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Steam = pressure. Figure out why the water is turning to steam and fix that ...
You could have a cracked head and it's blowing combustion gas into the cooling system. Is the coolant rusty colored?
Starter is probably toast after all the coolant that's been leaking on it.
I'm going to guess that your intake gaskets are leaking. Once you get low on coolant the sensor gets hot. Air in the system is making steam.
Back to square one. Figure out where the coolant is going. Check the back of the block above the starter and check the oil cap to see if it's orange/red. Check the oil for coolant. How many miles on it?