would a water pump be bad if it does not leak? what would make it run hot on 2003 Nissan Xterra

i can dirve arround town okay but when i get on the freeway for 10 to 15 min it gets hot, it starts to cool down if i turn the heater on i also checked the thermastat but there wasnt one there could that be the problem

Asked by for the 2003 Nissan Xterra
Thermostat could be the problem but I doubt it since it's running without one. Maybe put one in and see what happens? Also try flushing the radiator.