Recirc / Fresh Air problem in a W140
 

Ever since I got this car, I noticed that the heat / AC air volume does not seem high enough out of the vents. I can hear the fan really moving when set to high, so I know the fan is good. When I switch to RECIRC, I feel and hear a substantial difference in air volume out of the vents. I decided to pull the passenger side under-dash apart to see what might be going on. I see that the flap inside the car under the passenger dash closes and opens when recirc is turned on and off. I also checked the function of the recirc flap vacuum actuator above the pollen filter. It works properly, I can see that the door moves when commanded. I'm confused. I have this feeling that there is a flap, door, or some other obstruction blocking air flow when pulling fresh air. It sounds like the internal passage ways are under a lot of air pressure when in fresh air mode, with little air volume coming out the vents. Anybody have any ideas on this? p.s. the pollen filter was changed last year, and still looks good.

low fresh air flow replace filter?
 

Have you tried removing and replacing the fresh air filters?

All filters new except the expensive activated charcoal filter. Since that works only in recirc, and recirc works fine, I don't think it's clogged.

Those 140 cars are so airtight they would probably float. There is a big difference in air volume between recirc and fresh air. It is easier to move air inside of a space then to force it in from the outside. I would check that you had proper ventilation out of the car, I think I remember there being some exit flaps in the rear of that car, behind the bumper, but not sure, I haven't had mine for 2 years now. If you have had any mods done to the rear passenger area or had work done on the rear end, and changed any of the airflow exits that could have an effect, but otherwise it is normal. Happy benzing!
Brian

Activated charcoal filter can be turned on or off in either normal or recirc mode by button in early W140s, or automatically by smog sensor in a later cars.
I would doublecheck the recirc flap to make sure it opens all the way and stays open in normal mode. I had same problem you experiencing, and found the linkages from both vacuum elements to that flap were disconnected.
Mike

I checked the operation of the flap under the top portion of the assembly covering the pollen filter. It worked fine. Also checked the recirc flap under the passenger knee bolster. It also works fine. I will see if I can find the air exits, and see what condition they're in. I don't think that's it though, because I would expect air flow while in FRESH mode to increase with the windows open. It doesn't.

As a follow-up on bwheitmans' suggestion, lower a rear window slighly, and see if it makes a significant difference.

If it does, than his suggestion is on the right track.

Phil

Already checked as stated above.