Have seen a few posts on this but not conclusive. My cooling system has too much pressure in it, what ever the reason, should the expansion tank pressure cap release the excess pressure?

 

It could be something as simple as the radiator cap. ? Or are you sure your coolant system is properly bled? how do you know it has too much pressure? hoses swollen?

 

If it is bleeding out onto the street, from where? Most likely cause is a blown head gasket, thermostat, or a bad water pump. But . . . sometimes techs set up the overflow hoses between the radiator and the reservoir bottle incorrectly -- this causes the system to burp to the overflow bottle . . . see picture

 

While most cooling system problems are similar, I think Stu has a X300 and may get more accurate solutions in that section of the forum

 

I have two Jags now, x300 is good & ready for next road trip when we can go? X308 has issues, fingers crossed a new cap sorts it ?