my 1996 Chrysler Sebring has began backing up in the
coolant system I thought it was the heater core because I
could hear it boiling over so I replaced that it still didn't work
so I bypass that so far I replace the heater core the
thermostat and the intake gasket and it's still backing up it
never gets over half on the heat temperature gauge but when
I shut the car off its boiling over and leaking from somewhere
I'm not sure where what could be my issue?

 

First thing to do is find and fix the leak. Check the pressure cap. It's a cheap and easy replacement. Actually cheaper to replace than to test, so just replace it. If it won't hold pressure, it could explain the fact that you are not getting overheating until the engine shuts off but then it starts "backing up" (whatever that might mean). As long as the water pump is circulating the water, you could be getting adequate cooling, but when the engine (and the water pump) stop, there could be localized boiling, enough to push coolant out past the pressure cap. Make sure you have the proper amount and mixture of antifreeze in the system. If you have added just water to compensate for what has leaked out, you probably have diluted the antifreeze too much and it can start to boil over, especially if the cap can't hold pressure. Check the radiator itself: Make sure air can flow freely through it and that it's not internally clogged. You might want to flush the entire system. After a quarter of a century it could be dirty in there.