"What has happened to change this? "

What may have happened is something very common to the 240. The position of the recirculate flap is what allows the air from the cowl vent to push past your heater core. Without any vacuum applied to its actuator, the flap is held by spring in the recirc position and the outside air is closed off. Nothing will move the heat into your car then, but natural convection and conduction.

The recirculate vacuum motor is typically weak after 20 years because it leaks. More vacuum is needed to hold it in the fresh air position you need. This is the reason the older your 240 is, the more leaves appear out of that spot by your feet -- the door doesn't shut all the way.

When it gets weak, the check valve (usually black and white plastic above the intake manifold) needs to work well to keep enough vacuum built up in the vacuum storage bottle just behind the front console. The recirc flap is the first air door to suffer when the check valve gets dirty.

So, the easiest fix is to slip that check valve from its attached hoses and make sure it passes air only one way. Wash it if needed. If not that, you may have to search for a leak behind the dash. Ears or a rubber hose stethoscope can help, but your objective is to get that recirc flap closed in the fresh air position. When the vacuum motor eventually dies, a tennis ball wedged in will keep it closed. Replacing that motor is even more involved than replacing the electric blower motor.


--
Art Benstein near Baltimore

"You can say any foolish thing to a dog, and the dog will give you a look that says, ''My God, you're right! I never would've thought of that!'" (Dave Barry)