The “after” photo is what I had on my 2002 Plus 8. The black expansion tank where shown. 15psi cap on it and blank cap on the rad. The term “expansion tank” is very apt! It collects excess hot coolant to either feed back to the system when it cools or bleed it to a tube to drip away if in excess. By keeping this tank filled to the correct level it prevents air getting back into the system. That air is either trapped airlock expelled into the tank or fresh atmospheric air.
