I had this happen on a 350,000+ mile LLY in a rollback C4500. I replaced the one head and 5,000 miles later the other head did the exact same thing. Injector #8 and injector #1... Same casting position on each head, maybe a coincidence... The truck I experienced this with was running green coolant for some years with no SCA or DCA additives, I thought maybe electrolysis was to blame.