1997 Ford F150 coolant

hard to say. but since you have a wet manifold you could have a bad manifold gasket and the water is evaporating up there. or you can have a badgasket on the throttle body or a cracked manifold or throttle body. pressureize the throttle body and check. you should be able to do so by hooking up an air hose on the throttle body coolant hose and plugging the other

you may have a cracked throttle body if it's one of those heated throttle bodies Ford used around that time frame for icing