Engine Cooling problem
2004 Nissan Almera 4 cyl Front Wheel Drive Automatic 150000 miles

Hi
my car started over heating and i took it to a mechanic who took out the thermostat and replaced the radiotor cap. A month later it started over heating again and replaced the water pump. After doing that and bleeding the coolant system the car is still over heating. The head is not blown. when i feel the radiotor pipes the top pipe is extremely hot and the bottom one leading to where the thermstat was is ice cold and does not get hot. Even my car heater doesnt get hot anymore. they checked the entire system for blockages and they said they no blockage anywhere. Please can you perhaps tell me what the problem could be