Tom in Austin

In recent months, I've noticed some odd areas of powdery-white residue below my car. Pretty nearly underneath the rear differential. Since I don't remember seeing these before getting the C350 and since the garage is now 25 years old, it seems like they must be related to the Mercedes. But what could they be? Any fluid that might be leaking from the back half of the car would surely be a lubricant, and not leave a powdery residue. In fact, I can't think of any automotive fluid, other than maybe antifreeze, that could leave a powdery residue.

mrmotoguzzi00

is it sand ? salt ?

JustLooking

That looks like normal concrete efflorescence. Aka, minerals in salt form leeched from the concrete by water, and left after evaporation.

Tom in Austin

Never noticed it before we got the Mercedes, and not present anywhere else on the garage floor?

mrmotoguzzi00

Just for giggles, Clean up the floor and back the car into the garage for a while and see if ya still get the stains under the rear axle....

Alfadude

And if it would be coming from the car I would think it would be very easy to see evidence of something if you would look at the underside of the car. Just looks like it is leeching up from the concrete and could be a bit worse in that one area.