Ok I need HELP!!!! My 1999 Ranger with a 3.0 V6 is pushing the coolant back into resavoir and out of radiator. The motor is not running hot as the temp gauge stays right in the middle and have checked the sensor and it works fine. So far I have replaced the thermostat twice, replaced radiator,replaced radiator cap, replaced water pump, checked with a pressure test to check for blown head gasket and we started with 8 pounds of pressure and then went to 15 and then 22 pounds of pressure and did not loose any pressure so do not feel it is a head gasket. But why is it doing this I mean I drive down the road the truck runs fine but as soon as I stop and shut it off and pop the hood the coolant resavoir is full and bubbling over and when I turn the radiator cap it has back pressure but radiator looks empty so help has anyone else had this problem and what did you have to do to fix???

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1999 Ranger XLT Super Cab, 3.0 , Automatic, 2WD