I have a 2010 Subaru Forester with manual transmission and about 70k miles. Until 2 months ago, mixed driving resulted in 27.6 mpg. Abruptly, following refueling, the mileage dropped to 24.5 mpg.
There are no error codes indicated. Correct tire pressure has been verified, all fluids have been topped-up, correct weight of oil, and filter, has been changed, air filter is clean, mass flow sensor has been cleaned and fuel injection fluid has been added twice to the gas tank. The engine runs/idles well and there has not been any loss of power. The initial bottle of fuel injector cleaner resulted in an increased gas mileage to 25.5 mpg. The second bottle of cleaner has not seemed to make any difference.
It would be helpful if you described exactly how you get the data to compute your mpg. If you are relying on the gallons as determined by when the pump shuts off, your problem might be the variability of the shut-off point of different pumps.
I understand the point you are making, and I can see how, in certain circumstances, it may have validity. One of the methods I use is the pump reading to calculate gas mileage. However, at a specific time, two months ago, there was an abrupt (and repeating) 10% decrease in gas mileage. Since the car was new, this situation has never previously been experienced.
This condition only changed somewhat, when, after checking what I believed might be potential sources of the decline, I eventually added the fuel system cleaner. As previously stated, the mileage improved from an average 24.7 to 25.5 mpg. It’s worth noting that, although I use the on-board the mileage calculator as a reference, with each fill-up I do a mental calculation to verify its approximate accuracy. Both methods of determining fuel usage showed a similar abrupt increase in fuel consumption.
You said the air pressures are correct, but have the tires themselves been changed??
Also, have you had the alignment checked (not pulling is Not an indicator)???
Welcome to the forum…
Also might want to look at live data to see if anything looks out of whack… ect when engine cold to make sure it is the same as the ambient temp, a/f sensors look normal etc etc…
Also, did you change gas stations and or brands or Octane ratings??
Thank you for the welcome and suggestions/questions:
Tires were replaced 6-8 months ago and, until the new ones wore in, there was a drop in fuel economy. After a couple thousand miles, the normal mpg was re-achieved.
Alignment has not been checked.
“live data” will need to be explored.
Re gas stations: Depending on my location, when needing fuel, I tend to rotate between 2 or three (different brand) gas stations and I always use the same octane.
A reminder: this abrupt increase in fuel consumption was just that: abrupt. I had initially thought it might be related to the quality of gas I pumped just prior to noting the decline in gas mileage, but there was never any improvement related to subsequent refuelings.
Nothing I accomplished made a difference until the partial improvement when a fuel system cleaner was used…and I note no apparent problems with engine performance or idling stability, before or after the cleaner was used.
Driving habits is big. Going out to lunch at work is a killer. Driving 2 miles 1-2 times a week vs never doing it will kill your mileage. Road construction? Creeping thru 10miles of stop-go? Sharing car? Wife? Teens?
Yes, there are two seasonal changes in our fuel mixture.
The problem with attributing the abrupt decline in fuel economy is that it initiated in February, when the winter blend should still have been standard. Possibly the summer blend has now become available…but who knows?
I’m aware that the summer blend has more ethanol and hence mileage would decrease…but my overall experience doesn’t seem to coincide with possible changes in fuel blend.
So…great suggestion, but at the moment, I’m unable to verify that the changing fuel blend is at the heart of situation I continue to experience.
Good questions…absolutely nothing that I am aware of, occurred when the mileage dropped (I am the sole driver), other than filling up the tank at one of my familiar gas stations. That’s why I initially thought that there might have been a problem with the fuel from that fill-up. However, subsequent refills, as well as topping-up fluids, oil change, cleaning of mass flow sensor, verification of no error codes, and checking tire pressure led me nowhere.
No mechanical work has been done, nor any new battery installed, etc.
An 11% decline persisted since that time, other than a slight improvement after adding fuel system cleaner (now a consistent 8% decline in fuel economy).
Maybe your thermostat is faulty. Do you have a temperature gauge that reads temperature? A light to show overheating won’t do it. If you have a scan tool maybe you can read the temperature during operation. The car runs rich until normal temperature is achieved and running rich means lower fuel mileage.