15 mpg less after head gasket job

Discussion in 'Gen 3 Prius Fuel Economy' started by CharlesPrius, Aug 7, 2018.

  1. CharlesPrius

    CharlesPrius Junior Member

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    Had my head gasket replaced last week yay. Had thrown cyl 2 misfire code. Anyway that issue is fixed and no more coolant loss. I C Engine sounds good. However, before the job, even with the engine throwing codes and hesitating and losing coolant, I was still getting 52-55mpg. Since I got it back I'm only getting 38-40 on this first tank. I'm hoping it's just due to hot weather and a/c usage or very low tire pressure. But 15 is a huge dropoff. Could they have forgotten to hook up a wire? Or screwed with the throttle cable?
     
  2. ASRDogman

    ASRDogman Active Member

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    Could be many things. But maybe they drove the car some and/or let it idle to make sure it wasn't leaking.
    Check your next tank full and see what happens. Did they clean out the EGR system and intake manifold also?
     
    #2 ASRDogman, Aug 7, 2018
    Last edited: Aug 8, 2018
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  3. padroo

    padroo Senior Member

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    They probably had the battery disconnected and it has to go through its relearning procedure. Be patient .
     
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  4. cnc97

    cnc97 Active Member

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    As long as you have no Check Engine light, I wouldn’t worry about it too much. I just changed my engine. The first 20 miles, it was showing less then 30 mpg. I beat it up pretty good to make sure all was well. I topped it off with gas and reset the trip meter. I’m running 58.7 mpg at 200 miles post engine change.
     
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  5. bisco

    bisco cookie crumbler

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    how did you calculate 38-40? when i fill up and divide the miles driven by gallons pumped, it comes to an exact number.
     
  6. CharlesPrius

    CharlesPrius Junior Member

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    That's a good relief, thanks. I'm at about 55 miles since fillup and it's showing about 38mpg. GIven this is mainly stop-go driving and a/c on all the time (been unusually hot here).
     
  7. CharlesPrius

    CharlesPrius Junior Member

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    The computer readout/estimate (mpg) is what I refer to. Later I will do it manually yes, but the cpu has usually been within 1-3 mpg of that.
     
  8. bisco

    bisco cookie crumbler

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    how many miles have you driven since getting it back? did you reset the mpg when you got it?
     
  9. sam spade 2

    sam spade 2 Active Member

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    IF......your "before" figure was a long term average, it probably got reset and what looks like a sudden drop might not as bad as it looks.

    Give it some time.
     
  10. CharlesPrius

    CharlesPrius Junior Member

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    I would consider myself a dolt of the highest order had I not reset my trip odometer at my last fillup. So yes, the 36-38mpg I'm getting is for this tank only. So far it's at 70 miles. Hoping it's just a "the battery needs time to relearn / reprogram itself" issue and not something else.
     
  11. sam spade 2

    sam spade 2 Active Member

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    [QUOTE="CharlesPrius, post: 2765465, member: 153916" So far it's at 70 miles. .[/QUOTE]

    That is not enough time; not nearly enough.
    Get back to us after the first 3 tankfuls.......and a manual calculation. (y)
     
  12. cnc97

    cnc97 Active Member

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    Find a highway loop that you can easily access. 20-30 miles one way. Set your cruise and reset trip b. See if the calculated mileage is close to what you remember the car doing before the head gasket job. Also watch for the check engine light. It’s possible the when the head gasket was done the timing chain could be off one tooth. The intake VVT can adjust for that some in town, but the highway RPM would show it, and give you a check engine light.

    Did the shop do the job with the engine still in the car? I personally would not have done the job without pulling the engine, due to how hard it is to verify the timing marks.
     
  13. CharlesPrius

    CharlesPrius Junior Member

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    Thanks for the tip. I live in West Los Angeles so I might have to go to Arizona to find such a stretch of road (haha). No CEL yet, engine sounds strong.

    Pretty sure they pulled the engine. Head had to be machined a bit, and they sent it away for that. Though I wasn't watching while it was done.
     
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