Where to put antifreeze?

Many GM vehicles don't have a radiator cap but the coolant container cap is GMs pressure rated cooling system cap. The coolant container is pressurized too, so unscrew the cap after the engine cools down. Fill to the COLD level. After engine warmup, coolant expands with the coolant level rising in the container and lowers when coolant cools down. The remaining air gap and plastic pressure cap effectively prevents coolant from oxidizing for long life coolant. The air gap allows coolant to expand and contract without leaving the container unless the engine overheats with pressures higher than the pressure cap rating.