The bus uses AFC injection and off the top of my head I do not remember various specs, etc. but here are the most likely suspects to check since the airbox has been replaced.
Make SURE there are no air leaks in the intake system - from air cleaner to cylinder head it must be airtight.
Check the throttle switch.
Check the intake air sensor.
Check temp sensor II that is located in the intake tract.
Either a manual on AFC would be very helpful to have around or you might find the check procedures with a net search.