What is this missing cap '91 190e
 

This is a real newbie question. (I actually got pretty competent wrenching my three 80's and 90's Saabs, but I'm having to learn this world now.) I'm attaching a picture I borrowed and edited from the site http://www.2phast.com/190e/. I've circled in red a cap over a tube. My car doesn't have the cap, the tube is open to the world. Is this hurting me?

Thanks.

I'm having trouble telling for sure from the photo, but it looks like the dipstick for the auto transmission. Does the open "tube" lead back to the transmission area? If so, having that open is not good. Transmissions do not like dirt. I would replace the dipstick/cap and have your fluid/filter changed.

I'll have to check mine out later, but that's not the transmission dipstick.

That looks like the intake for the air pump. The air pump draws in atmospheric air and injects it into the exhaust. This helps heat up the cat, making it easier for the car to pass the european smog tests.

No, it's forward of that
 

No, the transmission dipstick is further back toward the firewall. This is something driven by the serpentine belt. Air pump makes sense.

cap
 

yes, that is the air intake cap for the air pump, it is mercedes part number
104-140-00-81
look at the picture below

Outstanding. Thanks.

MDB