This is my 99 S40 and I was always wondering about this opening (see the pitctures). I guess it links with the exhaust system. Is it a part of the catalytic converter?
I'm planing to wash the engine soon and do I need to cover this opening?

 

There is an air heater back there on the exhaust manifold (probably better to call it a "stove"), and originally there was a flexible pipe from that opening to another opening on the air cleaner. In cold weather, the air cleaner has a valve that can select warmer air coming from that stove. You do not have to worry about getting water into that, so long as the engine is cold.

 

Thanks for letting me know. So you meant, there should be a horse pipe connected with the air filter and this opening?

 

Yes. This forum doesn't allow me to post a message as short as the word "yes" so I had to add something else, but I meant to just say "yes".