2016 w/CVT- where's my gas going?!
 

I've had my 2016 LX (136k) for a year now and it's been solid. I average 34.5mpg mixed driving between Vancouver, WA and Portland, OR. 3 weeks ago I filled up my tank (87 octane, as usual) and within 2 days(!!) my tank was almost empty. The mpg display showed I was still getting 39.8mpg, but the tank was near empty, making me thing the gas isn't being burned in the engine, but it escaping elsewhere. There are no drips under the car overnight, no odor of gas at any time, and the car drives normally- same power, same sounds, nothing out of the ordinary except my fuel issue. It never popped any codes but I plugged in my OBD2 reader just in case. Nothing.
Took it to a local shop with excellent reviews and they said they couldn't find anything wrong with it except the spark plugs looked original and "a little wet" so they replaced those, cleaned the O2 sensor, and that's it. The problem is still there. Does anyone have any ideas? This issue happened literally overnight; fuel consumption was excellent for a year, then I went from 400mi per tank to 235.

Do you have a locking fuel cap? It's always possible that someone siphoned your tank, although I suspect it's tough in a Fit due to the distance from the gas cap to the tank.

very odd indeed. unsure how car reacts if gas "leaves tank" you would think MPG would decrease. but maybe not if just flow through injectors monitored. maybe fuel level sensor ? thats a logshot...i would agree with drew

i would check mileage when u park and when u come back. a few times

Many years ago a guy I worked with stopped at a gas station on his way home and filled up. He got home, parked his car as usual on the driveway. Next morning he went to work, but didn't make it. He ran out of gas a block from his work place. Best guess, they had followed him home from the gas station and returned overnight and siphoned his tank. Now a Fit is a small car with a small gas tank but it still might be enough to satisfy someone, especially when you've just filled it.

Catalytic Converter Thefts. Now Gas Thefts?
 

It's bad enough to worry about someone stealing our Catalytic Converter. Now we have to worry about gasoline theft?

The fuel cap isn't locked, but it does have a door. Besides, the loss of fuel is gradual and even, so it's not being siphoned. I've already gone through two full tanks and am on my third (only 1/3 tank left and only 115mi driven). I've got to make another appointment with a different shop. Hopefully, they'll have better luck. I feel like diagnostics is becoming a more nuanced craft as cars have become increasingly more electronically controlled.

Not surprised here at all when gas is $5+ per gallon here in SoCal.

Outlier guess: Your injectors are failing. That's why the spark plugs looked wet and your mileage has decreased. Surprised you haven't got code p0420 yet - raw fuel being dumped into the cat.

If you get p0420 before 150K miles, there's a chance your vehicle is covered under an extended warranty for the injector/rail assembly:

https://static.nhtsa.gov/odi/tsbs/20...08198-0001.pdf

That's an awful lot of gas to have lost in the engine in a couple days. Codes would have had to have been tripped on the dash by now if that was happening. Fuel leak is possible. It would probably only be leaking when the engine is running and so might not be noticed when the car is parked.

+1 on woof ...and bargainguy.....wow so odd,never heard of this

If it's going through the engine your catalytic converter should be almost glowing red with that much unburnt fuel going through. Is the exhaust hot?

Skip forward to the 7:50 mark of this video. Same problem as OP but different car.

plugging in the OBD scanner, can you see the "long term fuel trim" maybe not without freeze frame data, and you have no codes.

if the fuel trim is wayyy minus, i would guess it is a leaky injector.
if it isnt, its probably NOT the injectors.

just a thought.

Thanks for the input, everyone. I'm thinking if it was a bad fuel injector(s), it would happen more gradually, so I'm guessing bad sensor (MAF), bad FPR, etc. I'm trying to find a reputable shop that can get me in after I'm back from my next vacation (Nov 4-19), so I won't know for a while what's going on.

i wouldn't think the gas can be siphoned that easliy , it's quite a distance from the filler to the tank , probably some kind injector or fuel management fail . surprised it hasn't thrown codes and
the cat has to be taking a beating . and the oil is probably quite contaminated by now

yea....i was thinking about the oil too....i would CHECK the oil to make sure it isnt overfilled, that could be a real PROBLEM later on

The gas filling long neck to the gas tank tends to issue a full tank to the gas nozzle, ie, it will click sensing that the tank is full but it is not. I have overcome that problem by topping off (I don't need any opionions, thank you). I continue to fill very slowly until the gas is topped off. The spec of the tank I believe is 10.5+/- gallons. However, I have fill the tank up to 11.3 gallons. Since new, the mpg display is off, and getting worst from 2-4 mpg lower than my calculations. The way I calculate mpg is by using the ODO miles traveled between "full" tank of gas. To get an accurate calculation, you must top off the tank. You cannot use the nozzle click as the indcator of the gas tank is full. Most of my driving is highway and using the cruise control. I have been getting over 40 mpg all the time. If tail wind is favorable, I can get over 500 miles from a full tank of gas. The current ODO is about 128K miles, and because I rarely drive in traffic, I have not had a brake job since new. 2017 Fit LX CVT.

I think most of us recognize that the fuel economy value provided by the car can be off (mine reads both high and low but is generally within 1 mpg), and pretty much anyone interested in tracking their fuel economy does their calculations at the pump.

As for needing to fill the tank + the fuel neck to get an accurate fuel efficiency calculation, that's a bit like saying that I need to tell you the hour, minute, and second I was born for you to know how old I am. It's technically correct, but in practice that level of precision is generally not necessary. It's certainly not helpful in answering the OP's questions, as he or she isn't asking us why their fuel economy dropped from 39 mpg to 37 mpg, but instead they're wondering why their fuel economy has been cut nearly in half.

Your proposal also assumes that the fuel level when the pump shuts off the "full" tank varies widely, which I don't think is true based on how many miles my Fit goes before the fuel gauge hits certain points (e.g., moves off of Full, gets to 3/4, etc). That number of miles is very consistent on my regular long drives where I fill up at the same locations, suggesting that any difference in when the "full" click occurs at the pump is quite small and not relevant unless you really must know that you got 39.785 mpg vs me knowing that I got 39.8 mpg.

I see your point, but typically the shut off is around 8-9 gallons. I disagree with you. I know some nozzles are more sensitve than others. If that is so, then you are just guessing the amount of gas you used between fill up. You may be off 10-20%.. Why would you accept that estimate when you can get a reasonable accurate mpg calculation if that is what you wanted in the first place? Topping off eliminates one important varable in your calculation. I also found that the Honda reported mpg depends on how much gas you put in between filling the tank. If you want an estimated mpg, that is fine. But there are engineers who wish for precision. We didn't get to the moon with ballpark estimates. If someone tells me he is losing gas when there is no leak, knowing the amount of gas is in the tank would be my first concern. How did you determine that the tank is full?

You're saying that the pump typically shuts off with 8 or 9 gallons in the tank vs the 10.6 gallon capacity stated in my 2010 owner's manual? I've never seen that happen. Perhaps you have a problem with your fuel system (maybe from overfilling?), but I don't think that's a widespread issue.

You also say that the fuel efficiency reported by the car varies depending on how much fuel you put in the tank? So if I fill to 1/2 tank and reset the trip meter it will give me different fuel efficiency readings than if I get a full tank? That's a wild idea, but I'm going to give it a try because I love science.

I mostly do long trips in my Fit, I don't like to go much lower than 1/4 tank, and yet I routinely add 8.xx gallons when allowing the pump to choose when to stop. On a recent trip when I was distracted by story time with my niece and drove until the low fuel light came on (whoops) I pumped 9.84 gallons, which is perfectly in line with the idea that you've got about 1 gallon left when the low fuel light comes on.

I also get very consistent mileage, with less than 1 mpg deviation over repeated trips on the same route, and as I mentioned previously I can estimate to within 5 miles or so when the fuel gauge will leave full, get to 3/4 tank, etc. I'll also note that my calibrated Ultragauge reports fuel economy estimates that match those I calculate at the pump. None of that would be the case if I was seeing the 15-25% fuel delivery deviations you're suggesting.

To return to the OP, I'll reiterate that they aren't saying that they're losing 20% of their expected fuel economy or range, they're losing nearly 50%. Even if you're correct and their tank (suddenly) started shutting off at 8-9 gallons, their fuel economy and range would be dropping by about 25% maximum.

As for filling the fuel neck as you're suggesting, that's not a great idea. You can damage the fuel vapor recovery system and associated charcoal filters. That repair will buy you a decent amount of fuel. You're also much more likely to spill on yourself, the side of your car (there goes your pristine wax job), or the ground. You also need a bit of empty space in your fuel tank to allow for thermal expansion (e.g., fuel comes out of a tank in the ground at 50F, then you park the car on asphalt in the TX sun), but you're an engineer so I'm sure you know about that. Finally, in the unlikely event that you get in an accident right after filling up, would you rather have your fuel contained in the tank in a (relatively, this is a Fit and everyone else is in a giant truck) protected location or in a thin tube snaking up to the rear fender?

You're correct that we didn't get to the moon with ballpark estimates. We also didn't get to the moon by using the crew capsule for extra fuel capacity.