I removed two of the underside plastic shields to do some work on my 2004 XJ, still checking for any fluid leaks after short trips. Are these shields critical to be put back on right away, or can I drive it a month or so without them? Brandon

 

I don't drive with the main one on anymore because I found out it hides leaks until its almost too late. Your underside will be a bit dirtier, but that's it I think, plus it cuts down the time to do oil changes, etc, considerably.

 

The undershields are for aerodynamics and for some protection to the underside of the car. Not sure if you will notice much difference in economy with them missing, but the goal is to make the underside of the car as flat as possible. It certainly makes a difference at high speeds, well I could notice the difference anyhow.

 

On a different car I had, the undershield was in a left and right section. One side "went missing" and I got a dent and tiny leaking crack in the sump. While dealing with it, I noticed that rust and corrosion were very much worse on the unprotected side, not to mention the sump damage which would have been much less likely if protected. The rubber drive belts also get exposed to road grit.

It is a pity that the X350 shield is very hard for a DIYer to remove with no hoist or pit. I changed all my fixing bolts for 6mm flanged stainless with hex head, as the old ones were rusty.