Overheated ZX - any ideas?

[ChristoT]

if the fans is melted perhaps it failed altogether. could be atleast partly to blame

Yeah, it's melted the shaft, and it was banging against the radiator (not the knocking noise I heard on the road though). But at 70mph, the fan shouldn't BE on! What alarms me is how fast the system failed. That's why I fear a major fuckup.

Thermostat might be stuck shut, not opening and letting the hot water circulate, or a blockage somewhere.

One of the first avenues I plan to investigate.

A common problem on some cars is that the impeller in the water pump is plastic pressed onto a steel shaft and comes loose.  Any restriction in the cooling system is going to increase the resistance on the impeller making it start to slip.  Once it starts slipping it will heat the plastic very quickly and soon the shaft is spinning but the pump is doing nothing.  If that is the case, you only need to flush the cooling system and fit a new pump.

If it's the same as something else of course......

There's a really nasty burning smell from the rad cap, like scorched rubber. Not pleasent, and very worrying! I plan to give the pump a full overhaul.

On a ZX-4, the pump runs both the oil and coolant system. If that failed, I guess the engine could have overheated from oil starvation, and no cooling.

Certainly an avenue to investigate!