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Mass Air Flow Meter Replacement

(2001 Nissan Pathfinder)
in Flagstaff, AZ on May 27, 2010
OBD II DTC P0171 fuel lean bank 1, poor acceleration, poor fuel mileage. Worse when engine cold.

I have two 2001 Pathfinders. This one (P0171) was manufactured 09/2000. The other one (running perfectly) was manufactured 04/2001 and only had one minor problem which did not affect the operation in any way and was that the MIL, (Malfunction Indicator Light) which actually reads: "Service Engine Soon" had been illuminated on the dash for about 18 months. With the DTC code indicating fuel tank pressure problem (probable gas cap).

I swapped the Mass Air Flow Meter between the two vehicles (actually swapped the entire MAF/air filter enclosure assy).

Now the vehicle with DTC P0171 seems to have better acceleration, although the MIL is still lit on the dash.

Now the vehicle that was running fine has almost no power on acceleration and hesitates and cuts out at different speeds if the accelerator pedal is applied too much. The odd thing is that the MIL on the dash (probable gas cap) went out after being on continuously for 18 months.

I did a preliminary part number check at the Courtesy Nissan Website and both vehicles should use the same part number MAF so I am puzzled as to why the vehicle, which was running fine, should have such extreme power loss running with a MAF from my other vehicle (P0171) which only had mildly noticeable poor acceleration.

The pathfinder has a learning process for idle air flow and fuel/air mixture which can be initiated using the Consult II scanner, which unfortunately, I don't have. I do not know if the vehicle is capable of initiating the learning process on it's own nor what conditions will cause it to initiate the learning process.

When replacing the MAF on a Nissan Pathfinder does one have to do anything other than properly physically install it in order to get the ECM to establish the parameters for proper operation with a different/new MAF?

Thanks for your consideration of this puzzling symptom.

UPDATE: During a second trip the MIL on the dash would blink on or come on momentarily, then go off. The extreme loss of power continued throughout the second trip. After starting the vehicle and while waiting for the engine to warm up to begin a third trip, the MIL came on and stayed on. As I began driving I noticed that the vehicle now had the same symptoms that my other vehicle had, mildly noticeable poor acceleration.

Summary: It appears that during the first and second trip the ECM was doing it's best to cope with the malfunctioning MAF and when the ECM's efforts to adjust the air/fuel mixture went to the maximum limits without achieving the desired results, based on what the oxygen sensors were telling it, the ECM turned the MIL on and stored the DTC. Apparently it learned how to cope with the defective MAF and now the engine will run based on the parameters the ECM has established to cope with the defective MAF Meter.

I will now read the OBD II DTC and if the code is P0171 can I safely assume that the MAF (which was borrowed from my other Pathfinder with the DTC of P0171 stored) is defective?

A new MAF costs $442.03 plus shipping so I would really like to hear from you all on whether my analysis is valid or not. This is my Wife's car and she knows how to motivate me if you know what I mean. Please help. Thanks.
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