It seems that many of us (including = me) have been troubled by fresh (cold) air leaks in the
heater system. I = happen to have a complete (blower, heater core, ducts,
flap doors, etc.) = spare heater assembly from an 86 GL sitting next to me.
Examination of it = and my van, and a bit of advice from Darrell Boehler have
led me to the = following conclusions.

Fresh air coming into the van is supposed to be = stopped in two ways. (In a factory fresh van, = employment of both of these methods will stop all fresh air infiltration.) The
bottom heat control lever operates a large interior air plenum flap that supplies fresh = air
to the rear top ducts. Pushing this lever to full right = theoretically
closes this. However, foam around the edges of all of the ducts flaps does
deteriorate, resulting in air leakage even when the flaps are in = the closed
position. Repair of these foam edges does require = removal of much of the
dash to gain access. This is a project to be = avoided unless you have lots
of time and a comfortable (warm) work area.

It = should also be noted that if the wires that are attached to the = control
levers and run to the various flaps are bent, kinked, or misadjusted, the
flaps = will not operate properly, and this can be the sole or contributing
cause = of air leakage.

The second way of stopping = fresh air is simply to use the thumb knobs on the
two dash air vents. = Pushing them up closes those air flaps. They also can
have deteriorated = foam edges, but are very simple to remove and repair = if
desired.

Fresh air can also leak in if the duct connections to the = dash vents are not tight. If the
air leaks out prior to = reaching the dash vents, it does no good to close
them. There are 2 1/4 = " ducts that go from each side of the center air
assembly to the respective = dash vents. They can easily be loose at
either/both ends. This = can be resolved by using duct tape and/or radiator
hose clamps, depending = upon how anal you are feeling at the moment. Access
to the passenger = side duct is simple, just remove the glove box. Access to
the driver's = side can be gained from underneath the dash at the vent.
Access to the = driver's side at the center air assembly connection can be
done by someone with = small hands, if you reach underneath. Being a bit of a
contortionist = is also helpful. Alternately, you can remove the instrument
panel, speedometer, = etc. (not too bad a job, actually) and approach it from the top. BTW, I had loose hose connections on both sides. These hoses can also have holes in them, which = can obviously be repaired with duct tape, or you can replace the hose with = an original or make up a replacement from available = materials.

If you want to really stop fresh air infiltration, you can stop it at the source, and not have to be concerned = about hoses, flaps, and = foam edges. Fresh air enters the = entire van at the front, right behind the top = front grill. Simply remove the = grill and you will notice a large plastic mesh = which covers the fresh air intake = plenum. This can be covered with duct tape, = cardboard, or whatever you want. I used cardboard cut to fit, wrapped with duct tape for strength and moisture = resistance, and secured to the mesh with more duct = tape. Those of us in cold climates should be aware that duct tape does not adhere well if it is applied when it is = cold. One answer to this problem is to heat = the tape and van front by getting into a warm place for about 30 minutes (which is all it should take to place a cover = over the mesh). If this is = impractical, try heating the area locally with a portable heater or hair dryer (especially useful if you can get into an unheated
garage that isn't too frigid). Alternately, or in = addition to taping, you can secure the cover = by using wire to hold it firmly to the mesh and/or surrounding brackets.

After I followed this last approach, = my air leak was reduced substantially,
but was still not cured. Then = Darrell Boehler informed me that there are
two mice sized round holes, one on = each side of the mesh intake cover. So,
this weekend, as the temperature here will exceed 20 F for the first time in
several weeks, I will remove the top grill, locate those two = holes, and fill
them with insulation and/or duct tape.

The beauty of the = grill approach is that it is simple, quick, and easily
reversible when you = again want fresh air, or if you finally get a chance to
pull the dash and = fix any deteriorated foam.