It's called porous block syndrome but it's not really porous.
What happens IFAIK
The later engine management runs the engine as lean as it can for economy, the gearbos tries to keep higher gears as long as possible. Supposedly why 4.6s suffer worse than 4.0s
The cylinder liner gets very hot becasause of the lean mix and heats unevenly due to the slugging away in high gears.
The block surrounding the liner cracks, all V8s after the 3.5 have less metal round the liners which IIRC are cast in place, so if they move during casting the block is even weaker.
Once the crack appears the liner can slip down slightly, this releaves the pressure on the head gasket.
When the engine is running, combustion pressure keep the water at bay, but every time the engine stops when hot a small amount of water escapes the water jacket and gets into the effected cylinder.
A clean piston is a giveaway of water leaks, but this could be gasket or block (or even head).
porous block syndrome gets slowly worse up to a point, then it cascades into a nightmare. Thing is if it's got that it's too late anyway.
Bung a tin of cargo seal in it to cure the crack, then leave some K-Seal in for life to keep it sealed, the stuff does work I'm assured.
Now, if it's got combustin gas in the coolant I'd suspect a gasket. I've never seen Porous block first hand but I'm told the "leak" only goes one way and with a hot engine. One way to tell migh be to run with no radiator cap for a few days and see if the symptoms change at all.
P.S. I'm also told that LPG engines can suffer from cracked rings due to the speed of combustion, not sure how true that is but I know V8s don't like running too hot.
A cheap fix might be to bung a good 3.9 in rather than a 4.0 or 4.6, if it were a 4.0 this would be a no-brainer as reconditioners are paying good money fro 4.0 blocks that havn't cracked.
Otherwise it's a new short motor at about £1,700 plus fitting :(
