Well now I gave you the formula of : the boiling point increases 3 ?F/psi and with a 16 psi radiator cap then 3 ?F/psi x 16psi = 48 ?F. So the adjusted boiling point of water under 16 psi = 212 + 48 = 260 ?F. This is exactly what your instrument panel gauge reads. Therefore you are boiling some of your water off. Your radiator cap must be constantly held in the open position, venting liquid and vapor into the overflow reservoir. You should see the overflow reservoir cap or the pressure disc, located next to the cap, leaking at times. If you had Anti-freeze in the system, it would be easier to trace.