Thanks everyone for your input. Believe it or not the curved portion of the cap that goes down inside the surge tank is now one piece. Then, when you screw the cap back on (and yes) they gave us a new one)...it is very subject to move on you. The engine overheating was not the cause. Phoned Freightliner today and one tech was extremely helpful. He knew exactly what the issue was. He suggested we use some RTV silicone to rub on the small gasket that goes over the plastic surge opening. When we give it a try tonight, I hope it remedies this.