I have a '97 F150 with a 4.6L engine. What would cause the radiator reservoir to overflow and coolant to squirt out the cap? I noticed while driving home from work that the temp gauge went down after being at the normal temp range. Then it went back up to the normal range again. When I checked under the hood I noticed coolant on top of the reservoir and on down to the driver side wheel fenders. There are no hoses leaking and the radiator looks fine. The engine never overheated, and no coolant in the oil pan either judging from the dipstick. I plan to change the thermostat and flush the system soon, meanwhile I keep adding coolant.

 

A damaged cylinder head gasket or cracked head or block may allow air pressurized by the pumping action of one or more pistons to enter the cooling system. The result is what you see.

 

could also be the thermostat stuck closed.--- to test the head gasekt, drive around the block and get motor warm and thermostst open. park in the drive way, open hood, and look at the overflow tank. if you see gobs of little air bubbles, then you got a cracked head or head gasket.--- feel the radiator hose also to see that you are really pumping water and it is warm.