So, I read through the old threads and couldn't find a definitive answer. I recently deleted my AST with the help of Banzai Racing's AST Elimination Filler neck. Afterward, I noticed that the car was splitting coolant from the vent hole in the overflow tank. Apparently, I was supposed to replace the flat coolant housing cap with a pressurized cap when I deleted the AST which may have been causing the problem.

Pressurized cap now installed, coolant filled and burped using a Lisle funnel. After re-installing the new cap, I am still getting some drippage from the vent hole in the coolant overflow tank, which became full to the top after adding coolant (through the thermostat housing/filler neck).

Is this normal after filling the system? Is it normal for the overflow tank to fill when filling the coolant system?

 

you filled the engine with coolant with the overflow already full.......

what you have is going to happen. just let it continue and it will sort itself out. if you dont want to wait then you need to siphon out a lot of that coolant from the overflow

 

"Is this normal after filling the system? Is it normal for the overflow tank to fill when filling the coolant system?"

No and no. It sounds like you have too much coolant in there now.

With the pressure relief cap now on the filler neck, some coolant will be allowed to flow back and forth between the engine and the overflow tank as the system heats up and cools down. Once the system reaches equilibrium and has no more need for burping, the dipstick in the overflow tank should be at the "full" mark (about in the middle) with the engine cold. It not, you can add or remove coolant from the tank as needed to be at "full" when cold.

 

Definitely syphon off the excess. I would use a mityvac with brake bleeder. You don't want to have it almost overflowing and then on a hot day, track day, HPDE or whatever suddenly and without warning get coolant all over your front tire because you're running a little hot.

 

Yep. A system like ours has to have room to expand and contract.

Coolant gets hot and expands. Extra coolant pushes past the spring in the radiator cap and goes to the overflow tank. When the car cools down, the system contracts, and sucks coolant back from the overflow tank to stay topped off.

The overflow tank isn't sealed by design.

I usually keep the tank a few notches under full and you're good. Worst case jack up the car, remove the fender liner, and take the tank out to drain it.

If the tank is super jacked up they're about $100 new from Mazda.

Dale

 

The neck (where the dipstick fits) of the overflow tank is attached to the tank proper by a flexible foam rubber or plastic coupler that can rot and leak after a while. At least mine did. If the tank has too much coolant, it will slosh around and leak from the rotted coupler. I repaired the coupler and learned to keep the coolant level in the tank from getting too high.

 

True. The rubber grommet just to the right of that in the picture is the air hole that lets the tank breathe, it can't be sealed as it's a non-pressurized tank. It will barf out of that hole if the tank is overfilled, boils over, bad water seals, etc.

Dale