1991 190e 2.6, ac air cold on center vent, hot on side vents

We have had this same problem with this car a couple of years ago, I cannot remember what caused it, any input appreciated

You have a vacuum leak on one of the vacuum lines or the reservoir.

Or, I have seen the blend air flap potentiometer fail and cause this. I have had success cleaning it with contact cleaner and a readjustment.

I think that was the problem the last time, a vacuum leak. How do you find these? The soap and water trick does not sound appropriate for vacuum lines..

I have the same problem,I thought that was normal.

Ron
93 190e 2.6

Sometimes my air works ok. But if I really jump on the car I will immediately get hot air out of the side vents. I suspect a vaccum leak. But I performed all the tests in the Mercedes manual and the problem has not been located.

Hot air at side vents
 

Numerous occasions the problem occurred. The most common "for my experience" the vacuum motor behind the instrument cluster was defective. Fortunately this is the easiest one to test but not always the problem. That's what I would check first.

Now,
How do you check all of that??