Much thanks to the OP, Hot_Rod, for this really good "how to"; it was better and easier to follow than my dog eared and grease stained old Gregory's/Haines manual.
Thought I'd botched the job though, when the idle was high and the cooling fans were blowing flat out (engine cold) with the check engine light on... coz I'd forgotten to plug the temperature sensor back in, after unplugging it to get more clearance of the wiring loom when removing the manifold assembly. All good though, and driving better than it has in 2 years!
I had issues with the LPG setup and idle speed in general ,and whilst cleaning the PCV valve helped a little, it wasn't until oil became visible in the radiator that I found out that the inlet manifold gaskets are a common issue on the Ecotec. I had two rotten inlet ports and one water gallery between the 2 gaskets, with oil leaks at front and rear of the engine.
Long story short.... if you're having idle issues, and it hasn't already been done, changing the inlet manifold gaskets should be high on your "things to do list".