Pressure release cap.

Hi all, just gone out to bleed cooling system and start the diagnosis process of the over heating.
Was pleased that I got lots of air bubbles and heat back into the matrix and warm hoses top and bottom.

Then noticed the water was squirting straight out of the expansion reservoir before engine was fully up to temperature. Always thought the cap looked strange but I'm still not confident with just writing something off as wrong. But as a pressure release valve.....something is wrong here right?


Is this another skanky bodge by a previous owner to avoid more expensive work!? I guess I need a new one - I read somewhere that this is a fairly universal fitment thing.....or do I need a Daihatsu item? Pipe was pretty corroded too though so if anyone has a good one spare I'd be grateful.

I think I know where this road ends (as does HJ and other experienced users now) but see no problem with going through systematically as you'd do all this to check it's functional after a HG change anyway or you'll just blow it again and that WOULD suck.

The other question I have is, I was surprised at how quick I heard the crackling/bubbling as if it had boiled over? It didn't seem to squirt that much water out and it didn't take that much more when I tried to refill the system after cool down? Is this an Airlock symptom?

Yep that cap has been doctored by an idiot.

It is no longer a pressure cap, its just a "lid"

As the water expands it will just go straight to the header tank, and the system will never end up pressurised.

Problem is, if you don't keep the coolant system under pressure the bastard water boils at 100 degrees C.

Cylinder heads get right upto 130-140 degree's under normal use. Even hotter if there is no water in there to carry the heat away.

So the water boils in the head, leaving a space of gas (water vapour, oxygen, etc) and the gas can't carry the heat away so the head gets hotter than it should.

Also, with the cap doctored like this, you won't know where the system leaks because it never ends up under any pressure.

If you look on the other side of the cap, it will say "0.9" on it, this refers to 0.9 bar, so it holds pressure up to nearly twice the atmospheric pressure of earth, then it lets some pressure go. This is a safety feature because otherwise if the pressure kept increasing it might go BANG when it blows a hose or radiator open.

So with the cap like that your pissing in the wind,

Buy a new one from a motor factors, I buy these ones....



About 6 or 7 quid from a branch of the Uniparts UK motorfactors chain.



Next you need to clean up the seal points where the cap sits.


After that I would put your new cap on, then pressure test the system to see where you have any obvious leaks, look on this page for some tips on pressure testing:- http://www.daihatsuhijet.co.uk/hijet-overheating-problems-head-gasket-or-not/

If it turns out you have any leaks then fix them, and fill it back up with coolant mix, and see how it goes.

My bets are it'll overheat again sooner or later, and you'll have the head off it.

Ahhh bet that last line is music to your ears hey!!!    

Cheers, I don't mind the last line tbh, I'm quite up for it but obviously would rather not.

So going on your description does that explain why temp did not go above half before the 'crackling'. It was the gas in the head and nothing venting past the temp sensor to give a true reading?