Four problems related to frame rust have been reported for the 1998 Ford F-150. The most recently reported issues are listed below.
Frame so rusted can't get a sticker, original paint on truck. Ford was factory undercoated with rubber type undercoating. Which pealed off and left lots of rust on entire frame. Not safe for inspection. Told by garage, to junk it. Bought new in 1998.
In 2012, I replaced my gas tank because severe corrosion had eaten numerous small holes into the top of the tank so it could not be filled over 3/4th without leaking. Flakes of rust were falling onto the top of the tank from the frame x-member causing these holes. The frame x-member was not addressed and the new gas tank began to leak within a year and a half of replacement. The engine was just recently tuned up by a Ford dealer because it was intermittently vibrating. Within 6 months of this tune up, the engine lost full power with finding of grossly contaminated fuel. The 2 year old tank was pulled and the top of the tank was revealed to be metallic swiss cheese equivalent, this time with numberous very large holes that rain was easily penetrating into the tank via the cab-bed space opening. The x-member frame support was corroded in half. The dealer could not fix it and I paid for a second new tank and fuel pump($350 with shipping to hi), por 15 coated the top of the new tank myself, and paid a local body shop$1042. 00 to pull the x-member and replace it with a used part as Ford no longer stocks (likely rusted to bits in their warehouse). The new tank and used x-member are now installed and awaiting an engine mechanic to address why the engine won't start up despite new tank/fresh fuel/pump/filter. Likely another $500 to clean fuel injection/engine bringing 2 year total for rotted x-member and two gas tanks to well over $3000. Perusing your data base, the frame rust issue seems to be a recurring theme particularly for the 1997 year model. My assessment is there was something very rotten in 1998 f150 frame components and or corrosion protections used. Lessons learned: don't fix the tank until you fix the x-member problem first and Ford f150 frames/corrosion protections circa 1998 are getting a bad reputation which is likely deserved.
7/14/2011 - my mechanic replaced rotting gas tank straps 10/3/2011 - Ford dealer (after receiving recall notice on gas tank straps) applied reinforcement straps on gas tank straps. No indication given to me that these were not secured onto frame. 10/12/2011 - brought my truck into my mechanic for oil change. He noticed that the gas tank straps and reinforcement straps were not connected onto the frame on drivers side rear. 10/12/2011 - brought truck back to Ford dealer to fix the safety recall that last week they had applied. I mean, it is hard to believe that a week later, and the straps failed. Was told that they only install the gas tank straps and that's it. Whether they are connected to frame or not that is rusting is not their concern and is not covered by the recall. This is preposterous. Since the same metal was probably used throughout the truck and caused the recall of the gas tank straps -- a safety recall, shouldn't they be sure to apply these to known good structure and if such structure suffers from the same problem as the gas tank straps, shouldn't that be covered? I really think Ford should stand behind its products and its safety recall installations and take care of the frame that holds these rotting gas tank straps. The cause of failure is the same. Rusting frame is same rusting as the gas tank straps.
My mechanic told me that his father's 1999 or 2000 F-150 was rusting on the frame and was about to rust in two, a screwdriver could be stuck through the metal. This was discovered when the straps holding the gas tank rusted and the gas tank was almost touching the ground. So I asked him to look at the frame of my 1998 F-150, and he discovered the same problem. The main crossmember of the frame is almost rusted in two. The straps holding the gas tank are severely rusted. One of the bed crossmembers is already in two, and two others show severe rusting. My mechanic has told me that if the crossmember breaks, I could lose control of the vehicle. If I were driving in heavy traffic, this could pose a serious threat. The body of my truck is in good shape. If you did not know of the frame problems, you would not know to check these out. I feel that if nothing else, the public needs to be made aware so that they can have their vehicles checked out.