What is the effect (neg. or pos.) when driving with the cover/flap (to access the oil filter and drain plug) missing (completely gone, just an opening)? Long sotry short, I was driving back from Arizona once and discovered the access door under the car had somehow opened and was dragging on the highway as I drove. Removed it and never replaced it. Should I replace the door and just leave it alone. If it does not cause any issues, I figure not replacing the door. It's not a top priority for me to replace it as long as it does not cause any problems. (2010 III) Thanks for you input.
In your neck of the woods, I wouldn't think it necessary. Up here it helps keep snow, rocks out and probably helps the aerodynamics a bit. I think Tumbleweeds did a study and found no significant impact and that was the entire engine undercover.
I did some pretty accurate mileage tests at 70 mph with the 4 under body covers. These are the 4 large covers, 2 on each side, that go under the passenger compartment. There was no difference at all with the covers on or off. I suspect the oil change door or even the entire under engine cover would make no detectable difference. But as Spiderman mentioned they do help keep snow, dirt, and rocks out.
Changed the oil on Thursday, found the access door missing. I thought I had run over something a few weeks ago, guess it was the door falling off. I'm going to bite the bullet and buy the up-to-date OEM engine under cover (online, not from a dealer), cut off the access door, and use some pop rivets to attach some surface-mount lift off hinges (see pic). Should make future oil changes a bit easier (take the door off instead of swinging it down) as well as be a bit more secure. I prefer having all of the underside covers intact, if only to keep debris at a minimum. The front covers are bit scratched up since the car sits low, but I'm not going to go crazy replacing those since you can't see them anyway.
