Please play nicely. No one wants to listen/read a keyboard warriors rants....
Starting to think again it's expansion tank, I checked tonight, found moisture around the rim of the cap at the bottom.
Top of the tank is showing signs of the dye.... (before this tank was clear all over, with nice red GM coolant) the dye has affected the colour.
Again, bubbling sounds around cap, if my ear is close.
Post up another pic without the cap on showing the neck of the tank,as Entwood says cracks are common there
Expansion tank is prone to minute cracks around the neck where the cap screws on, these can prevent the cap sealing so that it leaks vapour when under pressure, it rarely leaks liquid as there is an "air gap" betwen the surface of the liquid and the cap. However, the loss of pressure reduces the boiling point of the coolant and leads to an increased evaporation of liquid. This could be the reason for your fluid loss.I advised he put a washing up/water solution in an old pump spray bottle a while ago and check for gas leaking round the tank and hoses.
One way of checking is to run the engine until well hot, ie, the pressure is at its potential maximum, and then to brush a soap solution around the lower edge of the cap, if it is leaking you will get bubbles.
A new expansion tank and cap is less the £50.00 ....
did you not try that Mr Tunnie
Despite what GM shove on the sticker, its not really good for 5 years. In the same way my XJ shouldn't have to wait 16k for an oil change. I generally like to change my 5yr coolants at 2-3yrs, and the older 2yr coolant annually (though the Kawasaki was the last vehicle to use that)
Coolant, like oil, starts to degrade from the second it goes in, and I know you're not daft enough to leave your oil in for 20k, as per GM spec. No offence intended with my earlier remark, but we know from previous posts that the Omega has been at the bottom of the priority pile. Just offering as a suggestion as to why the normally bombproof 3.2 MLS gasket has failed, *IF* it has. I'm not yet convinced it has, as a coolant to cylinder failure would often give massive pressure in coolant, and exhaust fumes in coolant bottle. Personally, I'd be getting my ass in the valley, and pressure the coolant to around 8psi.
As said, not offence intended in any way, and hopefully none has been taken.
HG on 3.2 is MLS, so shouldn't rot with abuse like an older composite. But given tunnies service schedule, its possible I guess.
But if the coolant isn't going anywhere else, it has to be going inside.
As a fellow coolant-weeping-V6 owner I can offer my own experiences. I'm still in the middle of doing this myself, mine is/was producing a warm/burning coolant smell - seemed to be toward the scuttle, but no vapour visible. HBV seemed dry, then again sometimes a smell seemed to come form the front of the engine, too. Hoses seemed fine, nothing visible anywhere.
With plenum, fuel rail and sandwich plate off, a bolt jammed in one hose, and a schrader valve in the other, neighbor's bicycle pump attached and the system built upto about 15-17PSI I saw a weep from the coolant bridge. Saddening as this was only done a year or two ago, using brand new dowty washers. Of course what's been happening is it's leaking water - no smell... but only after a while then being burned/boiled off as it sits on the block. It also looks like the auxilliary coolant pump seal has gone, though I can't rule out 100% the hoses to it are past their best too.
I had confirmed she was not leaking anywhere, simple process of elimination. Hoses=ok, HBV=ok, header tank=ok, cap=ok, My only other thought was the old oil cooler, however being under there not too long ago, again, this was discounted. After all, cant be the coolant bridge, as that's good as new!
Silly thought - if we are looking toward HG... could a cold pressure test as I did aid you? I'm wondering if you can build the system up to pressure, and assuming the system is holding pressure, would having the plugs out help? if there's something delicate that could be prodded inside each combustion chamber as a 'dipstick' (plastic tie wrap?) - if that comes away with even a spec of coolant on it, you've proved your HG's gone. Obviously depending the severity of the leak you may be talking hours/even overnight.
It's a rather heath robinson/left-field idea, but you'll end up taking the plenum etc.. off anyway to do the HG, so nothing to lose, really.
(i put red food colour dye in system)
No worries - more than welcome any time, whenever I'm here, anyway (which is most of the time!)
I've never used one of these things. 
