Not hot enough

[gs730287]

I have a 1998 Chevy Silverado and when I start it up, when it's cold out, and turn the heater on, it warms up but doesn't seem to blow hot air, just warm. Truck runs great, has I assume factory tstat. I've owned the truck since new. New heater core and radiator, put in yrs back. If 1 was warm and 10 was hot I'd say like 4

Any suggestions?

[TW97C1500]

Maybe the diverter door is not working properly and is letting in too much outside air.  On my 97, the door which diverts to floor, dash, windshield had fallen out on one side.

 

Another possibility is that the coolant level is a little low and either won't pump through the heater core, or it has let an air pocket form in the heater hose and won't let the coolant flow.