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Q: Gas mileage down

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I had installed new spark plugs on my 2005 Toyota 4Runner V6. I bought these at the dealer and had a mechanic I know install them. Than I noticed days later the mileage had gone down from 18mpg in the city to 16.2. Why might this be since I bought these spark plugs new from the dealer.

My car has 114000 miles.
My car has an automatic transmission.

First off, spark plugs are not going to affect fuel mileage. Also, how are you checking fuel mileage? If you are using the indicator on your dashboard or overhead console, this isn’t perfectly accurate. The only accurate way is to divide the number of miles by the number of gallons of fuel it takes to fill up your tank. You must begin the mileage count with a full tank then do the division after you have filled it a second time.

With that covered, there are a few things that will affect mileage in this way. The number one reason for mileage change is a change in driving habits. This can be more aggressive driving to a different route to work or play. Anytime you are climbing a hill, you are getting mileage less than 10 mph.

The next factor to consider is the weather. Wetter weather means thicker air which means the motor will require more fuel to maintain the correct and optimum air-fuel mixture. Cold air will have the same affect as colder air is denser that hot air. On that note, warmer air will result in better fuel mileage.

The caveat to this is reduced power under hotter conditions and more power under cooler weather conditions. There is a trade off with everything in automotive.

If there is indeed a problem that has been created after a tune-up, it will have something to do with the cleaning of the MAF (mass air flow) sensor and or throttle body. It is possible, but not very probable that the MAF sensor resistance may have slightly changed enough to cause a mileage drop. It should be considered that a vacuum line could have been dislodged resulting in more air being allowed into the motor and therefore more fuel will be required.

Another possibility is an oxygen sensor may have been damaged with the cleaning of one the MAF or throttle body. It is not very likely, but can happen in theory.

Lastly, check tire pressure and consider whether you’ve added anything to the outside of your car, such as a roof rack, bike rack, etc. New tires or bad dragging brakes could also alter performance.

As you can see, there are many possible variables. If you need help having this looked at, a certified professional from YourMechanic can come to your home or office to give the car a complete inspection and can help determine why your car is getting worse mileage.

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