I don't really have an answer as to why it does that. The fitting on the neck it's tube were meant to send overflow to the ground or it wouldn't be there in the first place is my theory. My 4.5 did the same thing occasionally. You might consider rigging up an after market plastic coolant recovery tank. Any auto parts store has them. Mounts nicely on the drivers side in the engine bay. No more mess on the ground. I guess those tanks were mandated by the EPA or DOT or both at some point to stop the pollution as all later cars have them as part of the cooloing system.
Of course, consider I have no idea what I'm talking about!
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STS