We have a '93 SR5 pickup that has a fairly new radiator (<2 yrs). It has been losing fluid for the last year. We replaced the cap as it was loose and not performing correctly. Seemed to fix the leak, but now the fluid goes ino the overflow and stays! I know its supposed to go back into the radiator once it cools down, but its not. Overflow is completely full and we overheated. Any ideas would be completely welcome!

 

I know you don't want to hear this,but it could be a blown headgasket. My truck was doing that when I suspected the head gasket was first starting to go.

 

To know for sure either you or a shop can do a block test. What it does is check for exhaust gases in the cooling system.
If you call around you should be able to find a shop that will charge you somewhere around $35-45 to do it.






Or, if you check for a block test kit on amazon.com you can purchase a cheaper full test kit shipped for about that.

With this one you need to purchase the dye separately.



I don't know if this works or not but someone on this site mentioned that you can do the following:
First thing in the morning when the engine is DEAD COLD remove the radiator cap and place a balloon around the radiator opening then start the engine. If it immediately starts to fill the balloon up exhaust gases are present.

If exhaust gases are present the most likely thing is a blown head gasket but there are a couple more expensive ways for it to happen. If gases are present you really need to stop driving it ASAP.
Some people will drive them until steady white is coming out of the tailpipe but that's not smart and is setting you up for costlier repairs and possibly an under performing engine once the head gasket is fixed.