I have a 1994 f250 4wd dually. It only gets abt one mile to the gallon. Last night it back fired and lost spark. We use it to plow snow. Any ideas?

transmission belt is worn out or slipped off pulley somewhere service drive belt then try it out

Check the whole air intake could be clogged, also check to see if spark is getting to plugs.

the mistake is washing the engine bay
water gets into every electronic connections and shorts out everything
use compressed air in all of the harness connections and use dewatering fluid ( wd-40 or the like )to remove the water

first i would check to see if your getting fire out of the coil using a spark tester if you are than it could be the rotor and distributor cap, if not its probably the ignition coil as they are notorious in chevys for going bad

Try with the vehicle in neutral, There may be a button on the shifter that has to be pressed and then pull the shifter back into 2 wheel. Let me know if that works.

check front pump on transmission, also check thrust main bearing on engine, but check transmission first.

Have you cleaned the spark plugs? they will surely be filthy after a headgasket failure... maybe even cracked by the water going on the hot plug... I recommend changing the plugs before trying anything else.

This is a very common problem with the 5.4 liter engines.And yes they do break in half leaving the outer shell still attached ,then there is a special tool that is required to remove the rest of the plug which cost about $100.How many miles on the truck??

Your cat is plugged, its not letting your exhaust leave the engine thus causing all these problems. You need to get it changed as soon as possible. You can get a shop to put in a universal one they are usually very cheep but do it soon. They can get hot enough to start a fire as you have already seen.