Outer Limits 4x4 Board

Ok got a big problem with a TB 4.8 litre Nissan Patrol (Y61 de 24 valve) on dual fuel.

THe car backfired (running on gas) which i am not sure if it caused the problem but anyway it has thrown 2 shims from the inlet valve from cylinder 2. THey have been throw out of the bucket/cam followers which has stuffed the cam shaft. Now before i start putting it backtogther etc i could do with knowing why it did this.

Is it related to the car running on LPG or is it a camshaft followup problem i am not sure
I htink the backfire maybe caused the valve to open leaving excess room for shim to fall out, however i am not sure if this would leave enough room for it to do such as thing.

cheers for any help

The trouble with an LPG backfire is that there is such a huge volume of LPG/air in the whole inlet tract. On the early GQ LPG installs backfiring was a common problem which wrecked many an airbox until it was realised that a very good spark was needed and many GQ's were converted to electronic ignition. I have an efi GQ and have had one backfire but it didn't do any damage as my LPG install has an expanding sock made out of wetsuit material which allowed the excess pressure to vent to atmosphere. The best bet to eliminate backfires is to have a sequential injection LPG system where the gas is injected into each individual port by its own dedicated gas injector. This means there is only the amount of gas in one port at a time compared to the whole inlet plenum. Victorian Autogas (LPG systems wholesaler) either are or have developed one to suit the 4.8 Patrol and on the other sequential LPG systems I've seen, (BA Ford, V6 Rodeo, Yota Hiace, Ford Territory) this is the way of the future for LPG.
The gas installers next door say it adds about $1000 to the cost of a normal LPG install. (it's own dedicated computer, wiring and gas injectors plumbed into the inlet manifold)
Regards Andrew.

LPG backfire = incorrect ignition timing

advance ya timing out a bit more... IE .....
if its 10dergees BTDC normally for petrol then advance it out a further 5 degrees so its 15degrees BTDC for LPG use.

Kingy

yeah usally it shoulud just blow the airbox off or soemthing but this has actually thrown out the shims (they are all bent and cracked well2 of them from cylinder 2 anyway) and the camshaft is now stuffed too. Anone else here of this happing with the 4.8 litres on LPG or even if they aren't on LPG