Engine 1990 Mustang 5.0 lean issues

What size injectors? What MAF? Is AFR gauge picking up one or both sides of the exhaust? Is there any holes or leaks in the exhaust before the gauge sensor? What AFR gauge are you using?

Is the AFR gauge calibrated properly? I have the Innovate dual wideband (DLG-1) and there is a free air calibration. If gauge is on and engine not running, gauge should read in the low 20's. On startup I am usually 16-17 at first and then as the car warms up it settles at 14.4-15.2 at idle. I have 24 lbs injectors now, but it was the same with 19s.

What is your fuel pressure set at ? Or is it stock FPR ? How about the fuel pump... is it stock ? Bad fuel pressure or pump usually causes the opposite result than you're getting. Just throwing stiff out there.

These cars are known to leak at the head to lower intake gasket under the intake.

Plug the tail pipes with rags. Disconnect the MAF tube and plug the intake ( after MAF sensor ) Disconnect the pcv hose and smoke test the intake. There should be no smoke coming out the valve covers or pcv hole. No smoke coming out the vacuum hose connections or components. I'd there is the intake gasket slipped at the heads. Pay attention to the injector o ring area too. We had a member here not too long ago that found his new injectors o rings weren't sealing in the intake side causing a lean condition. The injector o rings should be brown....or blue neoprene.

There are tons of videos on YT of how to make a DIY smoke tester. Baby oil works great....so does transmission fluid.

PS. A sneaky vacuum leak can be caused by the black blend door vacuum line the runs through where the ac lines go. Some people don't care about heat or AC and have a broken or leaking heater box component under the dash that causes a vacuum leak. It wouldn't be bad enough to peg a car lean though usually.