What would cause a head gasket to start leaking again, shortly after replacement?
Several things could cause such a problem. For instance if the head were warped, it will not provide an adequate sealing surface. Another cause is low spots in the cylinder head or engine block. This is particularly common when the leaking head gasket was not repaired in a timely manner. Other causes could be that the root cause of the failure, spark knock, detonation, overheating, etc. was not corrected. Head gasket failure is almost always a symptom of another problem.
