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Registered
2017 Forester XT Touring CVT
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236 Posts
Discussion Starter #1 (Edited)
@RickBH @DannyPTY @Legacy @aggie113


maybe all of your could help. i have 2017 forester XT. last week i swapped out the exhaust to a wrx stock midpipe and then put on nameless 5 inch mufflers.

the car seems to have less torque now, the MPG has gone from averaging 17mpg to 19mpg. the the car wont let itself go in to any boost pressure over 11psig. i have been testing in I and S. S# made it better. but thats not the solution im looking for. what is weird in that the car is getting MPG numbers more like what the factory advertised, yet the entire ECU is acting like its burning to lean.

so my car did the opposite of what the original posters did. stock wrx midpipe with nameless 5inch mufflers.

ill be going back to stock, im going to ask subaru to look for any weird codes. either its the exhaust or my A\F ratio sensors decided to go wonky at the same time.
 

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2015 Forester XT Touring
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653 Posts
What about vacuum? You should be seeing around -10psi when sitting in neutral and the car warmed up. If you're seeing less then did you fiddle with any of the plastic intake piping in the engine bay around the same time? The bands they use are notorious for not tightening back up properly once loosened.
Otherwise, I'd suggest testing for exhaust leaks but can't think of this as causing lack of boost. I know I used a good bit of RTV sealer when I connected my axleback setup to the STI midpipe (and hit it with a heat gun to be sure).
 

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2014 Forester CVT
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210 Posts
cat-back exhausts or muffler deletes don't do anything regarding performance, but make your but and cabin vibrate.
 

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Registered
2017 Forester XT Touring CVT
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236 Posts
Discussion Starter #4
I have done an idle relearn recently. it has been better for the past two days will update with what Subaru says.
 

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2017 Forester XT Touring CVT
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236 Posts
Discussion Starter #5
things fixed them selves afterward for the most part.

Also decided to stop by Subaru and talk to the Tech.

they install an updated CID which he said would solve some drivablity issues. after leaving subaru i install the STI Midpipe with Nameless Muffler. All good no issues. car drives great, has power, and MPG went up.
 

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Premium Member
‘14 FXTT; '15 Legacy FB25
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1,042 Posts
You won't be experiencing any more performance, nor any increase in MPG, from a cat-back exhaust.
 
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