Years ago I was riding with a friend in his '79 Cadillac and a loud knock started and the engine boiled over.
To make a long story short, he called a tow truck and we found out that the thermostat was stuck closed, causing the overheating. I asked the guy about the loud knock and he said that the engine had some type of flap in one of the water jacket's passages that would swing closed to prevent back flow of the heated water. That flap, or whatever he called it made the knock.
Anyway, new thermostat, no more knock, engine cooled ok. I never heard of anything like that before or since and can't figure out what would be the purpose of tha "flap", or why it would make such a loud noise. Maybe this helps?
Anybody ever here of that thing that the tow truck driver/mechanic was talking about or am I just dreaming. Anyway, the knock stopped after the new T-stat was put in.
