in the past three years I've replaced 3 heater cores. the first two were covered under warranty (extended which I purchased when I bought car) the last one was not. The dealer quoted me a $890 repair bill but was kind enough to halve it for me.... It was explained by the mechanic that this "electrolysis" problem had no solution until recently.... somehow, if copper was used in the replacement... it slowed down/or prevented this electrolysis. Unfortunately, there was no copper available for the most recent repair. The heater core is under another 1yr warranty. The heater core itself isn't too expensive.... HOWEVER, I was told it would take "20 hours" to repair.... hence the expensive labor. Why hasn't ford/mercury advised owners and dealers of thi potential problem? When at the dealership, for the latest repair, I was told that the problem is not that common, HOWEVER, another customer has had to make the repair as often as every three mos or so!!!!