mustang 5.0 running lean at idle

[notchback10s]

i have a 1988 mustang 5.0 with heads, cam, intake, gear drive timing, long tube headers, 70mm throttle body, stock maf, moates quarter horse tuner, wide band 02 kit! 24lbs injector, 39psi fuel pressure, and a tune for my mods. when i first start it up it runs rough and runs lean 17.1 and some times would go down to 20.0 i get on the gas and it jumps to rich, and idea you guys have would help me a lot

 

[93 5.0 GT]

bad tune?

 

[tannerc91gt]

Quarter horse= DIY tuning
sounds like you might need to retune....or learn how to tune

 

[fiveohwblow]

^what she said.

 

[84Ttop]

An exhaust leak upstream from your wide band w/ cause a lean reading. It could be a small leak, and when it heats up it seals and then reads correctly.