ahs2m2

Symptoms: Coolant level was checked and I drove the car for like an hour on the highway, temperature is at the normal level on the highway. As soon as I got off the highway, the temperature needle was rising slowly passing the middle line to almost red. I turned on the heater to full blast and the temp. gauge drops to normal but slowly rising again so I parked the car and raised the hood. Noticed that the reservoir tank was boiling (maybe it was just bubbling). Coolant in the reservoir tank was almost full which leads me to think that the car is losing coolant in the radiator because its overflowing to the reservoir tank. I brought the car to a shop and was told that my radiator was bad so they replaced the radiator with a brand new cap. Thinking that the car is fixed, I drove it home. It was perfect for an hour on the highway but as soon as I get off, the car overheated again. Fortunately I live close to the highway so I parked the car at home. Raised the hood and noticed the same symptoms. Reservoir tank is bubbling and coolant is overflowing out from the tank.
Same problem. Also noticed that both fans turn on immediately as soon as I start the car and they cut on and off at the same time.

Searched the forum , I was trying to do my own troubleshooting.
-Radiator cap - Unlikely because it is brand new.
-Radiator - Unlikely because it is brand new
-Thermostat - Maybe but both upper and lower radiator hoses are hot and not just one of them which leads me to think that the coolant is circulating.
-Air pocket in coolant - Maybe but I bet the guys at the shop flush them when they install the new radiator.
-Headgasket - Maybe but engine was recently rebuilt and a new headgasket was put on. No white smokes and no oil in the reservoir tank.
-Waterpump - Unlikely because when I turned on the heater the temp went down and the waterpump was recently replaced as well.

What else should I be checking? Any replies appreciated.

Da9Gro

save yourself the trouble and take it back to the shop and tell them they didnt fix the problem. and if they give you **** call the number:1-800-952-5210 its BAR(Bureau of Automotive Repair) well i think thats the california number... but call em and they'll have that shop shittin their pants

kicksremix2o6

its most likely air pockets cause coolant shouldnt be boiling unless air is involved or water pump.