i got a 98 ranger had if for little over a year now. it likes to trip P0171 or what ever system to lean code is( has done it about 5 times over the year) i have a K@N replacement filter with a thrush muffler on it. I read the K@N can dirty up the the sensor. i have not cleaned it yet. kinda worried about it since i hear it is very fragile. but would this code give me poor MPG and maby cause my engine to rev up on its own while the clutch is in or in nutrual? and help Plz

 

You will see a drop in mileage and your rpm's will max at 4500. Maf sensor is easy to clean you should be doing it atleast 4 times a year with your k&n in, or atleast every oil change. Mine idles lower than it should once it's warmed up.

 

I've been running a K&n drop in filter for 100k miles and never had to clean the MAF sensor. I just don't over oil the filter .
That high rpm problem would be a sticking IAC valve on the throttle body or a vacuum leak.
Clean the IAC first as they always stick sooner or later.

 

Since that filter was a complete waste of money, it will do nothing to affect your mileage.

The code is being thrown because your long term fuel trims has surpassed 20% enrichment. The PCM is scaling the fuel map because the output is leaner than expected. Due to the scaling, the actual fuel measurements are stoich, but the code is there to tell you there is a problem. compare your long term fuel trims under different cells of your load table to slim down what different things could be wrong causing the lean condition. Under high load (like on the highway) you may have a bad-maf if you see it very positive here (throttle is open, causing a vacuum leak) - at idle, if you have a lean condition it could be a vacuum leak (throttle is closed, maf errors will not matter much)

Knowledge of how the PCM/adaptive fuel managment system works greatly helps someone troubleshoot a problem like this.

 

don't forget fuel filter or a stuck open pcv valve and check for vacuum leaks

 

i read some where to unplugge the MAF and see if it runs differant and that how to tell if its u have a dirty MAF. i did that. she ran rough like it said it would. but she seemed to gain a a lil accleration and power. but she was jump as hell while shifting gears and the engein bogged like it was in 4x4 while pulling out backwards an forwards. i plugged it back in and she when back to being normal

 

This is a bit of an old post, but my 97 is having the same problem. System too lean, bank 1 and bank 2, but I smell gas when I park and get out. Would vacuum leaks cause that?

 

i belive it could. i did end up fixing the problem tho. it sucked up some oil from my airfilter and dirtied the MAF. Cleaned it and its been fine for over a year now