i have a 99 f150, 4.2 v6, 4x4 manual trans.. the problem is when im driving and put it in neutral and coast the engine will rev up and down repeatedly as if im depressing clutch, ( which im not). And while parked it idles way to high for about 2 minutes till it kicks down. now i've replaced the EGR valve, IAC Valve,IAC sensor, and cleaned the whole throttle body with nothing making a difference. also the check engine light was throwing some code, not sure which one, but it had something to do with bank 1 and 2 running to lean.? the only other thing i could think of, might be either the computer, or the catalytic converter,( which i really hope its not)..
and as an experiment i unhooked the IAC sensor and valve, the the truck runs and idles great when warmed up, but wont rev when i depress the clutch, and will hardly idle when its cold.

if anyone might know what the problem is i would greatly apreciate some help ,
thanks

 

the first thing to check out now would be for a vac. leak and make sure that it's not getting too much air. the codes for a lean condition are P0171 and P0174. when it's running too many rpm's the butterfly plate could be bent ever so slightly and letting the air flow be too high and the only way the engine can run is at a high idle