Coolant Doesn't Want to Return to the resivoir.
 

Havent heard the low coolant buzzer in a while and then I hear it this morning on my way to school. I know it wasnt low, I stopped checked the resivoir it was full so I made it to the gas station right by the school. I go in and buy a gallon of water and go park at the school. After school I come out and the resivoir was topped off and the upper neck on the waterpump, I couldnt see any water.. I fill it up and drive home. I got home let it cool down some and check the resivoir again and its still filled to the top. Why wouldnt the coolant return into the cooling system when it cools back down? I had an eye on the temp gauge the whole way after hearing the buzzer and on the way home and it was never up higher then it ever has been..

No doubt a leak somewhere.
Check out this simular post on page 3.
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/mysteriously-vanishing-coolant-but-doesnt-seem-like-bad-coolant-seal%3B-any-ideas-896870/

As blazr7s says, it could be a minor leak anywhere in the system preventing proper pressurization of the system. Check for an "air" leak in the small-diameter hose that runs from the filler neck to the overflow bottle. That can prevent the coolant from returning to the radiator as the engine cools down.

I havent noticed a leak anywhere. I was just under the car last week replacing my oil cooler lines and laid under the car checking for leaks as it was running.. I see no puddle under the car when it sits for a while. I will check into the air bubbles though.. I thought i had them all out of the hoses and system when I last replaced the coolant. Also I drove it last night to work 20 miles there and back and then back to school and back today and had no problems.

Yes, you have noticed a leak. This problem you describe is the first sign of a leak!

You have a coolant return problem caused by a vacuum leak. It could be in the coolant return hose. It could be in the water pump shaft seal. It could be any sealing surface in the system, so it could be *anywhere*. You will likely find the leak if you do a cooling system pressure test. If the leak does not show up in the pressurized portion of the cooling system, then you should test the return tube. Also you need to get a brand new radiator cap asap.

Good Luck!

Thanks, the cap that is on it was just replaced when I replaced the coolant a few weeks ago.. How can I do the test? or does it have to go to the shop?

Borrow the kit from advance auto parts to pressure test the coolant system. It includes the radiator cap tester.

had the same problem, kept smelling coolant when i popped the hood too, but no puddles. ended up replacing the cap, all of a sudden its good. then a few days later the hose on the back of the intake mani that turns down the the block cracks and starts leaking. chances are a small leak developed when you replaced the cap. check that hose first, it takes a lot of heat.

Is this the same on S4 and S5? I have a S4..

Ill go by advance auto on the wa home tomorrow and get the test kit.

i have an s4 as well.

The heater hose line that connects below the oil filter gets oil soaked and tends to fail. It usually blows out when you are driving in hot weather at speed...

First thing to replace on an FC when you buy it is the hoses, especially this one. It is probably not the problem with the coolant return, but if it hasn't been replaced now is a good time to do it.