Bad Header Tank Cap?
 

Hi all,

2007 STi. Rebuilt approx 10k ago.

These aren't the best pics by any means, but I think good enough.
I put a UV dye into my coolant a while ago, and searching with the UV torch tonight I found this. The standard genuine 1.1 bar cap I'm using is approx 5 or so years old.

Maybe bad cap, or hoses? Difficult to tell. It's not where I have spilled coolant when checking the level in the tank as I'm very particular about that.
I do have a virtually brand new genuine STi 1.3 bar cap from ICP (so it's genuine not Chinese etc)

But, not sure if those 1.3 bar caps are a good idea? I don't want to harm my mileage wise fairly fresh build.

Clean the area and give the 1.3 bar cap a go?

https://cimg7.ibsrv.net/gimg/www.sco...5edcb4b16f.jpg
https://cimg8.ibsrv.net/gimg/www.sco...fa40f40e07.jpg

Given the fact its a genuine sti cap I wouldn't have no issues using the cap mate but it all depends on what your using the for. If its just the normal everyday use with the odd bit of a blast, then I suspect you won't have no issues. If your round the track every week and running the car hard causing the temp to all ways run up hard etc then maybe not

Are you sure it's not the tank itself that has a hairline crack? That is quite common.
Personally I wouldn't ever use a 1.3bar cap; if you Google it you will see some very good reasons for not running one.
edit; the 1.3bar cap raises the boiling point of the coolant system, so it's designed for peak temperatures eg track work. The cons are that you should keep an eye on pipework and particularly oem rads for leaks from increased pressure, especially on older cars. I imagine it's not ideal for short journeys either.

Hmmm, I think I'll give it a good clean and check after a run with old 1.1 cap on, may be then able to pinpoint where it's actually leaking from.
My coolant in the overflow reservoir bottle does drop down a bit and needs topping up sometimes, but header tank is always full to the brim, so fingers crossed the leak is external.....

I've had it pressure tested a few times, but nothing obvious found, but on the plus side, no white smoke at start up either. I wondered if a coolant pressure test would seep coolant into one of the cylinders if my HG was going already, or a warped / cracked head, but it didn't appear to. Not that I'm paranoid about the 2.5 or anything....forged or not!

When my block cracked on my old build on cylinder 1 nearly 2 years ago, believe me that was a total white out....then had to drive back through my own thick fog to get the car back home lol.