I Just Replaced My Spark Plugs And Now I Am Getting Bad Gas Mileage

Posted in Internal Engine Parts
  • I have a 1999 Mazda Protege LX 1.6L Automatic 125K. I was getting about 30-33 mpg after I replaced the spark plugs I am getting about 23-25 mpg. Now I realize that it is colder out (Wisconsin), but I don’t leave my car running to heat up b/c that doesn’t actually work. So now what do I do to improve the gas mileage. Why is it getting bad mpg?
    The air filter is good, fuel filter probably needs replacing, but are there other things to do to get gas better gas mileage? Are there additives?
    Please any ideas are appreciated.
    I did not change the plugs b/c they looked fine. I know that they on right b/c the protege has a unique engine. I did get Bosch Platinum Plus spark plugs that do not require gaping. I did notice that it has misfires so maybe they aren’t appropriate for my car. I will go to AutoZone or Oreily’s to check it.

    There are 9 Answers for "I Just Replaced My Spark Plugs And Now I Am Getting Bad Gas Mileage"

    1. Undercover_cosmotelogist says:

      Did you replace the spark plug wires too? and you should change your fuel filter. use a fuel injector cleaner next time you fill your tank up

    2. John E says:

      Sounds like you switched one of your plug wires.

    3. Dennis says:

      If your car is not coming up to full temp.(because of a faulty thermostat) it will run rich and therefore poor MPG. It must get to 195 degrees.

    4. Bud says:

      ARE YOU SURE YOU HAVE THE CORRECT PLUGS? SOME AFTER MARKET PLUGS DON’T WORK WELL. ARE THEY THE CORRECT HEAT RANGE? DID YOU GAP THE PLUGS PROPERLY? I’M GUESSING AFTER MARKET(ANYTHING NOT OEM) WRONG HEAT RANGE AND GAPPED WRONG.THATS THE MOST COMMON MISTAKE

    5. Hank Scorpio says:

      You probably didn’t gap them right and/or are using plugs that the engine doesn’t like. Are you using autolite plugs by any chance?

      I know people run off to walmart and buy the plug packages off the shelf but those are garbage plugs. Just buy a set of ngk copper plugs from an auto parts store and put them in. Don’t mess with the platinum or iridium plugs you aren’t going to get a significant boost in gas mileage from using them and in a lot of engines they’ll cause misfires.

    6. Denfasr says:

      The warmer it is outside the better your vehicle will run. Yes, weather conditions will have allot to do with fuel mpg. However, Are you sure you used the correct plugs? and were the gap’s set correctly? Are the plugs fully seated? —- I would guess someone did not gap the plugs properly or used the wrong plugs. Or you had a hotter plug in the vehicle and replaced it with a standard plug which will reduce the mpg.

    7. Bandit_60 says:

      Did you gage the plugs before putting them in? it,s very important that all the plugs be gaged the same. if the gap on the plugs are too wide then the spark has too far to jump and then it takes more gas.

    8. Nick says:

      Your car may require iridium plugs, and i know they arent cheap, they are what the manufacturer uses to get their economy ratings for the car so they will pay for themselves

    9. Paul says:

      “Sounds like a Common Mistake” Miss Routing Some of the Plug Wires.

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