Merv, a shot in the dark here, but do you think you might have a leaking injector(s) which is bleeding off residual fuel rail pressure after shutdown. At shutdown as the engine spins down the unburnt fuel from the leaking injector is cycled through into the exhaust system and eventually ignites from a hot component.
EFI fuel rail holds pressure after shutdown due to the diaphram in the fuel press reg and check valves in the discharge end of the fuel pumps.
Jim
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