The problem isn't adding too much to the reservoir. That will just overflow once and spit it out. The problem is why you had to add any to start with. The common suspects are an inoperative fan clutch, a leak, and a leaking cylinder head gasket. Since you saw steam, an external leak is the best suspect. Look for wet spots, especially around hoses and clamps.
If there's a problem with the fan clutch, the overheating will occur at lower speeds, not at highway speeds where natural air flow is sufficient.
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Monday, August 3rd, 2015 AT 11:27 PM