What would cause my 99 F-150 4.2 stick shift with 13,000 miles to backfire? It only happens occasionally. I use 87 octane which is what Ford recommends.Thanks

 

Bob1, I would pay attention to how full your tank is when this happens if you just happen to fill up don't use that station for a while. Also if you run on a low tank a lot you can get condensation in the tank that will eventually rust the inside of the tank. I almost never let my tank get below 1/3. that is something to look for when buying used vehicle take off the gas cap and look for signs of rust in the filler neck, this spells t-r-o-u-b-l-e. You can expect clogged fuel filters and other problems. In summary water in the gas will cause backfiring.

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'97 F150 4.6L XL WHITE,FLEETSIDE R/C L/B, LEER SHELL W/FISHING POLE RACK, BED MAT,LIMMITED SLIP,245/75/16 DESERT DOGS ON POLISHED ALUM, WHEELS.
AIRAID, 3" CAT BACK OUT THE SIDE w/MAGNAFLOW MUFFER

 

I don't know how it works with fuel injected engines, but the only two times I ever experienced backfiring was once due to ignition (timing) and the second due to fuel starvation (clogged fuel filter). Both times in an older engine with a distributor and a carburetor.