Hello lads,

I have a 98 disco 3.9 V8.....it was spitting water out of the expansion tank after I parked it up. I changed the expansion tank cap for a new one (17 psi) and now it has spit the water out of the radiator plug (screwed into the rad top) which disintigrated as i screwed it out. I now have a brass one to replaced the old plastic one.
To give you some background, I work in Saudi and do 145 kilometers each way to work, the ambient temp is 40 plus at the moment.
The engine has done less than 80k kilometers, thermostat is new, rad was removed and flushed etc.when the engine was fitted. The viscous fan is working as it should and the AC is working well. The heater hoses are interconnected removing the heater matrix from the system altogether as is common practice out here.
The engine sounds perfect no knocking etc. and it does'nt do it all of the time, for instance last week drove it to work and back twice with no problem, this week drove it there no bother but it overheated 5 mins after I parked up after the drive back.
Your thoughts would be greatly appreciated

Does it have cats?

Yes it has cats !!!!

stu knows a bit more about this but HGF and even cracked blocks/slipped liners are far more common on the cat equiped vehicles- sorry!

I hope its not that, I found my non cat 3.9 was all healthy after changing the pressure cap- I had though it was HGF for a few months of local driving as it was over pressurising the cooling system.

first off. Replace the other plastic cap with another brass one at the heater matrix air/bleed point. The OEM plastic ones aren't a terribly good idea to keep on the engine, as youve already found with the one you replaced on top of radiator.

Check the thermostat is fully opening properly. If it is, next sop would be to do a coolant sniff test to find out if there's any combustions gas going into the coolant (ie head or heads or head gasket/s failure.
Is it firing ok? coolant and oil both look as they should? If porous head or headgasket failure is suspected it might be worth removing the thermostat completely then putting some 'irontite' in the coolant. sometimes it works sometimes it doesn't, and might just save the need to look for replacement engine parts.

Thanks lads for all of the advice.....I am getting another brass plug as you suggested Joe, and the ebay auction is being watched by the missus back in Blighty, thanks Mad Hat Man.
Now as to the rest...the engine sounds and runs very well...oil and water appear to be fine, it does'nt actually even overheat, the temp guage stays in the normal position.
I have done about 50kms since I changed the radiator plug for the brass one and it seems fine. So I will still fit the other brass plug and I have a spare 17psi cap (GM yellow one if you can get them in the uk) I have a spare stat, just in case although the one in the car has been checked and works fine. I dont think they actually fully close here at this time of year as the temperature has'nt been below 25 degrees c for about a month now.
I have my fingers crossed that this problem may be cracked now (oh dear i hope I don't regret saying that )
On a different tack altogether I had to buy 2 new front tyres for it on Friday, the car developed a bad steering wobble, as if it had thrown a lead balance weight off the front wheel......however when I was in the process of swapping the wheels I noticed both front tyres had developed bulges in the tread.......delamination.....caused by the heat!!! Bugger. Sorry for the long winded post and thanks again for all of the advice...I will keep you all posted