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98 Ford E150 4.2L V6 with P0171 and P0174 Codes: Diagnosis and Fixes

Model: Ford E150 Fault Code: P0171 Posted: 2007-09-13 13:29

Over the past few weeks, my 1998 Ford E150 4.2L V6 has been displaying both P0171 and P0174 OBD-II diagnostic codes. Initially, both codes appeared simultaneously, but now it's primarily showing only P0171. I've researched extensively on this forum and tried several common fixes with limited success. Living in Florida, I initially ruled out intake manifold issues due to the high ambient temperatures—codes typically appear when the van is hot. I frequently make short trips, and the codes have been recurring about three times per day. I first attempted a new fuel filter replacement, which only lasted one day before the code returned. Next, I added injector cleaner and filled the tank halfway; after that, the P0171 code reappeared. While at Advance Auto Parts, I purchased MAF sensor cleaner and began disassembling to clean the Mass Airflow (MAF) sensor. During this process, I noticed that the oil filler adapter—located on the valve cover and designed to extend the tube forward—is loose by approximately one and a half turns. I realized that these error codes started appearing shortly after I topped off the engine oil. Although I cleaned the MAF sensor (which appeared clean initially), I suspect the root cause may be related either to the MAF sensor or a lack of proper sealing at the crankcase area, potentially leading to air leaks affecting fuel mixture readings. My question is: Was it the MAF sensor that was faulty, or could the loose oil filler adapter and resulting crankcase ventilation leak be causing the P0171 code?

Related fault codes
P0171P0174
Comments (4)
Anonymous 2007-09-14 07:34

The issue could stem from either the MAF sensor or a vacuum leak at the crankcase, such as an improperly sealed oil filler adapter.

Anonymous 2007-09-14 17:01

I recommend continuing to drive the vehicle and monitoring whether cleaning the MAF sensor resolves the issue. It's unlikely that simply refilling oil would cause P0171 or P0174 codes, as these are typically related to air intake or fuel delivery systems.

Anonymous 2008-07-26 12:19

My experience showed that the problem was caused by worn-out rubber elbows in the PCV vacuum hoses. After replacing both with difficulty, the issue disappeared for nearly a year. However, I recently started seeing P1071 and P1074 codes again—more persistent than last year. I’ve cleaned the MAF sensor now to see if that helps.

Anonymous 2008-07-26 17:28

After cleaning the MAF sensor without success, I inspected the PCV hoses and discovered a large hole in one of those flimsy rubber elbows—exactly as before. This time, it was the elbow located at the rear of the engine, which required removing the center console to access. To prevent recurrence, I replaced it with high-pressure fuel hose and added a long loop to avoid kinks. So far, no return codes have appeared, and based on my previous experience, I don’t expect these issues to reoccur for quite some time.