I have a 1991 YJ. I just replaced the radiator, both upper and lower hoses, thermostat (tested fine - it opens), radiator cap, and thermostat housing. Of course fresh coolant. Before changing all, flow through hoses and radiator was fine, heat was great. Now, heat is great, upper hose gets hot but lower hose is cool. When t-stat opens, back pressure forces antifreeze to spew from low hose. Can't figure out why this is happening. Any suggestions?