my 98 f150 will start but won't idle, it just dies. I can keep it running by feathering the gas pedal. What is wrong with it?

Mine did this too. I cleaned the mass air flow sensor, the throttle body and IAC but still did it just as bad. I feared the speed sensor in the transmission next but before doing it I decided to check the spark plugs & wires with a multimeter and found arching had been going on with them and had burned a plug up. I replaced them all because I had arc damage to wires and two other plugs had readings that were near the end of their life span (also there is a tester you can use to test them). It had the standard wire set when I bought it so its a good idea to go ahead and buy the premium set and a tube of dielectric/bulb grease and put it on all wire boots both ends and make sure the coil pack has not gotten moisture around boot fitting or has any cracks to case. Also check the for vacuum leaks, under toward front and back under side passenger side of intake has small 90 degree vacuum hose fittings that go out fast if they are the older style stock fittings, the dealerships parts departments now have the improved versions, I had to replace them twice on my 98 f150 and now that I have the better ones I have not had those problems since.

If you do decide to clean those parts listed above use only (I like crc) mass air flow sensor cleaner on it and throttle body cleaner on that and the IAC and go ahead pull the PCV valve and clean it too (throttle body cleaner). And you can use the dielectric grease on all electrical connections just use contact cleaner (again I like CRC brand) before applying grease to clean them first (spray out the connectors and put the grease in and plug back up, while I was doing my plugs I did this to all electrical connections I could find while I had it there. By they way My throttle body & IAC worked o.k. but did benefit from the cleaning and gets better gas mileage now.

If it ends up being the speed sensor it is in the trans and can be replaced by removing the pan. My next thing to check was the safety neutral switch. The trans can be check by a shop that has the trans code reader computer which is not the standard code readers from auto parts stores. I would follow the steps above before the trans sensors which can cause the same systems you have and there is like five other sensors in there and they may never go out but could save some hassle later to just go ahead and replace them all while the pan is off along with filter and fluid too.