Likely Causes: Engine Block

Engine Block
Cracked Engine blocks are mainly attributed to the 4.0 and 4.6 litre engines built before 2002.
The reasons for this problem are well documented if you follow the links on this page.
Cracked engine blocks are also a common problem with the 3.9 and 4.2 litre engines due the age of the blocks, and the fact that they also suffer the 94mm piston size.

Exceptions
New engines supplied after 2002 are not prone to the same errors. This is because new engine blocks after 2002 are likely to have been ultrasonically tested for correct block thickness, and the later Cos-Cast engines are state of the art from brand new casting process.

Symptoms
Unlike a failed head gasket the symptoms of a cracked block normally begin with occasional high temperatures that will not re-occur perhaps for a week or a month later once the coolant system has been topped up and cleared of any air locks. However, continued high engine temperatures will cause further failures such as head gasket failure, leaking water pump or blown heater matrix seals and more.

Remedy
Such engine failures are not suitable for repair, not even with expensive ductile liners.
A replacement short engine is the only reliable option, but only if the replacement engine is Cos-Cast or a post 2002 casted engine.