[rose_surfer]

the answer above is an excellent explanation and is consistent with the way most freshwater cooled engines like yours work. To get the root of your question though, it doesn't sound normal to me to have your heat exchanger low on coolant. It's hard to tell how serious without knowing how low. But I'd start with topping it off and then checking again after some run time. Also keep an eye out in the bilge for any coolant. Also, if I'm not mistaken, on my 454s, theres a little sticker saying to top off the coolant at a certain rpm - 1200 or something like that. I guess the idea is for the engine to be circulating but not hot. You might check your engine for the same, or look it up on line. My engines are 1989 crusader CID. Finally, if you ever get (or suspect you have) air in your system (usually from breaking into it - checking a thermostat or something like that), you bleed the system by removing the plug at the top of the thermostat housing with the coolant full until coolant spews out. Just a few more tips on the system...