Some of these suffered from factory
defective head gaskets, and many were recalled to dealers for replacement after the defect was discovered.
Probably the most common cause of that is a head gasket with a leak between the two, which could be caused by high compression / detonation (there are several possible causes for that including incorrect ignition timing, low octane gasoline), an engine overheat, uneven head bolt torques and / or
a defective head gasket.
Not exact matches
I think they should accept some responsibility for the second instance of overheating and
head gasket failure since the electrical thermostat was probably
defective when they changed the two fans.