Petit AST Cap
 

I dont know if i have doen the right thing or not !!
I have replaced my Petit AST Cap with one that is EXACTLY the same (Bought it from a local parts shop and done ask me how did i find it :D), but its a 16lbs were the one that comes with the Petit is only 13LBS.

Hope im not harming my engine??

Mazda changed from a 1.1 bar or 16psi pressure cap to a 0.9 bar or 13psi cap in hopes of reducing the stress on the cooling system components to reduce the possibility of under hood fires. Some have done as you did but made sure they were working with new or up graded hoses. Can't say you'd harm your motor but if you did it without considering the results then I'd suggest a large and working fire extinguisher. Seems coolant when it sprays on the hot turbos burns pretty well with all the plastic and wires under the UIM. Nice find though since the Pettit cap is a typical domestic unit.

The whole reason for not having the coolant system under higher pressure (which lowers the boiling point of the coolant) is because the AST's tend to explode. Since you've replaced the AST, I wouldn't worry about it. It is a good idea to replace cracked or worn hoses though, so you should look for that anyway.

The AST wasn't the only reason (that wasn't the contributing cause of fires in the turbo area) the turbo coolant hoses typically fail first due to the high heat in that area, add fatigued hoses with higher pressure and there you go hose failure and coolant fire. Run higher pressure replace the hoses more often.

I'm a little confused... are AST's supposed to have pressure caps? Because my Greddy AST came with a cap that is just... well... a lid, not a pressure cap. It looks like a radiator cap but it's just an aluminum lid, which kinda left me questioning... which is it supposed to have?

Yes you need a pressure cap on the AST, which opens path to the overflow tank. On the other hand you MUST not have a pressure cap on the cooling system filler neck - just a "lid"

By the way, a 16 psi cap is OK and better than the 13 psi, if you cooling system is in order. The system should be tested at 20.6 psi for leakage, according to the shop manual. Higher pressure is better as water will boil at higher temperature.