Have a 99 B3000 lost power. Hard time maintaining 70 mph. Did minor troubleshooting. Replaced plugs, wires, fuel filter, and air filter. Codes were P0171, P0304, P0305. Left engine idling and removed plug wires from coil pack. Bank 2 all plugs when removed 1 at a time affected idle. On bank 1 I removed one at a time. Spark was present on each and every wire but removing them completely had no effect on idle speed or smoothness. I removed all 3 wires on bank one. Did not affect engine at all. They had spark. Took stethoscope and listed to injectors. They were clicking. Thinking somehow no fuel is getting to that bank? Thanks for any advice.

 

Misfires cause the Lean Code

O2 sensors read Oxygen Levels in the exhaust not fuel
Lean is too much Oxygen, Rich not enough Oxygen
When a cylinder misfires no oxygen is burned so it all goes into the exhaust, O2 sees that as Lean.

Ford uses a Waste Spark system, which means spark plugs are wired in Series, so pulling off 1 spark plug wire can effect 2 cylinders, so not a valid test like it was with distributors.

Next test to do is for vacuum leak
Engine needs to be warmed up
Locate the IAC Valve, upper intake front
With engine warm and idling unplug its wire.

IAC Valve will close all the way and engine RPMs should drop down to 500, or engine may even stall, either is good it means no vacuum leak.
If RPMs stay high, 700 and up it means you have a vacuum leak, it will cause Lean code and misfires

P0304 and 305 are misfires on Bank 2, drivers side, #4 and #5
Passenger side is Bank 1, P0171 = Lean on bank 1
So something is odd

Cold engine
Unplug the Coil Pack, you want a No Start
Crank the engine a few times, don't touch the gas pedal
Pull out spark plugs 1, 2, 3 and 4, 5, 6, if they are all wet then fuel is getting in there.

If spark plugs are wet........
Do a compression test on all 6 cylinders, all spark plugs must be out to test
3.0l should be about 165psi each, with all spark plugs removed for good crank speed.

If #4 and #5 are low then most likely valve issue

 

Thanks, Ron. I did the checks you suggested. Fuel was present, so I did a compression check. Bank 2 front to back 130-130-110. Bank 1 front to back 30-30-10. Ordered replacement heads from Cylinder Heads International for $150 each which included shipping both ways. Changed gaskets and head bolts. All is well. No codes and plenty of power.

 

Good Work

Thanks for posting the update