Leaking coolant from overflow tank
After that, you need to see why air is being pushed (or sucked) into your coolant system, which usually means blown head gasket. A simple pressure test of the coolant system can tell you that.
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OR...you have a blown head gasket and the associated piston is pushing air into the cooling system on the compression stroke.
OR........you have a very slow leak or a pinhole leak that is pissing coolant out while you're driving......then when the engine is shut down and cools, air gets sucked in through that small leak. Then after so long, as that air space grows in the cooling system....it can no longer stay cool while your engine is running and the overpressure gets pushed out through the cap and goes into the over flow tank.
Try a new cap.
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The alternator is on the other side of the front end than the overflow tank.....no way water/coolant can leak onto the alternator if the tank leaks.
And alternators are immune to water....doesn't matter if they get wet.
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