Backfiring through the manifold

Changed spark plugs and wires van ran great for two days. got fuel as I left the station car bogged down and began to backfire through the manifold upon acceleration. only so far we have changed the fuel pump and filter assembly the coolant sensor the thermostat because it broke and the TPS. we have used two cans of sea foam with no change. We have checked all vacuum lines twice this van does not have a MAF and are getting ready to redo the spark plugs to look for charing. Any thoughts or help will be very very much appreciated. Running out of money and Ideas. lol .Thank you