How does the AST work?
 

Info:
cap at radiator neck: regular pressure cap
cap at ast tank: 0.9 bar (13psi)

Ok so after fixing a bunch of leaks, I pressure tested the coolant system from the radiator fill neck and it’d drop from 13 psi to about 11psi and stay there. The cap on the ast is new from oreileys and I was wondering is that how it’s supposed to work? Thought it was supposed to stay at 13psi and only vent from ast at anything above that? Just making sure to see if the ast cap is doing its job

when I pressure tested from the ast tank, it stayed at 13psi so I know I have no leaks

Please someone let me know and if you can refer a ast cap my way let me know!

also I’m still stock twin turbos

Sounds like O’Reillys probably got in your pocket for a bad Chinese cap with a weak spring. OEM has always been my choice with such things. I’d probably just order one from your local MAZDA dealer to save time and hassle. If you don’t have a dealer close, give Ray Crowe a call. Trusted longtime source for OEM…
Crowe.Ray@aol.com
(703) 785-5554

Even though your filler (non-sprung) cap is sealing, if it’s not OEM I might just get both at the same time.

When running an ast you only use 1 pressure cap and that will be on the ast. The cap on the fill neck is just flat and has no spring. Deleting the ast is what you really should be doing. It hasn't been shown to actually do anything or serve any purpose other than fail and cause problems.

Hey guys yeah I meant the radiator cap at the filler neck isn’t pressurized my bad.

just ordered an OEM ast cap from my local Mazda dealer for around $56 yikes! Oh well better than a failing ast cap.

thanks everyone just wanted to make sure!

Damn, I figured $25-$30 but $56? Wow…makes me wonder what Ray gets for them now.
Still, I think OEM .9 bar is the way to go and it should last and fix the issue. Assuming the car is new to you deleting the AST PROPERLY is something you can consider. And you’ll still need that good spring cap. FWIW, I deleted mine some years ago with no regrets. The plastic AST, like the original radiator with plastic end-tanks, tend to eventually fail with age. All metal AST’s and radiators are available. But do your due diligence and search here on the forum on both issues.

Yeah I thought about it but my AST tank and radiator are aftermarket thankfully. I’m aware of the OEM ones failing especially the OEM ast along the seams. Although my ast is aftermarket, it’s still some sort of plastic material but shaped like the RE metal one. My radiator however, has metal ends so I don’t have to worry about those plastic ends failing. My old ast OEM cap failed hard, which is probably what resulted in my last overheat (plus those leaks I found at the coolant level sensor, and upper and lower rad hoses). My old ast cap couldn’t hold pressure at all when I pressure tested the coolant system. Would drop pressure insanely fast.

Pretty much bulletproofed the whole cooling system minus the lower turbo hose because that’s horrible to get to. No leaks though! Finally