Adding Coolant and Overflow Reservoir
 

I regularly check the Coolant Level on my 1993 FD RX-7 and even if its slightly low - I still top it up.

But I never add any coolant to the Overflow Reservoir - Because from the main coolant can - It eventually empties to the Overflow Reservoir.

Am I doing this correct?

Since the coolant system is sealed, it needs somewhere to vent as it expands when heated and contracts when cooled. That's where the radiator cap and overflow tank come in.

As the system heats up, it pressurizes and pushes against the radiator cap. Some coolant gets pushed out and into the overflow tank. As the system cools, the extra coolant in the overflow tank is sucked back into the system to maintain the level.

Really if you're checking the system you should check both spots - the fill cap and the overflow tank. The overflow tank should be in the middle of the Low and Full marks. If it's a little high don't worry too much about it, if it's overfull you'll want to get the level down. It needs to have room in there for coolant to expand in to it.

Dale

I have found that keeping the overflow tank to the correct levels on the dip stick on the tanks cap stopped the slightly low levels when checking the fluid at the fill tank between runs. Would try and see if it helps keep levels more constant for you as well.