coolant

Hi all, first post. A little different from other 'Engine Overheating' threads, as this seems a little quirky. I purchased my 197 last week and it's been a joy to drive. The garage had fitted a new radiator the morning of purchase, but every now and again I'll have an 'ENGINE OVERHEATING'...

Can anyone identify what the hell this is?! It’s black plastic and reaches from the just above the bottom hose connection of the radiator up to just under the coolant expansion tank. It looks like it should have a large hose on the top end, but there’s nothing on mine. It’s held in very loosely...