Likely Causes: Engine Block Head Gasket
Cylinder Head Gasket
If your engine is fitted with the earlier type shim steel tin gaskets then leakage is almost guaranteed. They should always be replaced with the composite gasket type because of overheating issues that may arise in situations such as long traffic jams. However if your head gaskets are completely blown through then replacement and consideration as to the cause is essential.
Symptoms of leaking Tin Head Gaskets
Normally you will suffer nothing more than the need to occasionally top up your coolant system. Topping-up will become more frequent if you make longer journeys in very hot weather, or perhaps stand in long traffic jams - in such events overheating will become a major issue.
Symptoms of Blown Head Gaskets
With such a situation you will normally be able to travel no more than just a couple of miles before the engine boils, pressurises, and ejects most of its coolant. This situation will apply to both Shim Steel (Tin) or Composite head gaskets.
The only choice is to replace the head gaskets and consider what has led to the failure.
With the 3.9 litre engines all the way to 4.6 litre engines this is most likely to be with the engine block itself.
There is always a deeper reason for a failed head gasket so please be completely sure what actually caused its failure before you go to the time and expense to replace, also never ever re-use your original head bolts, always fit new or better still fit an ARP Head stud kit.