with LED bulbs and thought was the cause but my roommate said it was always the case.

    Edit: I forgot to add that the spark plugs had not been replaced since we had the car so how long they were in the car was unknown.
     
    #7 KitsuneVoss, Jun 9, 2025 at 8:29 AM
  8. Trombone

    Trombone Active Member

    Joined:
    Oct 26, 2019
    286
    100
    0
    Location:
    Allentown, Pennsylvania
    Vehicle:
    2002 Prius
    Model:
    N/A
    That headlight behavior is normal in these cars. They have daytime running lights (DRLs) which cannot be turned off when the car is in gear. Regardless of whether the headlight switch is "on" or "off," the DRLs come on when the car is in Ready mode, the gear shift is engaged, and car moves. (I leave my headlights "on" all the time anyway; they shut off automatically when the key is removed from the ignition.)

    BTW, with car stopped and key "off," the DRLs remain on until you open the driver door. Not knowing this, I ran down the 12V battery when parked waiting for my wife at a doctor appointment. Fortunately I have a 12V power supply/tire inflator and can boot the hybrid system through the accessory socket, which I wired to be always "live" (using a fuse box jumper kit from coastaletech.com, an early supplier of after-market mods for the gen.1).

    As for LED headlights in a gen. 1, please read this thread: ANYONE actually install LED headlights in Gen1? | Page 2 | PriusChat
     
    #8 Trombone, Jun 9, 2025 at 12:08 PM
    Last edited: Jun 9, 2025 at 12:29 PM
(You must log in or sign up to post here.)