Hi all. Doing some TLC work on my '86 740GLE wagon today and noticed the fresh / manifold heated air flap on the air filter housing is in the manifold position. I set it out in the sun (70 degrees no wind and cloudless sky)and it's still the same. What tempiture should it change to fresh air at? The flap is springy. Thanks.

You could block the flapper in the fresh air position. I prefer to stay fully legal and original plus it can get cold here in Ontario, so I just do the hair dryer test when I check my air filter. Remove the air filter cover and air filter. Place a hair dryer in the box and point it towards the inlet to warm it up. You may have to remove the fresh air inlet snout to feel the flapper position (can't remember). The hair dryer is plenty hot to make the flapper move.

Bill

Block it opened to fresh air with a screw. That's one of the first things I do. I have done this to ALL my cars over the years. I could never tell that it came up to operating temp any slower. I would only worry if you live in an arctic region.

Good luck,
mark h.

Block it opened to fresh air with a screw. That's one of the first things I do. I have done this to ALL my cars over the years. I could never tell that it came up to operating temp any slower. I would only worry if you live in an arctic region.

Good luck,
msrk h.

If your system is anything like the flapper contraption in 240s, then this is a common problem:

1) it's undoubtedly the air box thermostat. This is a plunger (it sits vertically) in the box that would normally push your flap to close the "heated air duct" orifice and allow cool air from the other duct, originating in the cool air in front of the radiator, to be drawn in instead! IPD (and other sources) sells replacement thermostats.

2) But instead of replacing it, I suggest what many folks on the 240 forum have done, because the thermostat gives no indication when it fails (you discovered this by accident, by noticing the interior of your air filter box, but how often do you do that?), and when it does, it continually allows the "preheated" air through the box and into the AMM (Air Mass Meter) -- then the AMM is destroyed, and you're looking at a 'big bucks' replacement cost.
So what the 240 owners do is defeat the thermostat mechanism. It isn't really necessary for driveability (I've run my cars through the coldest New England snowstorms without warmup or driveability problems), but only to satisfy EPA demands for faster warmup to reduce emissions.
Some have just replaced the thermostat with a stick, to constantly push the flapper to cover the "preheat" orifice.
Others (like me) have take a dremel or hacksaw and cut out the whole mechanism (flapper, spring, levers, and the thermostat). Then we take some material (a little piece of sheet metal, for instance) and glue it over the "preheat" orifice to perpetually seal it.
Either way, it ensures that only cool, fresh air will enter the air filter box and (the main reason) no longer pose a danger to your expensive AMM.
The nice thing about doing this (making the changes inside the air filter box) is that the preheat hose is left intact, and it looks like it's still working so that in states where the inspectors bother to look under the hood, they see nothing wrong -- after all, the inspectors will never go to the trouble of looking inside your air filter box, given how troublesome it is!

Hope this helps.