I have been in the used car bus for 30yrs we just sold a 2001 dakota 4.7 had the same problem but it ate all the metal out of the thermostate housing and all the water ports then it took the headgaskets about 200 miles later like I said hook up the grounds like it sounds like they did and sale it to someone not on moparts or plan on possibly replacing motor electrolosis kills metal inside the motor with any water contact overtime