What might be causing my gas mileage to drop?

January 20, 2019 4:14 PM

I have an old car that's had some work done on the fuel system recently (details inside) and the gas mileage has dropped quite a bit since that work. Any thoughts on what it could be?

I drive a 2004 Mazda 3 hatchback with approximately 54k miles on it. Automatic transmission and I'm the sole owner and do mostly fairly minimal city driving. It's never been *great* with gas mileage for city driving, but decent. I always reset one of the trip odometers when I fill up the tank and do a little mental math to see how the mileage is doing, but unfortunately never wrote anything down.

Earlier this year I went through a lot of trouble trying to diagnose a check engine light (code was related to emisssions leak). Did the cheaper stuff first - new gas cap, replacing a valve that is known to fail on Mazdas - and it kept coming back on. Finally my mechanic pulled out the rear seat and inspected the gas tank and voila! The plastic nut that connects the fuel pump to the gas tank had a huge crack in it. My mechanic replaced it (at no charge after the other failed fixes) and the check engine light has remained off. Passed my emissions test, too.

But since then the gas mileage seems to have dropped quite a bit - like, below 20mpg which I don't think I've ever seen with this car. Usually it's more like low-mid 20s for city driving and high 20s for highway. Unfortunately because I never wrote down my mileage when I filled my tank, I'm just going off memory and I can't tell if this has been a slow decline that just hit a point where I'm noticing it, or it is as steep a drop as it feels.

For the tank of gas that covered the time including the repair, I figured maybe I lost some gas to evaporation or whatever considering they had to be working on fuel system. But I just went through a second tank since the repair and got less than 200 miles for about 10 gallons of gas.

I know I can take the car back in to the mechanic to see what's going on, but I'm curious if anyone has any thoughts about what it might be. This car is starting to be a money suck lately (although not more than a car payment, and I have other financial priorities right now so on balance I still want to keep it) so I don't want to bring it in if this is more of a "your car is 14 years old this is what happens" kind of thing.

I'm wondering if it's possible the mechanic screwed something up with the initial repair, or if messing around with the gas tank may have clogged up or triggered failure in something else in the system as a downstream effect. Or is this just a coincidence?

We just filled up the tires about a week ago after realizing the pressure was really low. Any chance that low tire pressure could make the mileage that bad?

Also curious if it's ok to just drive around a car with unusually bad mileage for a while or if there are some possible causes that if left unfixed could be catastrophic for the car. In which case I'd want to bring it in to be checked out or start troubleshooting on my own. Thanks!

posted by misskaz to Travel & Transportation (9 answers total)

Yes, low tire pressure can reduce gas mileage. So can winter gas mixes and cold weather in general, so it’s possible that it’s coincidental that winter arrived while you were doing repairs (if you live somewhere where it’s winter now).
https://www.boston.com/cars/news-and-reviews/2016/01/22/how-cold-weather-kills-your-gas-mileage-and-what-to-do-about-it
posted by Kriesa at 4:28 PM on January 20, 2019

<Spins the wheel of car trubs...>

... Vacuum leak!
posted by sourcequench at 5:57 PM on January 20, 2019

We just filled up the tires about a week ago after realizing the pressure was really low. Any chance that low tire pressure could make the mileage that bad

Absolutely, yes.

Is this on just two tanks of fuel you are doing this? Because just being colder could affect that.
posted by Brockles at 6:09 PM on January 20, 2019

When did this last repair happen? Around November, they start formulating winter gas, which, in my experience, can knock 10% off your mileage, easy. That, plus if you let the engine warm up a little when it's cold out, can add up to a significant drop in mileage efficiency. And then if your tires aren't inflated correctly, and it's cold enough out that they're dragging significantly, that can really send your mileage to shit.

Then there's the other thing: a couple tanks is not a trend. Ten tanks is just starting a trend. You should not base any suspicions on less than five tanks.

I just ran some numbers on the past three years of fuel ups, comparing mileage November to March, and April to October in my 1994 Honda del Sol, doing my city commute five days a week. Now: I'm a sweat hog, so I'm using the a/c pretty much concurrently with that April to October time frame. Despite that - and on this little car, A/C is a major power drag - I still got 10% higher mileage summer gas vs winter gas.

In a former life, I had to drive from St. Louis to the bootheel notch in Arkansas and back, every week. Same vehicle. Cruise control with one of two exact same speeds, filling up at the exact same gas pumps. Designed experiment to determine the effect of tire pressure, and pickup tailgate up or down. Incidental recorded variables of head or tailwind speed and temperature. I had about 20 runs of each combination of variables. Temperature didn't do much, as long as tire pressure wasn't out of whack. Higher tire pressure (within reason and safety), tailgate up, summer gas, and tailwinds contributed positively to mpg, to the tune of roughly 4%, 5%, 8%, and 8%, respectively.
posted by notsnot at 6:17 PM on January 20, 2019

My thoughts are along the lines of ... fuel pressure was artificially low due to leak, so the computer had to adjust it's fuel tables to a richer mixture (more fuel) in order to actually SEE the correct combustion characteristics in its analysis of the exhaust gases (O2 sensor primarily). So it is programmed now to run a too-rich mixture and that is causing you to use more fuel than necessary.

Now that the leak is fixed, the computer will adjust its fuel demands back down, but it will probably be doing this slowly, gradually, in order to prevent a too-lean condition (not enough fuel = pinging, which is bad and the computer wants to avoid this).

I would personally try disconnecting the battery for a good half hour or more - this will erase the KAM (keep-alive-memory) that contains all the messed up fuel maps it 'learned' while you were driving around with a pressure leak. Now you can start with a clean slate and hopefully it will adjust much quicker since it's starting from a baseline that is hopefully less messed up than what was in the KAM.
posted by some loser at 7:45 AM on January 21, 2019

welp, the check engine light just came on. i bought a code reader and will pull the codes in a little while but seems like this is more than just an issue of low tire pressure and lean winter fuel mixture.
posted by misskaz at 8:30 AM on January 21, 2019

The check engine code is P0128. Code reader details are saying the engine coolant temp is 140 (my understanding is it should be closer to 180-200?), although it's REALLY cold today and the car seems to warm up pretty normally, heat works and everything. Any chance this is related/would affect mileage? I had the thermostat replaced maybe a year or two ago for the same code so I'd be really pissed if it's a stuck thermostat again.
posted by misskaz at 8:52 AM on January 21, 2019

Yes, the colder as car is, the richer it will run (more fuel per unit of air consumed by the engine), so economy will suffer. That code indicates a stuck thermostat according to what I can find.
posted by Brockles at 10:25 AM on January 21, 2019

Update: I was right, something was wrong, and the fuel pressure regulator was leaking. The leak got significant enough to smell gas and the mechanic found the problem right away.

Turns out I do know my 15-year old car pretty well.
posted by misskaz at 9:25 AM on August 30, 2019