vw passat cooling problem
 

hi i was driving my vw passat 1999 1.8tdi last night the water light came on and then when i look under the bonnet the top water pipe had came off the radiator what would course this? i was able to get the pipe back on and fill it up with water and carry on and then noticed the heater was not blowing hot air out of the blowers. so i thought it could be air in the system so the next day i took the cap of the water tank and reved it for about 10 mins to get the air out but still no heat i took it for a drive no heat turned it off for about 20 mins and then went for a drive and they just started working, but now i think there is something eles wrong. the bottom radiator pipe is not geting warm but the top one is getting realy warm and i dont think the temperature gage works (it gose half way and then dose not go up or down at all even when the pipe came off), the car is fine doing about 30mph but about 70mph it gets really hot. iv hear it could be the thermostat how would i test this and will it damage the car driving it at 30mph when it dose not seem to over heat