Posting this for a friend ,He has a 97 ranger 4.0 pushing coolant into overflow jug . story goes ,he was driving the beginning of winter and noticed he lost heat. temp gauge dropped. he opened hood and reservoir jug was full and overflowing.First he changed thermostat and rad cap (aftermarket Stant).Same problem ,but he noticed if he starts cold and drives right away this happens,if he lets it warm up to operating temp before driving, it does not push coolant into jug.he has changed rad cap,water pump,thermostat ,flushed rad,checked coolant for combustion gasses with blue point tester,he has also put in some type of block sealer(made by permatex i think).He has had a pressure tester on it overnight with no coolant loss,removed spark plugs and have no signs of coolant,the truck does not have a engine miss,and no white smoke out of tail pipe.The truck does not overheat.Would a air pocket cause this?or is this the sign of a cracked head and or head gasket? I have searched the site for postings of this nature,and could not find any similar. Any help is appreciated. Thanks in advance Bob

 

This condition could be caused by air in the system. The heater core is the highest part off the coolant system. Also a collapse of the lower hose would be suspect. If the hose closes off, even partially, coolant gets pumped into the radiator but can not effectively be returned to the water pump. This results in a pressure build up in the radiator that causes the cap to let excess coolant go into the bottle.

 

Thanks for the reply Ranger Pat.I forgot to mention that he already replaced the lower hose,And I was thinking a air pocket also.I know there is a way to purge the system i will have to do some research

 

putting the front tires on ramps or on a good steep incline sometimes helps to get air out