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Originally Posted by djag  The coolant had turned rusty brown. This time Hyundai fellow tells me that radiator is choked with rust and carbon deposit. Radiator will have to be replaced, coolant refilled, and AC will have to be serviced. This would cost approx 10000/- |
You need another opinion! Carbon has no business being in a radiator! It is a product of combustion. Even rust in a radiator is unusual. Rust can only come from iron/steel components (engine block). How come coolant has not been inhibiting rust in your system? Did the car run for fairly extended periods with plain water?
And since the problem seems to be happening only at high speeds, there is something much more to it than what your current diagnosis shows. (Points to a leaky head gasket).
The AC service is a bit baffling. Possibly only the heat exchanger for cabin heating (being in the coolant circuit) would need cleaning. Nothing else in the cooling circuit should need to be touched and of course you should not be billed for gas/refrigerant charging. Please confirm this.