VT commodore help.

Yes, check/replace the thermostat first. VT's are renown for failure of the inlet manifold valley gaskets, so that could be where your coolant has gone. Ensure you use the OEM radiator cap as aftermarket ones can leak. VT's can also leak around the coolant crossover pipe located below and behind the radiator. The hoses were connected using spring band type clamps. In some cases, these clamps are insufficient, and coolant may leak from this area. Good luck.

lol, yep, my VT did that once about 2 months ago, hasn't done it since

my money's on thermostat

nope, still losing stupid amounts of water.

you want new heads or reco'd second hand ones.if you want more bang for the same dollar as new ones go grab a set of second hand ones and spend the difference on some porting and grinding etc, let it breath better.

You do have the heater on & set on hot when you fill radiator back up, don't you!

jezz43. sorry to say but it does sound like cracked heads. Very common for the Ecotec, especially the Series 1 engine. To be honest, get a pressure test done & if it is found to be either head gaskets or heads then i'd be replacing the engine.
The reason I say this is, if you pay a mechanic to strip it down to replace head gaskets THEN find it's the heads you have just wasted X amount of dollars to get nowhere so to speak. I know the heads have to come off to repair/reco but what has that just cost you as opposed to replacing the engine! (That's what i'm getting at).
Drury Car Parts up here will give a warranty as do most reputable wreckers. I would imagine a VT motor would be less to replace then getting the heads rebuilt, & the main problem you have IF it is indeed the heads will be finding some that arent cracked.
I used to work for Drury Car Parts & we had 11 heads crack tested to find 1pr that were any good.
Hey. i've seen a 1 owner 80000km Calais with full service history throw a rod for no apparrent reason, so sh*t happens.
Also you need to look at it this way. you may spend $1000 (possibly more) on repairing & reconditioning your own heads (plus any mechanics labour unless you can do it yourself) & you're fitting them back to an engine that's done 258000km. I've also seen this done only to have the bottom end fail soon after. Dont get me wrong, some Ecotecs will do 400000+km without a hitch, mates has done over 410000 now & has had nothing done to it except normal maintenance.
Not saying it will die on you but you need to weigh up the pros & cons of where or rather how your money is spent to get the best result for you.
I'd start with removing the thermostat, make sure it is fully closed when cold, boil the jug & tip the water into a container then chuck your thermostat in. Make sure it opens as it should. not jumps open all of a sudden. If it's a bit dodgy then replace it & see what happens.
From there is a cooling system pressure check to determine what could possibly be the worst scenario in this matter.
Good luck.

cheers guider1, i can do all the work myself easy enough. bought another set of wheels last week so i cant be arsed looking at the holden at the mo. will eventually get to it. i originally had the intention of replacing the 3.8 with a 5.0 V8, so maybe this is just the time to do it

very helpful site but the answers have depressed me. it seems like inlet or head problem .... thank you