When I start my car and its on the cold full line of the overflow. It heats up and fills to the hot full line. The problem is its not going back down to cold full. I have 2 core radiator, have tried 16,18,20 radiator caps. The car does not over heat just doesnt circulate. Ive had this problem for awhile i need to figure it out.

Also is their a way to check if coolant is leaking internally?

 

are you letting the vehicle cool down completely and checking level in the morning when it's stone cold??

 

Yes Its not a dd or anything.

 

Yes, check for white smoke out the exhaust.
Pull the plugs and look for 1 or 2 that are white.
Look for milkly oil. If the leak is minor you may find it under the filler cap. If it's more it will be on the dipstick. If it's major you will see it when you drain the oil.

They have pressure system checkers that take place of the radiator cap and pressurize the system while you search for the leak, if the pressure is going down.

 

See thats what I don't get, I don't see white smoke, milky oil. The car runs great, it doesnt over heat, so I just don't get why it does this to me. I need to get this taken care of its making me want to sell it haha.

 

When the coolant gets hot, it expands through the hose to the overflow tank because of the extra pressure. But for it to return to the radiator from the overflow tank, it depends on a siphone action. As the coolant cools and contracts, the siphoning action pulls it back up the hose into the radiator. If you've ever tried to siphone something, you now there can't be any holes in the hose you are using for obvious reasons. I suspect there is a hole in the hose that goes from your radiator to the overflow tank. It's probably in the part of the hose that is just inside the overflow tank lid since you haven't seen any leaks, but above the coolant level in the tank. It's probably sucking air when the radiator is trying to siphone the coolant back. Check/replace that hose and make sure you have a good tight clamp on the end of the hose that connects to the radiator neck... then see if your problem goes away. Hope this helped you out.

 

Bang on, also check to see if the hose is bent, or compressed. Pressure will push the coolant through it, but won't be able to suck it back. honestly though, an overflow isn't absolutely necessary. I don't have a return style overflow on my car, just a standard puke tank, where the pressure will push it out, but won't suck it back in. As the coolant gets hot and expands, the rad returns becomes full once again, only reason they started putting them in cars is for environmental reasons, doesn't affect cooling