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2007 Forester 2.5 X Prem 5MT
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Discussion Starter #1
The details:

2007 Forester 2.5x Prem
53,000 miles
For the last 2 months, mainly driven around down for short distances under 50 mph

I was calculating my economy pretty regularly over the summer and was getting 25+ mpg. I was also driving mostly highway at 50+ mph.

This winter my economy has plunged, to 16mpg for the most recent tank. It may be worth mentioning that I recently had the one-way valve removed as part of the Subaru recall at the dealership. For a few days after picking up my car from the service department, I could smell raw gasoline. I figured this was just leftover leakage.

Here are some other possible contributing factors to the decline in fuel economy:

It has been 30s and below for the past two months where I live.

I have been driving in snow every weekend.

My spark plugs have never been changed.

Transmission and diff fluid has never been changed.

I am running Mobil1 5 w30 and have been since I bought the Forester brand new.

I do not let my car idle for more than 5 minutes in the morning.

I just refilled and put in a bottle of gumout, so I am anxious to see the number next refill.

Is this just a product of crappy winter gasoline mix, or do I need to get my spark plugs changed?

Thanks for the input, as always! :Banane35:

Mmmmmm, I'm actually drinking a beer right now...Sweetwater IPA
 

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Premium Member
2008 2008 2.5i-2018 XT
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13,239 Posts
Could be a number of things. Gas/winter/driving in the snow dropped my mileage from 25/26 to 21/22 this winter. You certainly are not helping yourself by not changing the rear diff and if you have an auto the ATF. You need to do these two things ASAP.

But I doubt that's your problem. I doubt that the plugs are an issue either. But you need to do them come Spring. I have pulled plugs out of 2 Foresters with 35K and 40K miles and they both looked very very good. Should go 60/70 K miles..no problem.
 

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2008 LL Bean (4EAT)
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4,909 Posts
... mainly driven around down for short distances... economy has plunged, to 16 mpg for the most recent tank...
It has been 30s and below for the past two months where I live. I have been driving in snow every weekend...I do not let my car idle for more than 5 minutes in the morning....
Your "warm up" consists of a 5-minute fast idle running on a wasteful rich mixture because the engine is cold. An engine makes little heat at idle and takes a long time to warm up. This is followed by short-hop city driving in frigid temperatures that quickly re-cool the engine between hops. The engine may not have much time running efficiently at its full operating temperature of 190*F. You may also have a heavy foot.

There is not much you can do except to shorten that excessive "warm up" to not over a minute, which is all an engine needs and all the manufacturers recommend, and also drive more gently.

The removal of the one-way fuel valve should not affect mileage. The day after mine was removed, I went on a road trip and got the best mileage ever, 31 mpg on two tanks.
 

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09 Forester 2.5X
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711 Posts
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There is not much you can do except to shorten that excessive "warm up" to not over a minute, which is all an engine needs and all the manufacturers recommend, and also drive more gently...

Really? Let it warm up for a minute? The temperature indicator I believe would still be blue. (on the 09 it's blue, off, red) I've been letting mine warm up till that light goes off...
 

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2008 LL Bean (4EAT)
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4,909 Posts
Really? Let it warm up for a minute? The temperature indicator I believe would still be blue. (on the 09 it's blue, off, red) I've been letting mine warm up till that light goes off...
That occurs at the bottom of the normal operating temperature range, or around 150*F. It is your choice to postpone driving until the blue light goes out, but the engine will warm up faster if gently driven off after a minute, which is what car manufacturers recommend doing.
 

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2004 Forester XT Premium 4EAT
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30,106 Posts
For the last 2 months, mainly driven around down for short distances under 50 mph
There's your problem... you're probably just barely getting the engine into operating temperature.

Reminds me of a friend of ours who was complaining that his SVX would not get any better mileage than 15 mpg. He was only driving it for 2 miles every day, no highway driving at all. My husband took the car for an "Italian tune-up" and eventually drove it 1700 miles to deliver it to a buyer in Nebraska. By that time the gunky deposits had cleared out and the car was getting 33 mpg.
 

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2007 Forester 2.5 X Prem 5MT
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465 Posts
Discussion Starter #7
Could be a number of things. Gas/winter/driving in the snow dropped my mileage from 25/26 to 21/22 this winter. You certainly are not helping yourself by not changing the rear diff and if you have an auto the ATF. You need to do these two things ASAP.
I think you are right about this, I have been putting it off since the maintenance schedule only calls for an "inspection" of the gear oil every 30k miles. I have a manual transmission, which means I also have a LSD rear diff. I would like to stick with good synthetic oils. What would you recommend for mountainous driving conditions and good fuel economy, with the occasional aggressive rag session ;-) A good indicator that I need to do a tranny fluid change is that it's hard to get into 1st on cold mornings...

Some fluid recommendations I have gotten are for Amsoil, royal purple, Mobil1...
 

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none none
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I think you are right about this, I have been putting it off since the maintenance schedule only calls for an "inspection" of the gear oil every 30k miles. I have a manual transmission, which means I also have a LSD rear diff. I would like to stick with good synthetic oils. What would you recommend for mountainous driving conditions and good fuel economy, with the occasional aggressive rag session ;-) A good indicator that I need to do a tranny fluid change is that it's hard to get into 1st on cold mornings...

Some fluid recommendations I have gotten are for Amsoil, royal purple, Mobil1...
for the love of god do not put royal purple in that transmission lol. Subaru extra-S gear oil is probably the best out there. A lot of people have had goodluck with the plain old valvoline gear oil in the subaru trans. If you're getting any grinding there are a number of different "cocktails" of gear oil that may help. Do not use anything designed for a LSD in the transmission! Rear diff fluid, I like mobil1, its easily available and is one of the better performing gear oils for the rear diff. Just need 75w90, nothing special (it has a sealed VLSD, so it doesn't need any type of special lsd additives).
 

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2012 2.5 X Touring 4EAT
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48 Posts
In addition to what has already been advised I would suggest that you install a block heater if possible. Two to three hours will warm you engine depending on the outside temp. Works for me.
 

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2010 Legacy GT LTD 6 spd
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974 Posts
What about switching to a higher temp theromostat or blocking off the grille during the winter? Would any of those help the engine stay warmer and more efficient.
 

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2008 XS 4EAT
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9,878 Posts
Almost a year old thread and I'll add my comment about the condition of the air filter if none of the other things were done. :shrug:
 
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