After there is time for pressures to equalize and car to cool off awhile it should be 38 degrees. If not, it is either low on refrigerant or the vacuum pod on the outside air door is bad allowing outside air to raise the vent temp.

When first starting up a properly operating climate control system in one of these cars it will drop the a/c intake air temp by about 30 degrees, so it takes awhile to keep circulating the air until it gets to the 38 degree point.

Do not add more refrigerant until you have a set of manifold gauges so you can monitor the high side pressure. Without monitoring high side pressure you stand a good chance of ruining something in the a/c system.

Good luck,