A common cause of lean codes on the Ford 4.0 SOHC engine is the elbow on the PCV hose becoming soft over time and developing a tear and allowing un metered air to enter the intake manifold.
Had this exact problem and fixed for about $15. I used a "HPS HTSEC90-062-BLUE Silicone High Temperature 4-ply Reinforced 90 degree Elbow Coupler Hose, 100 PSI Maximum Pressure, 4" Leg Length on each side, 5/8" ID" and cut it down to size. It's also available in black. There are likely even less expensive options but I knew this would fit and could get on amazon prime.
Same problem on my '03. The caved-in PCV hose with the hole (crack) in the bend was hidden by another hose lying on top of it. Local dealer who had the now-obsolete part in the warehouse wanted $40.10. Today I bought just the L-shaped rubber hose off PartsGeek for $25.93 with shipping. Hope it ships quickly so I can get my truck all fixed up and before my Check Engine light starts flashing.
I thot some of the readers might want to know, if you happen to accidentally switch plug wires 5 and 6, you will get a P0174, high long and short term fuel trim bank2 only, and loss of power, but no check engine light, and no backfires, runs only a little rough,, ask me how I know :) the correct cylinders are noted on top of the coil , very faint on this vehicle but i was able to find them
