I have a 2010 Toyota Corolla. 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)
OP, Here's an example of a plastic engine cover on my 2001 VW Jetta VR6:
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.
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 .
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 :)
the "insulation" on the underside of a hood is not for noise, its a fire blanket. Its supposed to fall down on top of the engine fire as the plastic tabs holding it in place melt. Hood paint peeling is not from engine heat, its from sun damage and neglect of car. The engine cover is entirely there for looks, no other reason. cars ran great without them for 100 years. The first thing i do is remove mine and store it. It only gets in the way and hides leaks, holds heat into engine compartment, etc... for the guy who suggested it stay in place to keep fluids from splashing around....lol...you may want to actually be able to see those leaks early, and fix then....not use a cover to hide them? Noise reduction? not likely. My v8 nissan cover reduces no noise at all. They are for looks. probably one company started it and the others followed to not lose a sale to a manufacture with a "cleaner" looking engine.
I assumed they were for noise reduction, but I haven't noticed much difference with mine off. Since its usually warm to hot where I live I would prefer to have some airflow over the upper intake components, even if it's a small %. Of course when the weather starts to cool again I'll put it back on to make sure the engine warms fast, so as not to get a mil light for engine temp to low. Also its convenient when you working on problems to not have it on.
For your car, the covers do offer some protection to the components under them but you could theoretically remove the plastics without hurting your longevity. Engine noise will be more noticeable. In some cases it can be a lot more noticeable. At your drivers side front corner, one of the covers has an air scoop that directs fresh outside air to your air box. Removing that one might cause a slight reduction in performance but not necessarily longevity. Also, when the time comes to sell the car any of the engine compartment plastics you removed will be counted as missing parts and that can be a deal breaker. In my professional opinion, leave them on and replace any missing hardware. Most of the parts in your car are there for a reason.
Take if off. Engine runs cooler without the cover so your engine will be more efficient and run better giving better performance and better mpg. I know because I have run my motor with the cover on and off. Engine runs so much better without that lump of plastic restricting air-flow over the engine. Fact.
The primary function of the Engine bay cover is to absorb Heat. It reduces the amount of heat that goes on to your bonnet.
The improvement of mpg, I'm not so sure.
The primary function of the Engine bay cover is neither to absorb noise nor to make the bay look slick. But to absorb heat. The material used is plastic, plane plastic does not absorb sound, rather, the 'foam' on the inner side of your bonnet is used to absorb sound.
A little piece of plastic isn't going to even touch the volume level of an engine. It's literally just like the engines little plastic hat. As motors were produced in the beginning, they tried hard to keep them aesthetically pleasing. That took a bit of engineering to build the motor they wanted, and still make it look smooth and sexy. It also made it hard to work on, in that the cables and components had to be hidden out of sight, either to the side of, or below the engine. With the plastic cover they made it so that an unknowing customer could look under the hood, love how pretty it was, and buy the car. Of course it helps at car shows etc. too. If you work a lot under the hood of your car, you could leave it off,especially if it's past 200,000 miles, cause you'll be taking it off a lot! It'll also help to see leaks or issues easier. However, if you're proud of your car, you may want to leave it on, because when you pop the hood, it looks better. As far as airflow goes, your car is built with a cooling system that takes care of any heating issues. It still gets plenty of air and coolant flow. If there are spark plug cables or anything that runs along the top or is attached to it, you probably want to keep it on to keep those things in place out safe. As for your question, it's hard to know it's necessity without knowing the car and the piece that came off.
I had problem with my friends 2007 camry. All the ignition coil connector broke and disintegrate (cracked). I was suspecting the cause of the damage connectors due the plastic cover on top of the valve cover. With this plastic cover heat is much higher resulting to the brittleness of the connects. So i think its better to remove them to have good circulation of air on the upper part of the cylinder head.