I find I have to keep topping of my coolent every month or so, otherwise my buzzer (add coolent light) will suddenly go off. It seems that, after periods of driving the coolent will eventually make its way into the overflow tank. Why? It never over heats. I bought a new stock 13lbs radiator cap, I thought that fixed the problem. But surely a month later, the overflow slowly filled up. Does anyone have any idea why its doing this? I just bought the car a month n a half ago.

edit: 1986 RX7 GXL

 

Thermal expansion. Its like boiling water with a lid on the pot. Pressure builds up and will find a way to escape. As the coolant heats up, it expands and takes up more volume and creates more pressure. To prevent damage to your engine, the pressure cap relieves pressure at a given PSI. The fluid goes into the overflow. After the car cools off and you start it again, the vacuum caused by the lack of fluid should draw coolant back into the system from the overflow bottle. Does your overflow get full and start running over? When the engine is at operating temprature, look in the overflow bottle and see if its bubbling. If the tube is blowing bubbles in the bottle, then that is usually a sign that the rotors compression is getting past a damaged coolant seal and is pressurizing the coolant system and preventing the suction needed to pull coolant back into the system.

 

A small hole in the overflow tank tubing could also cause your problem- the coolant finds its way to the overflow with the system pressurized no problem, but when the system cools and vacuum is supposed to draw the coolant back into the system, the (alleged) hole in the tubing prevents this. Viola, the tank keeps getting more & more coolant (much rosier scenario than a blown engine seal)...

 

That was the first thing I checked for. No bubbles, with the cap off and the car on, no coolent spits out. So no problem with my coolent seals. I even ran some radiator leak through the system just to be sure im not losing coolent.

 

Thats something I haven't checked for yet. If there is a hole in the tank itself (top or side maybe above fluid level), I assume that would cause the same thing? I'll look for that later tonight.

 

Any updates, Wargoblin?

 

um ... i need to add coolant to the radiator once in a while and coolant overflow tanks gets all my coolant but wont go back in too
let me go check ...

 

GOOD EXPLANATION!!

Can we get this on the FAQ?

It sure beats the normal "coolant seals failed!" answers

 

yeah every time i hear "your engine is blown" i get depressed, but know mine is not so there is always hope!

 

As others have said...the vacuum is not pulling the fluid back into the rad. on cool-down. So you have a tiny vacuum leak somewhere. In the tubing as suggested, the rad cap itself, the rad end of the tubing, or there could be a rad/hose leak. This could be so small that you do not notice the leak when hot (or heat closes the leak).
My suggestion: change the small hose to the recovery tank, and put a gear clamp (the smallest size!) on the ends. After all, that hose is getting pretty old!
Failing that, a trip to the rad shop for a pressure test. that might find it.
Last resort: a housing seal leak. There has been suggestions on here how to effect a temporary/permanent fix by use of "Barsleak" or similar rad leak sealant. Try searching for that one.
Good luck,
Dave