The v6 is not good on emissions! You are indicating a rich condition. These engines need to be red hot when tested. Faulty thermostat, or temp sender or old lambda sensor could be a cause. & of course old spark plugs. Yours should have silver plugs in it for the lpg. If it did not run happy on lpg then don't be surprised that it will show to a lesser degree on petrol.
I know the V6 is absolutely appalling anyway, but from what the garage are saying, its an incomplete burn, so I am hoping its not down to the valves. (Duchies! That will be expensive.)
Another problem however has cropped up, the gear cable keeps falling off! So its stuck in first gear hold as the rubber has gone soft, the cheapest I can find a new cable is £131 (ouch), anyone know where I can find anything cheaper? Renparts don't have a V6 in stock or any automatic espace for that matter. Renault are a bit expensive, so anywhere else? I guess the breakers is next.
Part number: 60 25 316 578.