[Michael C Henry]

The thermostat bypas,the little tube between the manifold and coolant pump. It lets coolant circulate untill the thermostat opens.I've seen the steel nipple corrode shut. I've seen the short peice of hose ,which appeared normal from the outsie,witha blister on the inside. It would choke off the flow,.When the thermostat opened the rush of fliud would blow everything out of the coolant recovery tank. when I first removed the hose the restriction was obvious. After a while of sitting on the bench the internal blister dissapeared from the hose. A new chunk of hose and that problem was cured.It wasn't noticeable from the outside and was just stumbled on.Afterwards it all made sense.