Overheating repeatedly.

[FF3012]

Please forgive me if this is a duplicate issue somewhere else on the forum.
I have a 98 XLT with the 4.6 and lately it has been over heating on a daily basis. The temp gauge pegs out after about 15 minutes of normal operation and registers at about 212 on my digital OBD2 scanner. I can usually tell when its going to over heat as the temp will climb up to about 180, then rapidly drop to about 110 to 115, and then shoots up to about 212. Upon over heating I've noticed the upper radiator hose is pressurized but is cold to the touch. I normally pop the cap to let the pressure escape without puking the coolant. The upper hose then loses pressure but becomes quite hot right away. The truck will usually overheat once or twice this way everyday before the heat starts working normally. I've already replaced the hoses, heater cores, water pump, thermostat and coolant temp sensor.

I am at a loss as to how so much air keeps getting into the system.

Any ideas?

 

[stamp11127]

I'd pull the t-stat and check its operation in a pot of boiling water to make sure it is opening correctly.
Once that is confirmed to operate normally I would leave it out and see if the system still builds excessive pressure. If it does then I would check for combustion gases in the cooling system.

I use the tried and true "redneck" method of bleeding air from the system. Park on an incline with the coolant surge tank opening as the highest point in the system. Run engine up to temp and squeeze the upper radiator hose repeatedly until all the air has been removed.