ls/vtec overheating
 

ok so I bought a 2000 Integra gsr ls/vtec off my brother that he bought with a spun bearing and pulled the motor and put a ls/vtec together and put it back in.... the problem is it overheats. It starts up and runs fine but after about 15-20 mins when the car heats up it overheats... the fans never kick on and the lower rad hose stays ice cold. I replaced the thermostat and tested it before I put it in so I know it's good and bled the coolant pretty good. I figure since the lower hose is cold the hot fluid isn't hitting the senor and kicking the fans on... if I jump the fan plugs the fans come on. I'm thinking it's the water pump or something with the head gasket and it's a new gsr head gasket on now. Anyone else ever had this issue? Any input would be appreciated