That's what mine did when I had a bad HG/warped head. Under load, it would push coolant out the overflow, filling the tank then out onto the road. The other test option is a pressure test, cap the radiator with a gauge/pump, apply pressure and see if it leaks down. Then run the engine and see if the pressure builds from the engine. It t gets over 15 psi or so, the rad cap will vent and let coolant go to the overflow.

r.c.brown@ieee.org
TruckEditor@tlca.org
Project: 4Crawler
4Crawler OffRoad - 4x4Wire discount code