Explosive expansion tank

[Mitching]

The other day I finally got round to doing my anti freeze.
I drained it, hosed through the rad then topped it back up then ran the engine until the fan kicked in, then drained it once more before putting in about 3L of anti freeze, then topping it up with water.

I put the cap on the expansion tank, then turned the engine on until the fan kicked it, at that point my stepdad unscrewed the cap.
This made all the antifreeze in the system go crazy, it bubbled out of the top, as well as erupting out of the side of the tank (The side where it attaches to the strut, there's a small black shield covering it.
After that I duly topped it back up, after panicking for a bit that is.

But now, when the water temp gets up to half way, just as the fan is kicking in the tank starts erupting again!
This mostly happens when I'm idling after driving, as that's the only time it really gets hot enough.

Am I right in thinking it was coming out of a pressure release valve in the tank? Yet the valve is f**ked?
I did a google on it, and there's a thread on CGTI, RJ says how the valve in the cap is dodgy so should be replaced by a new, blue cap.
After some clarification, hope this is the case and I'm not just being paranoid as usual!

[topher]

most likely the cap. What was he thinking unscrewing the cap on a pressurised system? keep him away from your car

[Mitching]

That's what I was thinking!
I was sitting in the car, revving it a bit to speed up the heating process, next thing I see is water everywhere!

So him taking the cap off probably and the instant change in pressure caused the valve to go?
It's normal for the water to come out of the side of the tank?

[X4MGS]

http://www.golfgtiforum.co.uk/index.php?topic=106380.0

 

[Mitching]

http://www.golfgtiforum.co.uk/index.php?topic=106380.0

 

That's the same post I found on CGTI, he also had some links (which didn't work) to buy new caps.

[keg]

m8 my car is the same i have the cap undone part way just to back the pressure off just a bit
not so undone it can it run out of the top but so i can just let some pressure off untill i can get a cap
i don't recommend you try it but if you do just let off the cap a small amount at a time (you will hear a hiss sound) as the pressure is let off

[dadrathers]

I would certainly try a new blue cap. My header kept getting pressurised even with a new cap and I discovered that the narrow pipe inside the top of the radiator that feeds the thin rubber hose form there to the header tank was blocked. Once cleared it worked a treat as I then had a decent amount of water spraying out of that pipe into the header tank. So, you might like to check that out especially as when it boiled over it could have released all sorts of crud that could block this small pipe.

Good luck.

[cняis]

Mine did this, the cap was MISSING the seal! whoops!