>>Also, my bus has started to backfire some.  What causes a backfire?

Small holes in a muffler will bring out this problem in FI vehicles.
Too rich a mixture, weak plugs, bad valves etc in a carb engine, anything 
that lets excess/unburned fuel be sucked into the exhaust. Inlet back fires 
are scarier, and can lead to fires, due to bad valves/timing, may jam the 
control flap shut on an FI system.
Tim Smith