My sons 99 ranger 3.0 liter filled up the overflow and overflowed. So i flushed refilled an new thermostat. Thought it was fixed next day same thing. Put another thermostat in drove an hour perfect.Today he was drivn filled overflow again.He says itll start to get hot then go down then bak up.Another thing the radiator cap doesnt get hot like its not circulating.Is water pump possibly not pumping..DAVE

Do you have bubbles in the overflow tank when it does this? If so, combustion gas is getting into the cooling system possibly from a blown head gasket. If no bubbles, the truck is overheating, building up pressure and blowing it into the overflow. Until you get the overheating problem fixed, put on a higher pressure rated radiator cap. 22-26 lbs. When it blows all the coolant out the temp sensor for the gauge has nothing to register the heat with and will show a lower temperature even though the engine is overheating. Could very well be the water pump if you back flushed the radiator and it didn't help.

Ok for the bubbles theres foam in the overflow but i think from the prestone flush. I took thermostat out flushed again went thru all the stages to rinse out and refill. Drove 30 mins fine.shut off drove hour later started filling overflow tank again.Truck never really acts hot but just starts filling overflow.Thermostat still out sometimes the upper hose to therm is stiff like therms not open.The radiator cap never seems to get hot nor the bottom hose returning to radiator its warm not hot.Also felt lines goin to heater core neither were really stiff. Back to bubles when its fillin overflow its kinda like blowing in a straw in ur drink just like air.Were outta ideas any help please i dont like being without his ride..Thanks Dave

Just took off line goin to heater core while runnin water came out but not with the pressure i thought it should have. Would a weak water pump cause any of these problems?

The upper hose will be stiff whether the thermostat is open or not. Standard pressure is 12-16 lbs. in the cooling system. The bottom hose won't get hot and shouldn't because by the time the coolant has reached it the radiator has lowered it's temperature appreciably. I hate to say this, but I think you have a blown head gasket. Any radiator shop can confirm this by testing the coolant for combustion gas. I'd get this done before pulling the head. A weak water pump wouldn't cause bubbles to appear in the overflow.

Thank you so much for ur help..DAVE

Go to Auto Zone or Advanced rent a pressure tester. If a head gasket is blown the gage will pulse. A radiator shop will do a gas analysis appx. $35.00 but ask first this is a sure sign.

Ok ill try that tomorrow.I did just take both upper and lower lines off and clamp them off.lookin at opening where thermostat sets my son started it immediately looked like boiling water.Im pretty sure that answers my question to head gasket..Anyone disagree?