At redline of fuel economy gauge, a/c tries to suck in fresh air
 

Hi all,

I've blocked off the main air vent under the monowiper of my W124 260E because I couldn't stand the smell of exhaust fumes from the outside even if my a/c is on recirculate mode.

As a consequence, I must keep the a/c system in recirc mode all the time. Otherwise, it will not blow any air because the blocked vent keeps outside air from getting in.

When I push the accelerator down so that the economy meter's gauge is 3/4 of the way to the "gas guzzling" (rightmost) side, the a/c blower acts like recirc mode is off (I make sure it's on all the time). Once the needle swings back to the middle and back to the economical side of the gauge, air starts flowing again.

I know it's bad to block the main vent because it was designed that way to prevent mold, prevent the windows from fogging up, etc.

I can't floor my accelerator because the a/c tries to suck in air from the already blocked main vent.

Is this normal?

It sounds like you have a vacuum leak in the vacuum system for the HVAC. So when you are into the throttle, the vacuum suply from the engine goes away and you have no reserve in the cars vacuum system. Is there a one way, inline valve that is not working on the vacuum supply line to the cabin? I would think that your car should have a vacuum resivoir somewhere?
Best thing to do is to disconnect the vacuum feed line to the cabin and run a leak down test on it with a vacuum pump with a gauge. Do your vacuum door locks work after the car sits for a while?

Hi Motorhead,

Yes, the vacuum door locks work even after the car sits for several days.

I also suspected a vacuum leak but I'm not quite sure where to start.

My MityVac is on the way but I'm not sure what I need to dismantle first.

I believe the vacuum locks have a dedicated vacuum pump under the back seat.

You likely have leaking vacuum pod(s) that are causing the changes during lower-vacuum engine operation.

http://www.peachparts.com/shopforum/showthread.php?t=35373

Why do you get all these fumes? Are the rubber sealing strips and hood pad in good shape?

I guess you're right, brewtoo. I'll have my tech take a look at it when my mityvac arrives. :) I'll have to get a few vacuum pods as well.

The fumes are coming in through recirc flap through the main vent. There are a lot of trucks in my area and they are belchers.