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My 2005 Subaru Outback (3.0 flat 6) has a plastic cover under the hood which covers the engine.

Or it had -- I had some work done and the mechanic has given the car back to me without the shroud.

What is the point of these shrouds?

Should I ask for it to be put back?

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The cover is not purely cosmetic and serves several purposes, one is to reduce engine noise. You often notice there's foam on the inside and they fit over components such as cams and injectors.

A second function is thermal insulation. It prevents heat escaping from components such as the exhaust manifold and propagating to the cooler parts of the engine, such as electronics, electrical cables, sensors and wiring.

In some cases, they even serve a crash safety function. A press release for a Volvo engine cover states:

The tailor-made PU foam Elastofoam® I makes it possible to produce engine covers that are characterized by good sound absorption and thermal engine encapsulation... The part can absorb a lot of energy on account of the open-cell foam structure, thus increasing passive safety for pedestrians in the event of impact against the hood.

For long-term reliability, the cover should be replaced.

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    I believe you are talking about the cover underneath the hood rather than about the cover on top of the engine, as asked by the OP.
    – rexkogitans
    Commented Feb 27, 2019 at 17:20
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    @rexkogitans Nope. I am talking about the piece on the engine. Both serve similar overall purposes, but a hood liner can't isolate heat from other engine components. A engine cover can be smaller than hood liners, saving weight. For example, see this Tier 1 supplier, "example A" fits on the engine.
    – user71659
    Commented Feb 27, 2019 at 18:04
  • Thanks for the answer — it would be a little clearer if you said “not purely cosmetic “
    – tgdavies
    Commented Feb 27, 2019 at 21:11
  • I´ve never seen a plastic cover that can withstand even remotely the temperature produced by an exhaust manifold!
    – Daniel
    Commented Feb 28, 2019 at 9:34
  • This image from this page actually shows the engine cover being talked about. From what I can tell, Volvo is the only one using it though, so it seems like a one off and not an industry standard.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Mar 15, 2023 at 11:48
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Mostly the point is, to make it "look good".

There is arguably some protection for wires and pipes underneath against animals chewing on them, depending on how its implemented.

Also, on some models it may aide in airflow and cooling/thermal properties. Especially so if it is sitting around a top-mount intercooler. (On some models like the older VW V6 engines they may also be harmful, because the heat cant dissipate well enough from the valve-covers)

While it may be technically unimportant or at least not critical, I´d still ask to get that part back. First and foremost if you ever want to sell the car, potential buyers can get a bad impression if there are parts missing. This can affect resale value. Second, any mechanic should return a car in the same good state he got it in, unless otherwise agreed.

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    If the engine cover in either my Legacy or Ascent was misplaced, it would actually impact the efficiency of the intercooler in those cars. They serve a functional purpose in directing airflow through the intercooler so that the intercooler can transfer the heat it's retaining to the passing air. More airflow = more heat transfer = cooler intake temperatures. So, the ratio of "looking good" to "very functional" will vary from vehicle to vehicle.
    – Ellesedil
    Commented Feb 27, 2019 at 19:58
  • Thanks @Ellesedil I tried to incorporate this into my post.
    – Daniel
    Commented Feb 28, 2019 at 9:33
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The other function can be to control cooling airflow, so yes, get them to put it back - even if they have to buy one...

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On petrol engines the cover can help protect against water accumulating on top of the engine and interfering with the coil packs/ignition.

Rain/condensation can get into the engine bay and over time the water can build up around where the coil packs plug in to the engine. When the rubber seals around the coil packs get old they can start to let water through causing misfires. I know this because it happened to me

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