1994 Ford Tempo Very little warm air blowing

Very little warm air is blowing in car. The temp gauge used to sit int he middle of "normal" but now barely gets a quarter of the way up. It will go up somewhat, but drops again when you drive. Added more Prestone to the coolant jug, it was just about empty. Replaced the rad cap today.

When I take the rad cap off, there is very little pressure spray. In fact the first time I removed it yesterday, after the car had run for some time, there was no pressure buildup under the cap at all.

I'm thinking air block, but not sure what to try next.

Max