GM Vortec 4.3 V6's have had random problems with valve seats, it's nothing new, Running on LP had very little to do with it.
There is a larger chance that the intake gasket failed causing a lean condition on the effected cylinders and caused the valves to over heat and "tulip" also taking the seats with them.
How do I know this ? I have seen it happen on both gas and LP engines, GM intake gaskets are known to be trash, and cause vacuum leaks, it happens on the Vortec 350's, and the LSx engines if you continue driving when the check engine light illuminates for a lean/rich condition on OBD2 equiped vehicles. ( not until the last few years has there been a "check engine" light on industrial equipment, emissions requirements are getting tougher on everything, ALL LP and GAS forklifts since 2004 have had catalytic converters and since 2005 have had O2 sensors before the cats.)
I have seen this issue as both a GM dealership tech ( former) and currently I have seen it as a Toyota forklift tech, pisser is, not much has been done to fix the issue.
FYI, the 4.3 Vortec engines used in forklifts are INDUSTRIAL engines, it is not the same as one you would find in your S-10 or Mom's Blazer.
There very similar, but some of the casting and bolt locations are different. ( to keep you from using them in an automotive application, they are not EPA rated for highway use)
And Toyota is not the only ones using the engine, they are in Hyster, Yale, CAT/Mitubishi, and Clark lifts. MOST all of which run on propane, how much of a valve problem you think there is ?
:EDIT: 5.7 and 4.3 VOrtec engines are also used in ZAMBONI's, as are the LSx engines and still running propane.
