This morning, while testing the cooling system, I let the car idle for several minutes while watching the temp. It got up to 96 celsius, fans did not come on and the overflow bottle started spitting coolant.

Aren't the fans supposed to come on at 100degs? If so, why would coolant overflow happen at 96?

 

The overflow cap is pressure-dependent, not temperature dependent. Something is causing pressure in the cooling system to exceed the cap's spring pressure. Very possible the cap has failed and opens at a lower pressure than normal. Easy fix, the caps are like $5.

Otherwise the overflow bottle has cracked and it's spitting by the crack.

Otherwise, see if you're getting bubbles in the coolant with the cap off.. that would mean exhaust gases in the coolant, which will be more expensive.

 

It's a new bottle, but an old cap. I'll order a new one and see if it helps.
Thanks.

 

It seems the cap that the last owner put on the radiator had a lower psi than stock. 12 or 13 psi. Got a new stock cap and seems to have fixed. Tx.