Replacement stock type fan problem

[Stale Trooper]

on a '99 CVPI w/197,700 miles. a couple weeks ago, when it got very hot after a 20 mile run @ 60-65 (normal engine temp at speed, 80-85* F ambient temp), then caught at a stop light- It just started steaming like mad. figure my overheating must of been due to airlock in the block, burped it today and hope I got that fixed.
I also replaced the aftermarket fan (no shroud) with a stock-styled replacement w/shroud, stock wiring, etc. My question is this: when the fan is running, and I turn off the ignition, the fan also stops. This doesn't seem right. shouuldn't it stay on until the engine cools down? I also have a stray unknown black wire (12-14 ga) in the same harness as the stock fan wiring. no connector, just a raw cut off end. It's possible I also don't have 'low speed' on the fan,, could hear any difference during a static test in the shop.