Ok i keep having the same issue. I only have heat when I am driving. I bleed the system and the heater will work great for about 2 days. Then it will push a lot of coolant into the overflow tank and the radiator will be low and I end up back again where I started with the heater only working when driving. The radiator, hoses, themostat and cap are new.

 

do a compression test on the motor.

 

Sounds like you may have a bad head gasket, or perhaps worse. I would do a compression test. On the off chance that doesn't reveal anything, then I would pressure test the cooling system.

 

I don't think I have a bad head gasket. It is running great, no coolant leaks and no coolant in the oil or vice versa. Someone suggested there could still be a large amount of air forcing the coolant into the receovery tank.

 

Im guessing you have followed the bleeding procedures greeny has suggested.
a wild guess, are all your hoses tight? it may be a cheap fix, but if this isnt the case, it was worth a shot, and shouldnt take more'n 10 minutes.
another suggestion is check the antifreeze boiling point. it may just be getting hot and boiling out of the radiator into the overflow.

remember, these are just thoughts, and relatively easy to check.

 

1. So much coolant that the expansion tank overflows itself?
2. Do you check the radiator level when warm/hot under that "overflowed" condition?
3. The proper level of the radiator is only guaranteed when you can create a vacuum (or then pressure lower than atmosphere) inside the radiator by the cooling of the liquid, and that vacuum can suck on the contents of the expansion tank liquid to refill the radiator - you need a radiator cap with a good top seal and no gunk between the radiator top and the actual rubber seals on the cap breaking that vacuum
4. Sticky thermostat?
5. Good condition of the small diameter hose between the radiator top and the expansion tank?

 

the radiator cap is a small possibility...it may seal well enough against water getting out but not air getting in (?)

sounds like the system isn't permitting re-uptake of coolant from the overflow tank, which means you have air geting in from elsewhere...

check all your hoses for moisture (not necessarily leaks and drips, if it is a SMALL leaK/seepage)

 

There are 2 different rubber seals on the cap remember - one right on top preventing both air and water from moving in either direction, and then the lower one acting as a one way "valve" allowing only stuff out of the radiator - the reverse valve inside the cap opens with vacuum to draw the liquid from the expansion tank and if that vacuum cannot be made to affect that liquid (leaky top seal or actual tube to tank) then it will suck air iso liquid into the rad - every time after that any liquid gets dumped in the expansion tank the effect is repeated and the net result is that rad level drops constantly if the motor gets warm enough