1999 Cadillac STS Cadillac STS Head Gasket

Your best bet would be replacing the head gaskets, and at the same time installing thread inserts in the headbolt holes. It is the threads that fail resulting in an internal leak, the gaskets were never the root cause. I have seen next to none fail after having the threads repaired, and there were times where we performed 3-5 of these repairs a week. The problem with a used motor, you may be faced with the same problem at some point in time. There is no way to tell if the threads have been repaired unless the engine is torn down.