Alright, so I have changed radiator, water pump, thermostat, flush coolant, and did test of Block Tester BT-500 Combustion Leak Test Kit - Made in USA - and color is not turning yellow for head gasket issue. Can it still be head gasket? I did add aftermarket exhaust with catalytic converter removed. Why is the car still overheating? I see no signs for head gasket. One thing i did notice is hissing sound coming from engine after it overheats and i turn off the car, which i heard can be sign for head gasket issue. I want to sell the car soon and cannot sell it when it's overheating after 15 to 20 min of driving. Please help!!!!