I have a 91 Mustang GT 5.0 that has had a flat spot at about 3,500 rpm. It also pops out of the intake and has done this for some time. The popping and hesitation is more pronounced now that it is warmer outside. It happens when accelerating slowly at about 3,000 rpm and then launches out of it. Acceleration also stops in 3rd gear at 100 mph as fuel seems to cut off or lean out.
Everytime I upgrade to more horsepower, it becomes more pronounced. The only codes I have are for the removed items. I have removed the smog system and the EGR.
Upgrades: K&N cone filter, Pro-m 76mm mass air, Powerpipe, 65mm Accufab with EGR delete block, Edelbrock Performer, 24lb injectors, 1 5/8 Mac full length headers, Bassani x-pipe, Aeromotive Fuel Regulator.
TPS has been swapped and it is adjusted at .97v
The IAC has been swapped with a clean one.
Base idle is fine and at 600rpm
Base timing with spout off is 10*
Firing order is correct.
I have new plugs and wires. Gapped to factory.
Swapped MAF from 24lb C&L to Pro-M
tried different fuel regulator and adjusted from 38-44
Anyone have any ideas?