Low Mileage after Battery Disconnection

Discussion in 'Mechanical Problems & Technical Chat' started by greenhorn, Nov 25, 2018.

  1. greenhorn

    greenhorn Active Member

    Messages:
    36
    Location:
    CT
    Vehicle Model:
    Civic Si
    Body Style:
    Sedan
    My 2014 Civic Si had its battery die recently, and i got it jump started. After the jump start, i drove around at fairly high RPM's for a couple of days with late shifts to ensure that the battery got charged.

    One unfortunate side effect has been that My average mileage has gone down from 24 to 19. i got 19 initially - I assumed that was because of the lead footed driving, but even after babying the accelerator around, I am still getting the same mileage. Any ideas? Also not sure if this is related, but the engine seems a lot rougher these days
     
  2. webby
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    webby Administrator

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    48,698
    Check your air filter to see how it looks. Have you replaced it recently? How's the air pressure in your tires? Remove the negative battery cable for a few minutes and then put it back on. Start the car and let it idle for at least 5 minutes. Turn it off and then drive normally for a while and see if you notice any difference in how "rough" it is. Winter additives will also be in your gas since september, so you can also expect 1-2mpg loss with winter fuel. Still doesn't account for the mpg you're seeing.

    how many miles do you have on the car?
     
  3. greenhorn

    greenhorn Active Member

    Messages:
    36
    Location:
    CT
    Vehicle Model:
    Civic Si
    Body Style:
    Sedan
    I will check the air filter - haven't checked it since I bought it in august. Checked tire pressure and saw it was around 30, pumped it up to 35 or so. Mileage is 62K. I'm driving the same 5 mile commute to office every day I always have been.
    Is there a proper ECU relearning procedure? How long does it usually take? My last car didn't have an modern ECU, so hadn't bothered about stuff like this till now
     
  4. webby
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    webby Administrator

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    Removing the negative battery terminal and the idling as I mentioned above is the relearning procedure. Most have said they let it idle 5-10min after reconnecting the battery.
     
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  5. squiggy
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    squiggy Cartographer

    Messages:
    10,735
    Location:
    Michiana
    Vehicle Model:
    '12 Civic Si
    Body Style:
    DBP II Coupe
    Even if you don't do this, it will relearn on its own just driving around for a bit. Can be a bit scary seeing the idle fluctuate when stopping/coming to a stop if you aren't expecting it.
     
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  6. webby
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    webby Administrator

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    ^ also true
     
  7. Cman46290

    Cman46290 Active Member

    Messages:
    47
    After removing my resonator a couple weeks ago.. o disconnected my battery for 20 minutes or so... then after reconnecting battery... I let it idle for about 15minutes. Its idled... and ran perfect since. Avg mpg right now is about 26mph... 90% city driving
     
  8. greenhorn

    greenhorn Active Member

    Messages:
    36
    Location:
    CT
    Vehicle Model:
    Civic Si
    Body Style:
    Sedan
    I don't think I let my car idle immediately after starting the first time after the reset. I think i revved It a fair amount of times because i was worried the battery was going to die again.
     
  9. JAMES JANZEN

    JAMES JANZEN Active Member

    Messages:
    67
    First off. Your alternator won't charge faster with higher rpm. Second I'm not sure I understand the question or the point of this thread .you drove the car harder than before and are wondering why your mileage is worse?
     
  10. greenhorn

    greenhorn Active Member

    Messages:
    36
    Location:
    CT
    Vehicle Model:
    Civic Si
    Body Style:
    Sedan
    My understanding is that higher rpm's provide higher current which is needed when a battery is dead and charging.

    My worry is that I drove my car hard for 2 hours immediately after jump starting, and then started driving normally afterwards and even after 2 months, the mileage is still poor
     
  11. JAMES JANZEN

    JAMES JANZEN Active Member

    Messages:
    67
    Alternators don't charge more with higher rpm. Computers adjust to rpm .If they did your battery wouldnt last too long on the highway . If you put a voltmeter on your battery at idle it will read around 14v. It will show the same at higher rpm.

    How many miles are on your car? Is it due for spark plugs. How's your air filter? Tire pressures? Do you live in a cold climate and idle your car more than before when it was warm? Have you disconnected your battery for a few hours and reconnected it to see if it changes the mpg?
     

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