That is strange. On the Miata, there is a small hose running from the bottom of the thermostat housing to the water pump intake. When it is clogged, the Miata temp gauge will do exactly what you described. It's because the thermostat element is not "seeing" hot coolant from the cylinder head flow around it (it's closed), so it takes a while to heat up and open. In the case of the RX-7, I see only the hose from the thermostat housing to the AST... but since that coolant would eventually flow to the overflow tank, and not back to the water pump input, that doesn't make any sense. Is there an internal casting path inside the RX-7 thermostat housing that flows a small amount of coolant around the thermostat element and into the radiator hose?? And can that get clogged? If it did, would that drive coolant back into the overflow tank (with or without an AST)?

 

I honestly don't know.All I did was remove the tstat and the antifreeze,flushed the system and added a bottle of blue devil from autozone.I haven't put the tstat back nor used antifreeze since and the max the temp would go on PFC is 89c.I have added a small amount of "leftover" water mixed with blue devil(very small amount left)once and so far so good.To me it's no doubt that I have a bad coolant seal,but so far,knock on wood,everything seems to be fine

 

If there is a bypass path, and it was clogged, and your flushing cleared it, I would expect your quick over-temperature followed by a fall to normal would have gone away.

 

Have to apologize for my stupid errors in post #26 above. Of course there is a connection from the bottom of the AST to the bottom of the radiator (i.e. input to the water pump). Was writing from mistaken memory instead of well-documented coverage of cooling system. So it would be possible for either that hose or the thermostat housing-to-AST hose to be clogged and prevent the thermostat element from being heated rapidly by hot coolant from the engine. Your flushing probably cleared the hose(s) and solved the initial overheating with return to normal.