rx7 s5 turbo. I keep having the annoying leak from the thermostat housing cap. I replaced the plastic piece with the billet one and its still leaking. I put a 1.1 bar cap on it which is higher than the .9 bar cap i have on the radiator and its still leaking when the pressure is built up. I've zip tied it down because every time i pull it off the metal tabs on the cap look bent and I thought that would at least stop it from leaking, didn't fix the problem. I'm almost about to RTV the cap in place and see if that works.
does anyone else have this problem because I cant seem to stop the leak.
and I know its not the seal on the thermostat housing because if i run my finger arround the lip of the cap I find its wet, which means its coming from higher up, and the cap is the only thing that's there.

 

Is the aluminum piece worn down on the little nubs that the cap grips with those tabs? I too have the aluminum piece, no issues sealing here.

Really it could only be the aluminum piece or the cap. You've already tried replacing the cap with another pressure cap, maybe try again but with a flat-cap? Assuming your overflow nipple is on the radiator, of course. That's the current setup I'm using, no issues here.

 

stock setup, and the neck piece is billet and its practically brand new. I had another 1.1 bar cap laying around and drove it around a bit and it seemed to hold for some reason even though the previous cap had no cracks in the seal that I could see with the naked eye.
maybe it was because I didn't wipe the old coolant off the seal enough? freaking weird......

 

It is weird.

One thing comes to mind; the cap has two seals. There's the lower one that sits on the end of the spring, and there's an upper one inside the cap part. Make sure both are clean and unbroken.

If they're both clean and solid then I'm kind of out of ideas.

 

ok, I THINK I figured it out but its really freaking weird and requires a bit of explaining so I'll leave a short answer and a long answer explaining the former.
Short answer: Clean the cap thoroughly and let it dry because if you leave even a little bit of water on the outer lip its going to leak.
Long answer: Because fluid is "sticky" in a sense (as in water likes to stay attacked to water. If you know how a toilet works you know what I mean) whenever the cap vented the water pressed up against the seal, if there is a bit of water in between the neck and the cap it will push the old water out and replace it with coolant that was just vented, causing the cap to leak and keeping the seal broken.