Whoa, sounds impressive.

That would have to be the cheapest car to run EVER.
In Australia, Propane is quite a popular fuel (it is btwn. 1/2 and 1/4 the price of gas) pretty much all taxis in Oz run on Propane...

I should be able to provide some more info on propane systems (I know a guy that does the conversions) and I will ask them what they think would be causing it...