Apparently I overlooked or forgot the part about replacing the transmission because of a fuel mileage complaint while thinking this out.
Per VDCdriver, I’m also VERY skeptical about that and feel there has to be another reason for an entire transmission replacement. Subaru is not that benevolent.
If this car has a manual transmission maybe there’s a slipping clutch issue. Knowing the minutiae might help sort it out.
I would change the thermostat and cross my fingers.
Take a drive for a couple miles on a highway then let the car coast down to <30mph before using the brakes.
Feel the rims for one warmer than the others; be careful!
@VDCdriver @ok4450 I don’t have the transmission invoice with me at the moment but I have the follow up appointment that says: cust states that mileage is still low after trans was replaced states is getting 280 miles to a tank check for stored codes none found at this time. recommend fuel consumption test overnight test drive beginning mileage 31,587 ending mileage 31643. trip total 56.3 miles. total fuel used 1.7 gallons = 33mpgs
I have no idea how they got that mpg on the test drive. but that’s what it said in digital read out too when I got it back.
transmission was the only thing they diagnosed when i dropped it off the first time complaining about mileage issues. which is weird because they replaced it within a week with a brand new transmission ( they couldn’t find a rebuilt one) without doing a fuel consumption test.
How are you checking your mileage?
@“the same mountainbike” Both digital read out and manually from fillups.
“I did track manually as well, and it matched up a little under the digital meter: first full tank I tracked =287.9 miles on 13.929 gallons = 20.67 mpg (at least 80 miles were highway in one day when I criss-crossed the metro area) Smaller range was 57.9 miles on 2.917 gallons = 19.849 mpgs”
The only thing I’ll add is that taking the freeway across town is not really a valid measure of highway driving. You really need to get it out on a road with a constant speed after fully warmed up. Take it up to Duluth and back for a mileage check for a more valid test. An extra start up, stop light, slow down in traffic etc., can reduce the average mileage for miles. I suppose it is possible that they broke a wire or didn’t reconnect a sensor when they did the air filter, but other than that it has been really cold here in Minnesota and driving a cold car takes a lot more gas.
Your trip computer likely got reset and you are reflecting the current average in winter not the overall one you had prior.
Nothing to do with dealer work unless it happens to be dragging a brake or something.
I just rolled 10k on my trip meter which resets my average MPG on my 07 MDX. It was 19MPG(including summer) and now has dropped to 15MPG for the last week of driving in excessive cold.
I doubt the transmission got replaced b/c of the mpg complaint. In investigating the mpg complaint, they probably noticed a symptom of a known design or manufacturing problem affecting your transmission, and just decided to address that while the car was in the shop. Otherwise they knew you’d be coming back in a few months anyway, once the transmission noise got worse.
One thing you might do OP is see if you can find a copy of Consumer Report “Reliablity Guide”. It lists known problems for newer cars. Some of these have warranty extensions that go along with them. It’s not just TSB’s. Instead I think they call these kinds of programs “customer interest” programs. So it might have been a customer interest program that got your transmission replaced.
@bing I don’t ever take trips to Duluth so using that as my mpg average vs my normal commute average wouldn’t be too fair. A point I should make is I’m trying to return to MPG I had previously, not match a number it shows in the EPA estimates. That day of driving was St. Louis Park on 94 to White bear, back to Plymouth on 694, across again to Shoreview on 694, and back down through Minneapolis to St. Louis park on 35/94.
@andrewRA I was driving away from dealer on a 500 mile trip meter and the MPG’s started dropping rapidly enough that I reset it thinking it was some sort of glitch and now have been tracking per tank/fill ups. I normally reset odometer every 1 to two fill ups so I’m comparing current MPG’s to the fact that my average trip meter had never before dropped below low 27’s no matter the weather.
@GeorgeSanJose Here’s the invoice for transmission:
Customer states fuel economy has dropped since last service performed, please inspect and advise. Inspected and found no codes in system but found noise coming from Trans/Differential assy. Looks like what you said, they identified and fixed a problem, but not the problem I stated. they told me this was the first transmission they had ever replaced in a Crosstrek.
Bad thermostat.
Dragging brake.
OK, go to Mankato or Albert Lea or someplace where you can run 100-150 miles at a constant speed. Too much hapanstance going from Plymouth to White Bear etc. Like I said a couple stop lights, a start up or two, slow down in traffic, etc. can really throw the average off. I understand you want to be where you were, but you don’t really know where you were exactly. Doing a controlled run will establish the best case during this weather condition. Then if it is way low, you’ll have something to complain about with substance. Fully warmed up, get out on the open road and punch the reset button and see where you are 50-100 miles later.
I have a possible solve: I replaced the spark plugs on my 2016 Outback and saw a 5mpg drop afterwards. I then cleaned the mass air sensor, cleaned the throttle body, reset the computer idle three times, ran 2 tanks of fuel system cleaner, ran premium gas, took it to the dealer… Nothing worked. Then I decided to have another mechanic replace the plugs again, with the hope that he might somehow, even if inadvertently, fix the problem. And lo and behold, it turns out that even though we had torqued the first plugs to Subaru specs, it turns out that it was not enough to crush the spark plug washers all the way down. Now that we’ve done that, I’ve got my mileage back. It feels great to say the least. My theory is that if the plugs weren’t torqued down enough, then some air might be leaking in. Just a theory, but whatever, I’ve got my mileage back!
I would submit that if they disconnected the battery or cleared the computer, it would take a while for the computer to re-learn the fuel settings again. I saw this when I replaced an O2 sensor and took the car on a highway run. Mileage continued to climb from 14 to 27 as I drove. Use the computer for mileage readings and not the gas pump.
I’ll leave it to the mechanics to answer if air would be drawn in but loose plugs would make a heck of a racket, and anyone that does not know the washer needs to be crushed should not be trusted. I’ve known this since I was 12 and not a mechanic.
If the plugs aren’t making an air-tight seal, reduced mpg could indeed be one of the symptoms. Usually you’d also hear weird noises, notice reduced engine power, and see a check engine light f as well. @Larrychase8_190798 makes a worthwhile post though.
You’d think that the combustion flame shooting out past the spark plugs would draw attention too.
But this was nine years ago, I doubt we’ll ever know.
Necro-thread. From 2015.