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2007 Forester Sports XT 4EAT
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2018 Forester XT Limited CVT
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Depending who you ask and about which car you ask, you might find that leaving the sheet plastic attached under the engine helps with cooling and fuel economy. The air is supposed to flow a certain way and if that thing isn't there, it just kind of buffets around more so in the engine compartment. With it there, the air flows through the radiator as designed, and with airflow going as designed, your fuel economy may be a bit better.
 

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2008 LL Bean (4EAT)
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5,155 Posts
Had to remove the plastic thingy that sits under the engine of my forester. Should it be replaced?
Mine has kept the underside of the engine clean as new. It also creates negative pressure which keeps the engine compartment cooler, and in the case of an XT, contributes to intercooler efficiency.

When Subaru deleted the tray from the Foresters in 2009, they kept it on the XT turbo model.
Need for XT Undertray:

Peaty, who consulted for Subaru, said:
... the way the plastic is configured it affords some measure of air management. I suspect there is some negative pressure under the car at the points of the opening drawing the hot air away from the turbo side that is forced in the scoop past the IC...
And cpt kernow posted:
Here's a good article from an Australian web magazine. It's part of a series. There's abit somewhere that says when testing airflow on an Impreza without an undertray, air was coming out of the bonnet scoop until speeds of 50mph were reached. Meaning the IC was acting as a pre-heater.
The article he posted said:
...what an eye-opener these measurements were! Remember, the higher the pressure difference across the intercooler, the more air that will be flowing through it... The table above and this graph show that without any undertrays in place, the air from under the bonnet is likely to have been flowing out through the bonnet scoop! That is, there was a measured higher pressure under the intercooler than on top, even at 80 km/h! No bloody wonder intake air temps went up in this configuration – the air was coming in through the radiator, being heated, then passing through the intercooler and out the forward-facing bonnet scoop!
An XT without an undertray or a skidplate is using the hood scoop as a "reverse-flow intercooler preheater exhaust." If one has an XT, it sounds beneficial to follow the wisdom of Subaru and keep something under there to make the intercooler work right.
 

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2015 Forester 2.5i Limited CVT
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I keep them on. If there’s a leak, I’ll be able to spot it when I remove the undercover. I’ll put some white oil absorbent paper on if I even suspect a leak. And like others have said, it protects the engine from road debris and excess water, salt etc…

There’s an opening or hatch for oil drain. My Stahlbus valve gives me quick access.
 

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2021 Forester Limited
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It's a 2007, the warranty is long gone...

It was missing from my 2010 Impreza when I aquired the car. It had all the stuff there to attach it, but I never had any issues. IIRC, the cost was almost $90 for a replacement, so I never got to it. Not sure if any fuel savings from the decreased drag would have paid for it.

Current fuel prices, more than likely. :ROFLMAO:
 

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2015 Forester Limited
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42 Posts
My opinion - You won't have any real problems removing and keeping it off. Your biggest reason to put it back on is keeping the engine bay cleaner and protected from everything that spits up from the road
 

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2008 LL Bean (4EAT)
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... You won't have any real problems removing and keeping it off. Your biggest reason to put it back on is keeping the engine bay cleaner and protected from everything that spits up from the road.
There are absolutely no problems if the under tray is removed. Subaru provided it because, unlike most engines, the boxer is splayed out low against the road with its whole underside exposed to water, mud and salt.
It is easy to damage, or lose a few fasteners, and once it is gone few people will spend for a new one. So by now, many old Subarus have lost their under tray and are doing fine without it. But I cherish mine as it has kept the engine and compartment very clean.
 

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2005 2.5X 4EAT 2017 2.5i Prem CVT
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584 Posts
We almost lost the one on our '05 when a quick lube place pulled the whole thing off instead of using the little swing-away door for the oil filter. When they put it back they had used the handful of fasteners that they didn't break while pulling the cover off and left much of it hanging. Got some new fasteners from the dealer parts counter and we never went back to that quick lube place. It's been dealer or DIY for oil changes ever since.
 
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