Anybody can check their rad cap ? Mine was replaced at my last tune-up, but was replaced with a OEM one, not the Mazda stock.

Now the cooling system seems unable to hold pressure, coolant is constantly overflowing to the overflow tank and never returns (so I get the Low Coolant buzzer ALL the time), and yesterday I got a slight overheat (it was almost 35 degrees Celcius outside). When I went to check the radiator/overflow tank, the overflow was FULL (it was at the "Full" mark the same morning). When the engine got colder, the coolant returned to the radiator.

The rad cap is for 0.9psi, but its lower disc seems to be smaller than the neck of the rad, and the rubber disc at the very bottom can move almost freely (i dont think it should move at all). Not only that, but the rubber disc is quite bigger than the rad neck, so it kinda gets stuck between the metal disc of the cap and the neck of the rad, and is probably pulled to one side (so I think the other side is "exposed" and never trully sealed up...

EDIT: I had to ground the Low Coolant sensor in order to make the car "streetable" (i.e less annoying ) because I would get the Low Coolant buzzing after 30-40 minutes of spirited driving....

 

mine says 13psi but mine is not the original one either

 

Sorry, It should be 9psi....

The thing is that either a) the cap is not fuctioning correctly (yesterday there was cooolant coming out FROM THE CAP ITSELF, not around it but from within the CAP ), or b) the cap never fully seals properly, and lets coolant through no matter what the pressure is (or does not allow coolant to return properly).

The things to check are whether the rubber disc on the the very bottom of the rad cap is exactly the size of the rad neck (or if bigger, how much bigger ?), and if it can move at all on its hinging point (mine can move around about 1cm )

EDIT: I went and took a look at the cap. Its actually "0.9kg/cm2", not in psi. A cap for 9psi, that translates to.....0,62kg/cm2 Is that possible ?

 

Actually, OEM (original equipment manufaturer) means the same thing as stock Mazda.
The "0.9" means 0.9bar or 13psi, which is the right pressure for most cars, includinf the FC. It sounds like you might have been given the wrong cap. Just take it back and swap it.

 

Then I stated it incorrectly: the cap I got is not labeled at all, and is VASTLY different from the Mazda one that was on the car. The 0.9kg/cm2 equals 13psi (I guess, more or less), but the thing does not seem to seal at all....I suppose it is just a cap that happened to fit the rad neck for the FC....I just ordered the stock one, just to make sure

 

Your symptoms of overfilling the puke tank are classic for a defective radiator cap. Replace it immediately.

What is happening is the pressure is building up causing the coolant to overflow, but when the engine cools there is not an airtight seal and the coolant is not sucked back into the engine.

 

Yeah, thats what I thought too....the cap just allows coolant to over-flow and go over at the overflow tank just too soon/too easily, and then it either blocks coolant and prevents it from going back, or takes in air instead of coolant, so the radiator sucks in air as well as coolant.

Anyway, the stock cap should fix all these (I hope....) Damn this issue is dragging for more than 2 months now....and now the car overheated -just a tad- because of this