Technical questions re: 2005 Mitsubishi Express Van

I have some questions regarding recent technical problems with my Van.
* I noticed about one month ago the Van was getting harder to start each day.
( The van is both petrol/LPG)
* When the Van was running on petrol, it would run really rough in idle,
stall when stopping at lights etc(this was just happening on Petrol, not LPG.

My mechanic suggested doing a minor service, replaced ignition leads, spark plugs -2.5hrs labour - hoping this would fix the problem.($530)

* 3 days ago the Van started to violently jerk-rough idle whilst running on LPG, the Van is also blowing white smoke and leaking raw fuel from the exhaust

I witnessed the mechanic do the following;
# The mechanic used a special computer to check all the injectors were working properly and mentioned something about an "oxygen centre". He stated injector number 1 was not working sufficiently, and that the cylinder is flooding with raw fuel & "somehow" end up in the exhaust system.
# He advised he has never seen anything like this before.



*How long does it take to diagnose this situation?



cheers
2005 - Mitsubishi Express WA Van - AUTOMATIC - LPG & Petrol - 2.4L
Australia