240sx coolant boil problem

[damienknight]

I have a 1990 240sx automatic. I have the stock KA24E engine in it. I'm going to convert it to manual, but first I need to figure out how to fix my damn problem. Here's the deal:

When I drive it around, the coolant will overflow in to the reservoir and start to boil. I've alread tried flushing the coolant system, and when I refilled it the radiator only took 4 quarts of fluid, instead of the specified 7 1/4 quarts. I tried running the engine with the cap off to see if there was air in the system, and the coolant started overflowing from the radiator after idling for less than a minute.

I'm just looking for advice as to what steps to take next, if anyone's had this problem that would be great. I'm afraid it might be the head gasket, but I want to try other things first, thanks.