Vauxhall corsa c coolant pressure problems

[Trickett43]

Hello
im new to this site, im an apprentice mechanic and im having trouble with my sisters car just wondered if anyone can help!
okay its a 1.0l vauxhall corsa c and a few months back it had a bad head gasket failure which i replaced and it runs and drives fine but if you run the car then leave it for a day or so and open the coolant cap it hisses like its been holding pressure! it struggles to get the heater to blow hot could this be a blocked matrix adding to the pressure problem? Or could it be head related? has anyone had this issue or have any ideas of what it could be?
Thanks jake

[Jimmyarm]

If the system is holding pressure like that then there is probably a crack in the head that was missed on the test or more likely a fracture in the block somewhere.

Have done plenty of engine swaps on these for a similar sort of thing.

This assumes its been bled up properly, when filling these engines remove the expansion bottle and hold it up high (Vauxhall made them intentionally easy to remove).

[Steve_D]

Jimmyarm said:

If the system is holding pressure like that then there is probably a crack in the head that was missed on the test or more likely a fracture in the block somewhere.....

Surely this comment is back to front.

If the cooling system is holding pressure for a day or more then there can be no cracks or gasket defects or the pressure would disipate.

Steve

[Trickett43]

Thanks for the reply, i was thinking the head maybe cracked or something but surely it wouldnt hold the pressure but it holds a fair amount for a while, could something cause the system to over pressurise, or is it just the nature of the engine which has done over 100k miles because it drives fault less.

Jake

[littleredrooster]

Isn't it a hiss of vacuum rather than pressure - i.e. air going in rather than out?

This would be natural as the system cools down and contracts, and indicates a blocked one-way valve in the cap.