Hi, I just purchased a 1990 aerostar today, it is a bi-fuel (or duel fuel, whatever is correct) it runs on gasoline and Natural Gas with a switch on the dashboard to switch from either one, The guy i bought it from told me that the CNG was only working at high RPM on the highway and what not, and that otherwise the engine would just stall out if you turned it on. (fuel cylinders full) i switched it over and it just stalled out and would only run 1 second and shut off every time i started it again. Could it be that the Mixer is gone? or a vacuum hose or something?... there is also a tap on the filler unit that i have to turn to "OPEN" when i fill it... does this tap have to be open when i want to run it on CNG? If anybody knows i would appreciate any help, as i am not familliar with alternative fuel systems.