hi Folks,
Yesterday i went on a 300 km long drive car was cursing very well on cng 140 kmph speed but after 2 hour drive we were on some slow track where my car got heated up to H suddenly then we just stopped needle came to middle position and then we drove for 10 mins and reached a petrol pump following were things i noticed.
1) Radiator pipes were hard they were holding pressure.
2) expansion tank was full and it was over flowing how ever coolant was not boiling at all i used castrol premix coolant .. before that we were using water and never had this type of issue.
then after half hour i topped up a liter of coolant and radiator was full again all with coolant and expantion tank was empty then again we crused for 1 more hour with 140 speed and again when we reached slow track car heated up and coolant was over flowing the expantion tank .how ever this time it almost empty the radiator i removed coolant and put water in it.
1) why coolant was overflowing althoug it wasnt boiling.
2) even water in over flowing without boiling.
3) radiator pipe is hard mean radiator cap is holding pressure.
4) radiator was cleaned 6 months back and there is no chocking m sure.
5) in city on drive of 30 to 40 mins in never heats up or lessen the coolant in radiator.
6) needle suddenly goes up to H from middle there is no slow risings and some after few minutes in comes back to normal at its own .. is it how it works >?
how ever i witnessed lot of rusty water in expansion tank i guess radiator should be changed.sorry for the long post but i know there are experts who will help me out 
