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Discussion Starter #1
The moral of this story is not to trust the rusty nasty little screws that (don't) hold the plastic undertray on. I had been tempted to fit a couple of cable ties where it meets the bottom of the bumper last time I was under the car as the screw threads seemed a bit flaky, I didn't, I should have.

I drove over a particularly bumpy piece of road causing vibration, the next thing I know I can hear a loud noise (sounds like wind noise suddenly amplified ten times). Stopped and pulled over when safe to do so (was on a busy road so couldn't stop instantly). Turns out the noise is the sound of the front of my plastic undertray dragging along the road. Upon inspection it was only still held on by the rear 3 screws which I undid to remove the tray.

Luckily this tray acted like a "catcher" in a snow-plough kind of way, unluckily, various other plastic items were lying in the tray, including plastic air ducts and a plastic "protecting" grill.

I also removed the plastic strip from the bottom of the radiator which had got a bit mangled and would no longer stay on properly, likewise I cut away another rubber strip that was hanging down underneath behind the bumper.

None of this stuff was essential to the running of the car so I was able to drive it home but now I'm wondering what some of this stuff does and how to replace bits that may or may not be needed.

I'm told the undertray helps aerodynamically but most cars don't have them and I noticed no adverse affects in driving back or on fuel consumption so in reality this has little effect. Likewise the air ducts, most cars don't have them and I don't drive the car in such a way that my brakes get hot and need extra cooling airflow.

The main purpose of this post is to ask you guys what is the purpose or function of the plastic "protecting grill" ? What does it actually protect ?

Any underside pics showing these parts in situ would be gratefully received.

See item 23:
http://www.realoem.com/bmw/showparts.do?model=BN32&mospid=47617&btnr=51_3275&hg=51&fg=75
 

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Discussion Starter #2
Update

I had the chance to get under the car today and have a look around.
Basically I saw steering gear, load of pulleys, belts and pipes, all of which looked expensive, none of which I was happy to leave exposed to the elements, dirt, grit, road salt and stones that they may have come into contact with.

I refitted the plastic part to the underside of the radiator (it was much easier at home than by the side of a busy road).
BMW quoted me £118 for a new plastic undertray so I decided to refit my own, which although a little scuffed wasn't too badly damaged.

I re-used the 4 remaining original "twist-fit" screws and sockets at the back and on one side after cleaning them up with WD40 and making sure they would do up tightly, then drilled a few holes into the plastic along one side and under the bumper and secured it with a few large cable ties.

I feel happier now the protective shield is (even more) securely back in place.
 
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