high pressure in cooling system even 24 hours after last start

[Uriil]

Saxo SX 1.4 petrol automatic
I had VW passat B2 and VW bora before. On these cars I was able to open coolant reservoir cap without major pressure difference.
When i first tried it on my Saxo 15hours after last start, pressure difference was so big that coolant splashed from the reservoir.
I bought new cap thinking old one doesn't release the pressure, but same happens with new one. When i open it carefully like bottle of Champagne i avoid splash but coolant level goes up.
Is it normal for Saxo or Citroen in general? If not do you think it's blown head gasket?

Another related question, when i started Saxo engine with cap removed and coolant level around MIN it worked fine until the moment when thermostat opened: coolant splashed massively our of reservoir. Is it normal? it doesn't look normal to me, but maybe French cars are special?

Thanks

[ozvtr]

Those two statements combined lead me to believe that you have a pressure leak from the combustion chamber(s) into the coolant system.

I have worked on three cars with TU3 engines in them. I have replaced all three head gaskets and had two of the heads machined. I have learnt that overheating warps the cylinder head. The OEM cylinder head gaskets are of poor construction. And if you have reason to remove the cylinder head, get it checked for warpage and get it machined flat! No matter how good it looks!
The engine is of the "wet liner" type and there is not much of the cylinders, or the block, in contact with the cylinder head. So you need to make every bit count.

You could remove the thermostat and purge the coolant system of air. Start the engine and see if there are bubbles in the expansion tank. You should initially get bubbles as the system purges the bubbles but they should eventually stop. If they don't stop there is most likely a combustion leak.
There are also test kits that detect combustion gasses in the expansion tank.

Under normal circumstances, the coolant level in the expansion tank will rise as the engine warms up, if you don't have the cap on the expansion tank. It might even overflow. HOWEVER, it should slowly rise, not explode! Also, it will fall when the radiator fan comes on. It's just the gasses that are normally in the liquid coolant expanding and contracting.

[Uriil]

Those two statements combined lead me to believe that you have a pressure leak from the combustion chamber(s) into the coolant system.

I have worked on three cars with TU3 engines in them. I have replaced all three head gaskets and had two of the heads machined. I have learnt that overheating warps the cylinder head. The OEM cylinder head gaskets are of poor construction. And if you have reason to remove the cylinder head, get it checked for warpage and get it machined flat! No matter how good it looks!
The engine is of the "wet liner" type and there is not much of the cylinders, or the block, in contact with the cylinder head. So you need to make every bit count.

You could remove the thermostat and purge the coolant system of air. Start the engine and see if there are bubbles in the expansion tank. You should initially get bubbles as the system purges the bubbles but they should eventually stop. If they don't stop there is most likely a combustion leak.
There are also test kits that detect combustion gasses in the expansion tank.

Under normal circumstances, the coolant level in the expansion tank will rise as the engine warms up, if you don't have the cap on the expansion tank. It might even overflow. HOWEVER, it should slowly rise, not explode! Also, it will fall when the radiator fan comes on. It's just the gasses that are normally in the liquid coolant expanding and contracting.

You sound experienced with head gasket issue on that specific engine. Could you comment my logic.
In addition to facts described above we have:
Oil has no milky "cream" in it, level of it stay the same
No white smoke in the exhaust
Engine is not overheating (but rpm rearly reach 2500, short 5-6 miles mainly, cool english weather)
No bubbling in coolant (i checked it when refilled new coolant)
Coolant slowly disappears 100ml in 300miles
I think if it's blown head gasket or warped head gasket, it only created hole between coolant and cylinder(based on symptoms) and is not significant. Probably can last for 1-2k miles

Second question, what do you personally think about head gasket liquid sealant? Do you think it can work in my case?

Thanks

[PaulC5]

Being a petrol take out the spark plugs and see if the cylinders are spotless as coolant leaking in cleans them out. With the coolant level dropping so much it must be leaking to the outside or into a cylinder. At worst in a good engine it might drop 300 ml in well over a year, 10k miles.