Take a look at this head gasket we pulled from a 5.0L we rebuilt. The “snake” you see sticking through the head gasket’s cooling passage is a strip of silicone sealer. Somewhere along the line, possibly from an intake swap or water pump replacement, someone got sloppy and let this get into a coolant passage. Crud like this slowly blocks off water flow and causes problems. The older the engine, the worse it can be. Flush your cooling system annually!