80% goodman furnace. There is no a/c with this unit. New high limit switch. Furnace runs a complete cycle, but blower motor stays running. t-stat new, wired right. Blower housing clean. Return air might be undersized, a little. Will jumping low limit tell me anything? Do not know flash code.

First of all is the thermostat set for fan-"Auto". If it is "On" then it will run constantly. Some thermostats have a switch, some are a little more fancy.

Is this a mechanical fan/limit control (once you take cover off there is a "dial" with three adjustable set-point tabs) or a simple limit switch along with a roll-out switch? Is this oil or gas? The mechanical control's first set-point tab is fan-off (the second tab is fan-on, and the third is the limit which will kill power to the burner, but keep the blower running). Is this set too low? Does it eventually stop?

If there is not a fan/limit control then I am assuming that the blower is energized by some type of electronic circuit board. Are there LED's on the circuit board? Does it show that the "W" is lit (this is what makes the call for heat). If it is a timer board then you can try carefully pulling off the W wire wire (low-voltage). The blower should stop after a delay. If this is the case then it could be a bad board.