84 300d heat problem

when i turned the heat on this winter for the first time i got very little warm air. it seems to blow warm with the fan on slow however this does not heat the car well. As soon as i change the fan speed to high the air becomes cooler. could this be a faulty pump? or a bad heater coil? the car has 270K miles. Any help would be greatly appreciated!

Thanks!

check your monovalve insert. located on the firewall just off center toward the passenger side. Remove the 4 screws, take the insert out and inspect the rubber end of the plunger for rips.