rubber grommit has blown on radiator

hi all have problem with overheating just got back drove about 10 miles noticed the check gauges red light illuminated on dash checked under hood and found at the top of the radiator neck where the rad cap sits at the back of that tee piece their is a rubber grommit which keeps blowing out and im losing coolant i have tryed flushing out the rad and cleaned out resovoir bottle but this keeps blowing the grommit now on last trip this has completely blown and i cant find it.i have a cherry 4l 99 auto any help would be appreciated thanks. tony

Can you get a picture of said grommet?

The only things on/in the radiator neck are the cap and the fitting for the overflow hose. There's no grommet by design. Have you replaced the cap?

As Blaine asks, how about a pic so we can see?