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Over heating, Starts with water loss. The early 3.9 and 4.2 (pre 95-96) engines where, basically an over-bored 3.5 casting with 4mm extra on the diameter of the liners, this caused a reduced thickness of aluminium between the water jacket and the cylinder bore. The subsequent water loss problem normally starts off as just a water light that appears once a month or so, Then once a week until it becomes a permanent feature, The normal unsuspecting owner, will have by this time paid for heads to be skimmed and gaskets to be changed by which time they have spent a lot of money already. So, although the engine functions fine it is a permanent worry leading towards a Total engine failure.
Many would be excused for thinking that the overheating is caused by running the engine in a Hot climate, or with a radiator problem. or even insufficient cooling fans, or Oil cooler but we can assure you, this is not normally the case.
3.9 & 4.2 The true & main reason these engines run very hot is due to the fuel/air ratio, or fuel mixture, that is controlled by the engine EFi computer, (ECU Chip) from the factory this was designed to run very lean through the mid range, to make altitude driving or mid range emissions (tested in some countries) less of a problem, the upshot of this however is when these engines are used on low quality or low octane fuels or when indeed the engines are upgraded with items even as minor as free flow air filter, or exhaust headers, & Cam/Head upgrades, although all of these are only mild upgrades they will make weak engine run even weaker, and the problem will get closer!!
The solution is simple. (If its not cracked already) fitting our Optimax or Tornado Eprom (ECU Chip) will give your engine the near perfect fuel / air ratio, it deserves thus, lower engine internal temperatures and give, without other modifications to all 15% efficiency [power and economy & also allow the upgrades you have already done, to be beneficial at last, instead, of being detrimental. 3.9 & 4.2 ECU info "Click"
4.0 & 4.6 (New shape)Suffer a similar but normally less dramatic problem, although these engines are much stronger, "The Same" Air/ fuel ratio problem will arise with low octane fuels, you will not normally however suffer cracked blocks, but you will cause the Liner to shift from its seat or cause sticky valve guides (partial seizure) and the solution is much the same as the 3.9 & 4.2 above, an ECU re chip will sort it out and more Gems ECU info "Click"
All cracked engines (blocks) are not normally repairable due to the fact that it will crack behind other liners, even if you could effectively repair the one at fault
If you are undecided about where the problem actually is, then the next job would be to remove the heads if you see nothing then suspect the worst. (The surest way to tell if your engine is suffering from this problem is the fact that you can find nothing obvious wrong) you could have the heads & block pressure tested if you are still not convinced).
