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Hyundai Elantra 2007- Horrible gas mileage. Went from 32 mgp city avg to 15 mpg city Please Help

I have a Hyundai Elantra 2007 with 123,000 miles on it. Three weeks ago the engine About two weeks ago the engine light turned on. I took it into an auto parts store so they could use the digital code reader, it said the upstream O2 sensor needed to be replaced. I replace it and still have bad gas mileage, the second time I had it read with the code read it said both of the O2 sensors were bad so I replaced the downstream one week later. There was still bad gas mileage and the code reader still said it was both of the O2 sensors, the auto part’s representative said when the engine light is on that the computer turns off the O2 sensors so he recommended to erase the codes on my car with the code reader (I pushed the button and he looked away). Still nothing changed so I disconnected the negative terminal on the battery to restart the computer. Still horrible gas mileage with no engine light anymore. Gas mileage averages about 15 mpgs in city and 32-35 on the highway. The highway seems about right but I’ve never seen any mpg below 30 for city mpg before this problem. I also cleaned the mass air flow sensor with mass air flow cleaner after dealing with the O2 sensor. All preventive maintenance is kept up by me and I have had no problems with this car before. Water pump replacement, timing belt replacement, and transmission flush have occurred in the last ~50,000 miles.

I forgot to include that one week before the engine light turned on the rpms fluctuated between 1000 and 1500 rpms when stopped at a stop sign. Normal rpm with car in drive and stopped at a stop sign is 900-1000 rpms.

Also, the spark plugs are new and I have always used Mobil One extended synthetic oil. Usually replace it around 10000 miles.

What method are you using to calculate your “city” mileage? Is it cold where you live?

Winter gas will cause your fuel economy to suffer somewhat. How long did you leave the negative cable off the battery? I would leave the negative cable disconnected overnight if possible. That may get the computer parameters back where they should be. I did this on my 2005 Dakota a couple of months ago and it’s been running great since.

Your car is running rich. Something must be very off to cut your mileage in 1/2? An erratic idle is sometimes tied to a vacuum leak. The O2 sensor is not happy. IF engine is running rich, you may be dumping a lot of excess fuel into cylinders and potentially damaging your cat conv. Which is a bad thing. $$$. You should have a shop scope the car. Is idle rpm ok. Is exhaust mixture ok? Are all O2 sensors registering correct values during operation? Perhaps a good shop is next.

“I pushed the button and he looked away”

That parts store person doesn’t have much understanding of this system.
It’s OK to erase the codes.
The car’s computer will simply start testing things next time you drive and set the error codes again when it finds something wrong.
It only takes 30 seconds max to reset the computer by disconnecting the battery.
There are no codes that directly say a particular part is bad.
What were the codes? They look like PXXXX, for example P0123
There could be a bad sensor or a problem with the wiring to it.
It could be a sticky thermostat that’s keeping the engine from fully warming up.

Circuitsmith has a good idea about a sticking thermostat. How is the cabin heat? Is the temperature gauge reading where it usually does.

Also consider that the Coolant Temperature Sensor or its wiring may be faulty. If the CTS is always indicating that the engine is cold, the engine will be running in rich warmup mode all the time. If the wire to the sensor is unattached or broken, the Engine Control Module will also think the engine is super cold.

Hope this helps.

Were the plugs changed to correct the fuel mileage problem? You might put all the events in a time line that can be followed. The spark plugs that McParts stores push at you are usually a poor substitute for the originals. And when a McParts counterman reads “O2 sensor fixed” they might mistakenly think that the O2 is failing when the problem may be a leaky injector. The codes are of little use if they aren’t understood.

This could be caused by at least a dozen different things. First of all, it’s possible you’ve already fixed it but don’t realize it. After you change the O2 sensors, it will take some time for the ECM to adjust to the new readings. Give it a week or two and at least 200 miles since the last O2 sensor was change, then measure the mileage again.

Second, take full advantage of the car’s self-diagnosis feature. You’ll probably be dollars ahead to take it to a shop who has experts on advanced ECM parameter diagnosis, with knowledge of the procedures and access to the tools to do it. Get their opinion.

If it still isnt fixed, or you just want to go it alone … hmm … If this were my car, being the cheapo that I am, I guess the first thing I’d do is bring all the routine engine maintenance up to what the owner’s manual recommends. This is good insurance against a costly trial and error procedure that may ensue otherwise. If any engine/xmssion service is past due, have it done. Especially engine/xmission fluid changes, replace the air filter, and check the igition timing and idle speed that they are at spec. Then I’d go one by one through the vacuum devices and their connections, testing them for leaks with a vacuum guage/pump. I’d also measure the intake manifold vacuum at idle to make sure it is at spec. I’d check the EGR and PCV systems. Finally I’d measure the fuel pressure at the rail. I’d expect there’s a good chance the problem would be found by this time.

I’m having the same exact problem here, elantra 2007, 34000km, I just bought it used. Averaging 15mpg in the city, I can literaly see the full meter drop as I drive. It’s has been pretty cold these days (~-25C), I took it in a garage to get it check and everything looks ok, O2 is good, injectors are good, spark plug ok, idle speed ok, I don’t know what to do next…
If you find it please let us know!

If all else fail you might cobble up a snorkel to feed hot air from the exhaust manifold or the radiator shroud into the breather. The cold air intake may be counter productive at near arctic temperatures.

City mpg is bad, but hwy mpg is ok?

@RicBrown can you post the exact fault code?
Was the engine even going into closed loop?
An exhaust leak upstream of the O2 sensor could cause it to read the extra oxygen, in which case the PCM would interpret a lean condition and command the injectors to stay open longer.
Any other fault codes?
P0171 for example?
Was it an “insufficient activity” code? In those situations, the O2 sensor problem is sometimes the symptom, not the cause.
Has anyone verified that the ECT sensor is sending the correct data to the PCM? If the PCM thinks the engine is cold, it’ll command a richer mixture.


You might change your coolant for cooler temperatures 70%/30% ratio, and the normal 5W20 oil for Hyundai Elantras might not be rated for -20C. The thermostat could have gone bad early too. I’m probably going to change that next, I’ll let you know if it works for me.

At some point a methodical, by the book diagnosis needs to be done or one can guess forever.

The bit about the parts guy is pretty funny though.


Thanks for the tips! I was thinking that the thermostat might be stuck half open, leaving the engine too cold. I took a picture of my steady state operating temperature, is it normal that it doesn’t reach the halfway mark?

I just got the car so I don’t really know what the normal is (and I can’t contact the previous owner).

Any progress on your problem Rick?

I got myself an OBDII scanner and my coolant temperature looks OK, oscillating between 80 and 87C. You can see a log of a recent trip here at -10C :

I’m kind of lost, MPG on highway looks ok to me (around 7.5l/100km ~ 32mpg), but city driving is just killing me (~19mpg)

You city mileage could be a combination of factors, short trips in the city, cold temperatures, long warmups before driving and the winter mix for the fuel. Long warmups are the worst thing for the mileage. 2 to 3 seconds is enough, just drive easy for the first three minutes, but get moving right after you start the engine.

How sophisticated is ur scanner? It reads coolant temp? What city mileage were u getting before? U make it sound like 25-30mpg is ur former mpg. And u get 32mpg hwy now? So ur city and hwy mpg was similar before?