I was changing out my air filter and in the process of cleaning out the air filter box I took a closer look at the source of this funny square patch of cleaner air filter media. This clean patch sits over this small rectangular box in the corner of the filter box. I've seen this odd thing before in previous air filter changes but what I noticed this week was that there was a hole to the atmosphere at that corner as you see with my image in an image below where I'm basically probing my finger from the back of the hole into the body of the air filter box.

What's this for exactly? This hole is surely by design, what purpose does it serve? Am I missing some sort of foam plug or something that should be plugged into that hole? Is it like a vacuum modulating area for the turbo of the vehicle such that a maximum vacuum is clipped by the presence of such a hole? I should note that the portion of the air filter box that's shown is the dirty air side which is the bottom of the box. The top receives the filtered air that goes first to the MAF sensor and ultimately the intake. The vehicle in question is a 2005 Volvo S80 5 cyl 2.5L Turbo engine B5254T2

Dirty air filter with small clean patch Showing the air filter box and its hole

  • Does the hole look like it was there originally or does it look like someone had drilled it out? Maybe some person that had the car before you tried to install some kind of sensor and they removed it? Nov 26, 2017 at 13:34
  • @sjfklsdafjks it definitely looks original to the part as far as the machining/casting. What's not clear is whether or not there was always a void there or something else.
    – jxramos
    Nov 26, 2017 at 17:41
  • Well for starters at least I think we can rule out a vaccum line since it’s on the dirty side, however I really don’t think a plug would be there either because what would the purpose be on the dirty side?! Maybe Paul can help us lol Nov 26, 2017 at 17:48

1 Answer 1


It's not exactly an answer, but if it's any consolation I've got the same aperture (and it's so dirty it looks as though it's never had anything fitted. The only thing our cars have in common is the manufacturer

enter image description here

Maybe it helps prevent water being sucked in through a low down intake by providing an alternative air route for trickle amounts of air (e.g. Chugging through a lake a low revs)

One thought struck me; perhaps if you can find a microfiche parts diagram you can see if there's an obviously named optional part that fits here on the assembly diagram

  • I was looking up if FCP Euro had an air filter box for sale and I was going to ask one of the reps to pull one from the shelves and inspect if it had such an aperture. They don't sell it however, at least not for my make/model so I never managed to confirm if something came installed with it. Your image corroborates with mine that it's void by default. Very curious lil thing, what a mystery.
    – jxramos
    Nov 28, 2017 at 20:59

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