There may be air leaks in addition to possible engine temperature issues.

Aging of the flap seals in the main air plenum (which are controlled by 3 of
the 4 dash air control levers) creates gaps which allows fresh (cold) air to
sneak past.  Since it is a huge pain to remove the entire heater/plenum
assembly and renew the flap seals, I have found it expedient to go for for
an easier solution .

You can cover the main air intake (behind the upper grill) with cardboard
and duct tape.  This greatly reduces the amount of fresh (cold) air that
gets into the heater plenum.  In effect, you have just upgraded to a
recirculating interior air system.

However, don't cover the small side air intakes as this will completely stop
all fresh air from entering the van.  This will allow too much moisture
(from breathing) to remain in the van, thus allowing condensation to form on
the cold windows.  Remove your cardboard air intake cover when it warms up
in the spring.

Also check for air leaks from loose fresh air ducts under the dash.  These
are the ducts that supply fresh air to the outer dash vents.  Tighten the
ducts and secure the connections with duct tape (what else?).

I did both of these easy fixes last winter and my van now stays warm as
toast, even when temps are well below 0 F.

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Sky Blue Waters)