Is the brown gunk oily/creamy in consistency, or is it dried onto the stick in the overflow?
If it's dry, it's likely to be rust, from ineffective coolant (too old, too diluted, or no coolant used at all).

If it's wet, and oily, then it could be an indication of a head gasket issue letting oil into the water, but it would be very rare - not only rare on an Ecotec, but also rare because you're more likely to get water into the oil as well. So check the engine oil dipstick to see if it has a "chocolate thickshake" appearance. If it does, then it's a sure sign of a head gasket or warped head (normally caused by overheating).

From my experience with VS-VY Ecotecs, when they do have head issues, it's rarely involving oil - it's normally coolant to cylinder pressure (they like to crack between the valves in the LH head at the rear) which will only show up as steam beyond warm-up, and a slightly rusty spark plug.

When this cracking has occurred, it's generally not enough to blow bubbles in the top of the radiator, but they consume maybe a litre a week or less for a daily commuter. This cracking usually shows up within weeks after a heater tap has let go - which is common in these cars now - and the lack of coolant in the head is the likely cause of the cracking.

In your shoes, the next thing I would do is pull the spark plugs out & inspect them - they can tell you if a cylinder is oily, getting coolant/water in, or running OK. From there, you can decide the next avenue in the diagnosis trail.

I'd suggest being s sludgy liquid (and not oily) it's built up corrosion, caused by really old coolant that's well overdue for a change.

The alloy radiator & cast iron block combo in V6 commodores is well known as a galvanic cell, and coolant loses its anti-corrosive properties as it ages.

I'd be flushing the system really well, and refilling it with a quality coolant, diluted to the recommended level.

Edited by user Saturday, 12 September 2015 9:13:02 PM(UTC)  | Reason: Fixed up spelling

Thanks Dr Terry.

I didn't get the car back until late today.

Tomorrow I will pull the plugs to inspect them as I cant see any harm in doing so. If all is well I will proceed to flushing thoroughly and putting some quality coolant in.

Mick / Dr Terry - in your opinion, are the radiator flush additives worthwhile, or do without them.

Regards,

Mick

Thanks Mick

Tour Director - thanks for that. I always use distilled water to dilute coolant. I will continue to do so.

Regards

Mike

Gents,

Thank you for all replies.

I pulled all the plugs and they looked as good as plugs could.

I then opted to take it to the local radiator shop who charged me a reasonable price to add a chemical flush (Tectaloy Muckout), drain, flush and fill with coolant.

The guys said that although the existing coolant looked ok, there was heaps of rust present on the initial drain.

If I detect any further gunk I will report back here, but in the absence of a further post, it can be assumed you guys were correct and my problems are solved.

Thanks and regards,

Mick.

Alloy head on the Astra John. Ecotec is still the tough old cast iron, and doesn't suffer the same issues as alloy.