Gas Mileage Sudden Drop

I have a 2004 Subaru Forrester with a 2.5L engine (non-turbo) and about 137,000 miles. I usually get just a bit more than 300 miles on a tank of gas. About 3 weeks ago, I took the car for a major tune up (and they also replaced a valve cover around the spark plugs area). After the tune up, the car was running great and I was still getting about 300 miles to the tank. Within the last 10 days, I noticed that the gas mileage on the car has decreased - I am now getting closer to 250 miles per tank. The mechanic rechecked the fuel lines and test drove the car two days ago but did not see any signs of leaks. The weather was near freezing for about a week but has warmed up again to the 40’s. My driving habits have not changed and I am driving the car the same way as before. I am stumped as to why the gas mileage would drop that much. Any ideas on what might be happening?

Other than the cold weather, no. See how the next tankfulls do and then check again when temperatures are in the 60s. Right now, there hasn’t been enough time or weather change to say anything useful.

After 9 years of use and over 130k miles, it is very possible that the cooling system’s thermostat is stuck in the open position, thus causing the engine to run cooler than it should. When this happens, the fuel/air mix becomes too “rich”, and gas mileage invariably plummets. So, if the thermostat has never been replaced, I would suggest having this done. I like to replace mine every 4 years or so, in order to prevent problems.

There is also a possibility that an O2 sensor has stopped functioning properly, thereby causing an improper (overly rich) fuel/air mix. Yes, this should have resulted in the Check Engine Light turning on, but, even if the CEL is not lit up, it might be a good idea to check for stored trouble codes.

I agree with @pleasedodgevan2
Whether the day temps do moderate a little, the night temps are still quite low if you do any driving then and your car is still especially cold in the morning. As @VDC says too, a cooling system check wouldn’t hurt either as that can raise havock with your mileage as well.

Along with those^ very good suggestions, Subarus do ‘learn’ certain things by driving over time and make adjustments accordingly. There are actually hacker (ie ‘aftermarket’) tools out there to make those adjustments manually for those cars and make them perform better.
Anyway, it is possible that they replaced certain sensors (specifically, the A/F sensor) on the car and it still uses the ‘learned’ settings from the old sensors so they are way out of whack.
The car is basically making adjustments that it shouldn’t.

Eventually, it will catch up to reality but that may take a while because the adjustments are quite slow.

A common thing to do is to pull the terminal off the battery and leave it off overnight when those sensors are replaced.

How many tankfuls of gas have you gone through in the last 10 days? Sometimes a change in the weather along with a change in your routine can have a effect on your mileage that is just temporary.

I consistently get 360-380 miles on a tank of gas in my car, but every once in a while it will drop into the 320-330 range like it did the last two weeks, I only got 319 on my last tank, but I won’t worry about it unless it doesn’t return to normal in the next month or so.

IMHO it’s impossible to get any accuracy of mileage readings when trying to measuer by the tankful. There’s too much variation in the fillups. The only way to accurately measure mileage, and the only way to determinie a change in mileage, is to divide the amount added into the mileage driven and repeat the process three or four times.

I’m sorry, but IMHO it,s necessary to properly determine mileage before determining whether a problem exists.

All of the points made in the thread are legitimate in that they will affect mileage, but there may not even really be a problem.

I too see changes in tank to tank MPG of 8 - 10% up or down. Can this be explained by different quality gasoline tank to tank? We price shop for gasoline so we buy it all over (NW Louisiana/East Texas).

“Can this be explained by different quality gasoline tank to tank?”

In a word, no.

It is probably the result of differences in where you drive (hilly terrain, as opposed to flat terrain, local driving as opposed to highway driving), and/or differences in ambient temperature (the engine uses more gas when the temps are lower), and/or differences in driver behavior (using drive-up windows sometimes, but not on other occasions), and/or a difference in the amount of weight that the vehicle is carrying.