No Air From Center Vents Under Acceleration?
 

I have an interesting problem:

Whether the AC is on or not, under acceleration or if the car is going over 75 mph, air will not come out of the center vents. It will only come out of the end ones. As soon as I am not with my foot on the gas pedal, or if I am below 75 (not on a hill) it blows normally. Any idea what this could be? I have a 1998 C230 by the way. Thanks in advance! :confused:

After some research, it seems to be vacuum related. Anyone know the best place to start looking?

You have a vacuum leak somewhere in the miles of tubing running around the engine compartment and/or under the dash. The leak is probably in the tubing feeding the climate control system. There is likely also a vacuum reservoir associated with the climate system. Look there first. The leak could also be elsewhere and just reduce the vacuum available to the climate system.

BTW, the engine produces less vacuum at higher throttle settings - e.g. under acceleration or at higher speeds. Hence the intermittent nature of the problem.

The car will run and shift better once you fix this problem.

over on the right side of engine near the firewall there is a vacuum distributor.it is black and has several coloured vacuum lines attatched.in the front of it there is a ball bearing used to block off main passage.they all crack where the ball bearing is and i bet yours is no exception.this is causing the vacuum loss in your system.

Thanks guys I will start checking the lines tomorrow!

If you press the "EC" button & then accelerate, does the center vent stay OPEN?

Of course the air won't be cooled but will show if the fresh air vacuum diaphram is the problem.