Is there a catalytic converter in the exhaust still?? Or back boxes?

Take the exhaust off at the turbo / downpipe an run it with no cats/back boxes to test it

I had this exact problem before on an escort gti !

The honeycomb stuff in the cat had broken up into chunks and 1 big piece had pretty much blocked the exhaust pipe completely, the gas had nowhere to go and pressurised the coolant system and all my water hoses and the rad blew up like balloons and rock hard to feel,

Took a lot of working out but ended up taking exhaust off, smashing all the bits of cat out the pipe put it back on and car was fine,

Marc