I have recently brought a Vt Commodore that won't run on petrol but runs on gas

check out a faulty headlight switch

There appears to be a short internally in your steering wheel.

What's the throttle body like? Any carbon build up

Since this is just a Buick 3.8 V6 the firing order is
1-6-5-4-3-2

First check petrol flow and pressure , if pressure ok ,check flow, if flow is low replace fuel filter, If the fuel all checks ok .There is a relay that disables the injectors and petrol pump when on gas, this may be at fault.

It may be a problem with the TPS switch (Throttle Position Switch). Go to a local mechanic with a diagnostic tester to confirm.

I would suggest having a qualified gas installer check the installation in the vehicle for leaks from connections and damaged lines and filler cap. He will also check the converter unit to see if it is working correctly and does not have corrosion holes in the unit that is allowing improper gas usage. If the engine is running ok except for the gas consumption I would focus my attention in that area. You can have the fault codes read but from your description I consider that you will only be replacing a minor sensor.

Hi dave hill, not really sure on your problem but I have a VN commodore that I purchased that had the fuel pump crook and did the same thing of not starting on petrol. Owner said to hold in lpg button when starting . I got the fuel pump replaced. Mechanic said it was caused by not running petrol through enough and running always on LPG.

hi pull the dip stick out have a good look at it if it looks like it as a milky coller on it mixd with the oil on it then the head has to come off and be fixd

Inlet manifold gasket loose (valley gasket) my vt was running like **** at idle only , tightened the 12 bolts near the injectors runs perfect now if that doesn't fix it the gasket may be wrecked $70 kit or get it done
Water was getting into the throttle body and into the cylinders causing irregular and irratick loping and rough idle it had me thinking it was all different things like leads , air leaks , bad fuel , bloked injectors , computers , coils
You'll probably find some loose one their is a torque sequence but try just tightening them start with middle side 4 first hope this helps and saves people money