its probably all furred up or full of old goo
if pipes are getting hot in and out it confirms that heater matrix will need replacement
be sure to completely flush out rest of system too though or it will bung up again
radiator leak fixes (snake oils) often block up these matrixes quicker than you can say shazam, this is one reason why i always implore fello fiatforum users not to deploy these products
poor or no antifreeze causes this issue too