need everyones help with lean condition

[whytry89]

im having trouble with my 89 stang its 5.0 5 speed, it has headers , offroad h pipe and flowmasters, 373 gears

the car runs really strong when its cold like when i leave for work early in the morning and pulls hard

but when the car is warmed up or its warm out it pings or spark knocks only when its warm though

i cant figure out what it is i know the car is lean the plugs were brand new and i put them in over a month ago and checked them yesterday and they look like i took them out of the box no coloring at all i bought one of those scanners from the parts store but its not bringing up any codes its saying the system is fine

if anyone knows anything or can help get my pony running strong please help