2.7L V6 & Lambda troubles

[sheppy99]

I am running a '93 Rover 827 Sterling, also I think known as an early generation Legend with the 2.7 litre 24v V6 engine. Currently I am using LPG (propane mostly) as the fuel and have noticed that it uses noticably less fuel when the check engine light is on caused by having a faulty lambda sensor in the side of the engine not used to control the LPG system. Can anyone give me a clue as to how the ignition timing differs when the check engine light is on caused by a faulty lambda sensor? The LPG side has its own controller so all PGMFi is doing when on LPG is controlling the ignition timing and variable intake manifold.

Thanks in advance

Kevin