Best thing to do is do a Cylinder leak down test to see it the head gasket is not blown. Odds are if the coolant is blowing and bubbling out the over flow the head gasket is blown.

Same deal happened to me. I flipped because of the money I already dumped into it and a simple rad cap was my problem. Good luck with it.

+1 on the radiator cap. Might as well try it to make sure. Also, remember to burp your coolant system as well since the coolant got pushed out.

Deffinately try the new cap and make sure to get all the air bubbles out of the system. If that works then great, if not, do a leak down test. I have had people tell me my head gasket is blown on several different occasions and it's always something stupid like getting a new cap or I just didn't burp the system good enough.

Like everyone else has said definitely try the cap first. Have you flushed and burped your system lately? That might help as well

I'm having the same problem with my 2g GSX. When I let my car run it gets hot and coolant starts bubbling up and pushing into my over flow. It got hot and cracked my radiator. So I plastic welded my radiator and burped the system and got a new radiator cap and replaced thermostat. None of that fixed my problem. Do you think it could be my water pump? I heard that if my water pump was bad there is two little wholes that coolant will leak out of to tell you the water pump is bad. I can not see it leaking out of the water pump because the water pump is in an odd spot. I don't want to go through all the work replacing the water pump if it's my head gasket that's bad. Any ideas? Help please..