Sudden drop in gas mileage, 2012 Elantra with 9000 miles

Does anybody know why all of a sudden the mileage indicator on my 2012 suddenly dropped about 5 miles/gallon, both highway and around town? The car has only 9000 miles on it. Has there been a service bulletin regarding the “Eco’” mode? Having the “Eco” mode on or off doesn’t seem to make a difference.

Got enough air in your tires? Is the parking brake off? Is your teenage child driving the car?

Have you actually measured the mileage drop by filling the tank and recording the mileage and doing this again with the next fill up?

Mileage indicators can be very inaccurate. So first find out if your mileage has actually dropped.

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In a newer car like that, I’d be suspicious of three things. First, the thermostat; second, the ECM’s coolant temp sensor; and third, the evap system. You might ask your mechanic to check those.

Thank you. GeorgeSan Jose sounds like he may be on the right track, but I’d love to hear other possible answers. Thank you for your responses.

Tires been rotated recently? Have you checked the pressure in the tires?


I assume the car still has new car warranty . . .

Have you spoken to the dealer about this problem?

Have you had any CEL’s (Check Engine Light)?

hit a monster pothole, or hit a curb hard enough to knock the alignement way off?

Under warranty, Hyundai replaced everything but the engine. The idle never smoothed out, and the better city mileage lasted about one month, then reverted back to 20 mpg!

Maybe the engine computer got re-programmed with newer software, which worked for a while, but then the engine-parameter learning function took over which reduced the mpg back to where it started. There are some problems that the engine computer can’t compensate for, like if the injectors are not balanced, and therefore each one doesn’t inject the same amount of gasoline as the others for a give pulse interval. The engine computer doesn’t know how to compensate for that, and that will affect the mpg probably. Same with the engine airflow, if one cylinder gets more air flowing into it than the others, that might affect the mpg, and would be difficult to diagnose. Engine compression is another one, especially if it is unbalanced. If you feel you aren’t getting the same mpg as before, fuel pressure & fuel injector & compresssion balance testing and fuel trim testing is where I’d look, presuming the basic stuff already posted above has been looked at.

George- you already replied to this thread, 5 1/2 YEARS ago!

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I thought about it some more over the years … :wink:

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Thank you, George. That’s the best explanation that I have received so far. This problem has also occurred in my newer 2014 Elantra
That’s why I raised the issue again. I’ll ask the dealer to check out these issues. Thank you SO much, George

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Well again, thank you very much, George

I just had the same issue with my 2017 Malibu hybrid. They’ve cleaned out the throttle plate. Hopefully, that may also help the issue with my 2014 Elantra. I’ll have my former mechanic do that as soon as the warranty is over, since Hyundai never addressed this issue.

Three different cars with the same problem suggests the issue isn’t in the cars.
What region are you in? What are the typical driving conditions (city vs hwy)?
Are you a gentle or “spirited” driver? Miles per year? Where do you get gas?

California; half city, half highway; old fart gentle driver; 12,000 miles/year; usually get gas from Costco or Arco. I’ve tried all of the name brand gases, and there’s no difference. I’ve even put BG fuel injecter 5 times in the tank, and no difference. Today, the Chevrolet dealer cleaned out the throttle plate and put the injector cleaner directly into the injectors; no help!

How long are these trips ?

If you’re only drive 5 to 10 miles at a time, you’ll never get good mileage, the engine is never getting up to proper operating temperature.

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Mileage is better on highway trips, but 10 miles/gallon less than the first 3 years.