Air pump M 104
 

Air pump 19940 E320 engine 104 blows air from the top backside of the pump and make a loude blow out sound when air pump runs, its looks like it is a hose should be connected but i guess its not i guess there is only one hose that goes to the exhaust and that one is ok.

There is only one line from the air pump, the pump has an opening with a grate on it that it sucks air from, then that air is pushed into the exhaust when the engine is cold, this enriches the exhaust with oxygen to assist the converters catalyst while the fuel mixture is rich. Mine sounds like a vacuum cleaner in the morning. I am not aware of air going the other way. If this is happening you need to check the bypass valve on top of the manifold as it may be defective and allowing exhaust into the pump, this will ruin the pump. The hot exhaust if it hits the pump will burn it out. Good luck!
Brian

Just so you know that air pump is very expensive, quote I got was ~$800 for the job.

That sounds like the startup airpump noise my '93 M104 makes. You will know when it goes south , screech, scrunch, bad bearings, horrible sound.
My '92 did go south suddenly, while out of town. I simply unplugged it.

I eventually replaced it with a re-built unit from CVJ Axle in Denver, about
$400, if I remember correctly. There is also www.mbpumps.com, which I just learned about on this forum. Airpump for around $200 - 250. They also sell just the bearing kit.

As previously mentioned, if you replace the pump, replace the manifold valve.

Steve