1996 Dodge Ram heater not working well on my dodge diesel

I recently bought this truck and the heater is blowing luke warn air on low fan speed when i turn the blower up it blows cold air. I grabed the heater hose on both sides and they felt like they were both about the same temp, both hot. at first i thought it was the theromstat because the engine wasnt getting hotter than 160 degrees, but now im getting 190 degrees with cardboarrd in the grille. Any sugestions thanks, NIck