I have a '99 explorer sport 4.0L. I just purchased this vehicle. Had to replace the fuel pump and starter. Now that it runs I'm getting multiple codes. Started with 6 codes, 3 saying system too lean bank 1, and 3 saying system too lean bank 2. Checked for vacuum leaks, new pre-cat oxygen sensors, new spark plugs, checked wires, checked and cleaned electrical connectors that have to do with fuel system. Cleared the codes and drove it for a day. Smothed out no codes. Next day it ran rough. Threw new codes 3 saying misfire cyl4. Next day it added 3 more saying system too lean bank 1. PCM is functioning properly. Any suggestions? I'm headed towards taking off the intake manifold check for cracks?

 

Oh, and the misfire is only periodic. I can feel it come and go.

 

Could you please provide the exact codes it gave?

 

3/p0171, 3/p0174. Then 3/p0171, p0304
Been doing a ton of research and have ruled out MAF, injectors, IAC, DPFE. Checking fuel pressure today. Have checked for vacuum leaks multiple times, though this seems the most likely culprit. So, after fuel pressure gonna go over it again. Having trouble figuring out how to check the lower intake. I'm thoroughly stumped.

 

A cracked or broken PCV hose can influence performance also.
How did you rule out the MAF?

 

Sorry, forgot to add, that I already checked the PCV and hose. Did a voltage test on MAF. Fuel pressure fine. I'm down to the EGR. Connected vacuum line to it while the car was running(was told to test this way by a mechanic friend). It did not try to stall out. This means it's bad, right? Took it off. I think it is stuck closed.

 

Update
I cleaned the EGR. Got it un-stuck. Cleared the codes and drove it awhile. Still misfiring.
New codes
3\p0304, 3\p0174 (system too lean bank 2)
Now what?

 

Last update
Turns out the coil pack wasn't putting out enough juice. This causes unburnt fuel to travel passed the o2 sensors and tell the PCM the system is too rich. The PCM cuts down the fuel and causes a too lean condition. Well, glad to have that figured out. Thanks to all the other posts about these codes. I was able to research it, and figure it out 😊