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[harpersimon]

Still trying to establish whats causing my symptoms here - but have a  theory..
I have carried out one system flush, run it abit, flushed again and to be fair the coolant looked pretty tired and crappy, but I am still getting it forced out of the pressure cap
Since the car temp fluctuates alot up and down (goes up.. fans kick in.. comes down.. goes up.. etc) more than I think is 'normal' - and when I turn the car off the coolant fills the header tank, and forces its way out of the cap - would I be right in thinking that system pressure is not being maintained, and therefore the coolant is boiling over/causing excess pressure which then escapes through the cap? (have had this on other car makes but wanted to check if this could be the cause of my issue)
My theory is that system pressure is never maintained totally 'sealed' and the coolant is running too hot.
The only escape I can see (having peered into where the HBV is, and examined the patches of coolant on the floor!) is from the header cap - which I am about to replace as its not that expensive, but the question is are there other potential system leak points/weak points?
Thing is I am assuming if there was a pressure leak elsewhere then I would also see a coolant leak from that point, so have pretty well ruled out other leaks.

[Teebee]

If you have coolant on the floor that might help diagnosis. If it's the cap the leaking, fluid should be close to the cap position. My car still has an intermittent leak under the gearbox, I thought HBV but now i'm almost certain it's the pipe going into the coolant bridge which is completely hidden from view behind the inlet manifold. I suspect in my case I will have a replacement car before I fix it 

[harpersimon]

sorry if I wasn't quite clear, but I defo get fluid out the cap. Can watch it when I pop the bonnet! - just wondered if thats a cap failure, or it doing its job to let out excess pressure .. or something. I am running out of ideas.

[aaronjb]

If it's overheating and pushing coolant out of the cap it could be overpressurising due to a headgasket leak between cylinder and coolant (you don't always get oil/water contamination with a HG failure) - you can test that if you do a dye test with a kit from a motor factors..

Alternatively it could be a faulty seal/pressure relief on the cap - a dye test is possibly easier than a new cap unless you can swap with someone..

good call.. Ill have a pop at doing that..when I am able to get near the car again in a maintenance capacity (new baby :-) )

Women can sort that out  (waits for sexist replies  )

You can tell I don't have kids.. I read that as the car being the new 'baby'! 

[harpersimon]

LOL!
Its not overheating at all.. on the guage. Just seems like a pressure leak causing the fluid to boil up and overflow when the car stops. when Running its fine.. The net result is similar to what happens when you remove a coolant cap when the system is hot, only a lot slower. I can see the coolant trying to boil when I pop the hood after a run.
I may have a breakthrough though. While examing the cap, I noticed that there appears to be a crack around about a third of the tank filler neck.. so am going to get a replacement header tank. Can't do any harm.

[harpersimon]

Little update to help those with similar issues. WORTH TRYING!
I replaced the cap, and still had the issue - but then got another Tank and replaced that and presto, issue seems resolved. There were obvious cracks around the neck of the expansion bottle that let in air and hence return the system to atomospheric well too soon when it s hot. Thanks for assistance.

[Andy H]

Sorry both getting confused there, no there is no release valve, as mentioned above its a sealed system.

Has anyone ever put rad weld in the cooling system? Also when its up to temp, does the big pipe from the top of the rad to engine get hot?

The cooling system isn't totally sealed. There is a pressure relief valve and an air admittance valve built into the cap.

[harpersimon]

Its more pressure relief than air admittance though surely - as air in the system will make the coolant boil at normal temps (as was my issue) i.e. shouldn't be letting air in!