A recurring P0171 and P0174, System Too Lean Banks 1 & 2, have been a common problem with the Ford 4.0L engines (VIN Code " E", Plastic Intakes) Not only are there recurring codes, other symptoms may include, stalling when hot or cold, rough or erratic idle, excessive fuel consumption and misfire codes may also be present.
1. The most common cause is the failure of the seals between the upper and lower intake assemblies. Resulting in a gross vacuum leak, causing the above symptoms. Some very quick and easy ways to diagnose this issue are:
a) With a can of Carb Cleaner, spray around the mating surfaces of the upper and lower intakes. (Please use caution while spraying! Do Not spray on hot exhaust manifolds!) If the idle " settles out" or increases there is a leak at that point and the seals should be changed.
b) If you have a scan tool and can view O2 data stream then use the same procedure and " view" the O2's as you spray. You should note that they both will go " fixed rich" for a short time. Indicating a leak at the seals.
2. The second most common cause, and this is not limited to the 4.0 engine, is a contaminated Mass Air Flow Sensor or a M.A.F. That has failed completely (A good portion of the time it will not set a code!)
a) A quick fix for this is to remove the M.A.F and with some electrical cleaner or isopropal and Q-tips, gently clean the small coiled wires inside the M.A.F. If this does not cure the symptoms, then further testing is required.
Though there are other causes for these symptoms, these are the 2 most common. Please feel free to contact myself or any other of the professional moderators for additional assistance!