Alright here is my problem, very simple. The coolant that makes it to the overflow bottle sometimes doesnt make it back into the radiator.

Yesterday I was driving to work and my coolant buzzer went off for a few seconds. So I pulled into a gas station, let the car cool for a bit and took off the cap. It was obvious that the fluid level was low, but there was a good amount of coolant in the overflow bottle.

I bought a bottle of water at the gas station, an filled my radiator back up, no problems since. But, why would this happen, why would coolant go into the bottle, but apparently not back out???

 

Look for leaks. Especially at the rear of the engine under the upper manifold or at the heater hoses. The coolant level should not drop that drastically unless there is a "leak" somewhere--either externally or internally. And BTW, use coolant/water mix of 50/50 when topping off the rad. Boiling point is higher than straight water when using a mix.

I'll let someone else answer your question since I'm not entirely clear on how the coolant is transferred back into the rad from the overflow bottle, or even if it is supposed to.

Edit: From Wikipedia; "When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so the setting of the spring on the cap determines the maximum pressure in the cooling system. When the pressure reaches 15 psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system. When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled."

 

I know mine does it because of coolant seal failure. How many miles are on it? How does the coolant in the overflow bottle look? It could be like Go48 said though just coolant expanding into the overflow tank.

 

https://www.rx7club.com/showpost.php...1&postcount=11

 

Well, it doesnt happen too often. This was the 1st time its happened in at least a month, possibly longer.

The motor has roughly 70K it, its a Mazda reman

 

is your fan shroud on and fan blade? or have electric fan?
idk? don't know what you have on your car... somtimes that would cause it to over flow the bottle and buzzer since its not cooling

 

that would undoubtedly indicate a slow leak somewhere. i have a similar problem, the water level in my overflow tank is slowly receding not at an alarming rate just a tad bit perplexing. i think mine may be leaking somewhere around my water pump housing but ohh well. i frequently check my level so no worries.