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What is the purpose of the large plastic cover that stretches over my valve cover? Does it have a particular function? Are they just for looks? Or are they designed by the engineers to protect wiring? I believe its called an engine cover. (I bought a car that was missing this piece and I need to know if its critical)

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You've asked a generic question that's hard to answer with anything other than "it depends." If you provide some specific information about your can and its valve cover, we could give you a specific answer. – Bob Cross♦ Dec 6 '12 at 20:12

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OP, Here's an example of a plastic engine cover on my 2001 VW Jetta VR6:

2001 VW Jetta VR6 Engine Cover

I can't speak for your car in particular (unless you specify what it is), but in my case it has a number of non-critical functions:

  • Aesthetics. This looks far nicer than seeing a bunch of exhaust headers (stock ones are never pretty), oil seepage on my valve cover, fuel injection and other wiring, etc.

  • Routing of spark plug wires. It keeps them organized and away from any hot engine components that would melt or deteriorate them.

  • Keeping fluids below the cover. In a car with 150k+ miles, I usually see more oil splatter and coolant being thrown around, up on the hood, etc. If I were to take my cover off, I'm sure I'd find some oil, grease and/or dried coolant under it.

Now, if I were to remove it, I wouldn't be too concerned except for my longer than necessary spark plug wires that I'd zip tie out of the way. However, it does look nice, so if you care about that sort of thing, buy a replacement.

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Good info, I just looked and my vehicle (2010 Corolla) routes the spark plug wires underneath were the cover would be located. So it sounds like mine is more for looks. Are they called 'engine covers' on the market? – David Walz Dec 6 '12 at 20:38
That's all I've heard them called, and what they're referred to on forums, so I'd have to say yes. – Ehryk Dec 6 '12 at 20:39
+1 for aesthetics and keeping fluids from getting everywhere. Not sure routing spark plug wires is a main function - not many do that like the VR6 cover, most hide them under the cover :-) – Rory Alsop Dec 7 '12 at 0:07
@DavidWalz, it would be a good idea to add your specific car information to the original question. Right now, that info is hidden here in the comments. – Bob Cross♦ Dec 7 '12 at 4:02
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I believe the most common use is noise reduction. As engines got smaller they rev'd higher. The mufflers got moved to the back of the car. No more engine exhaust roar. The result is engine buzz,noise,clacks etc. The cover muffles the injector clicks,belt noise etc. It also just looks cooler to see Vortec, Turbo,SFI .

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Interesting point, I hadn't thought of that. These covers do seem to cover more area that would be needed for just wire guidance and protection. – David Walz Dec 6 '12 at 23:53
I agree with the 'looking cooler' reason, but I've never heard of 'noise reduction' as a reason to have one. Do you have any sources or references on the sound reduction from a plastic cover? – Ehryk Dec 7 '12 at 4:07
Remove one and notice how much louder the engine noise is. Also it is atypical for manufacturers to spend money for a WOW factor that is under the hood thus mostly unseen. – mikes Dec 7 '12 at 12:54
up vote 1 down vote

I have a VW Jetta 1.8T. The factory service manual produced by Bentley (for Volkswagen) specifically calls this component the upper sound absorber panel. To an extent, these are eye candy, but their primary function is for sound dampening.

That being said, I know many people remove them when they are displaying aftermarket components, or keep their valve cover looking immaculate. I had a valve cover gasket leak up until recently, so my sound absorber is best left in place :)

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