I've had my truck a bit over a year and it's time to get it e-checked.
It's a 98 Ford ranger 2 WD (can't remember the engine size but my friend had said at one point "Oh, that's the small engine"

Anyways, the check engine light's been on the entire time I've had it with the 0171 lean code as well as the tcc error code (which was fixed)

I've replaced both o2 sensors (now realizing that probably wasn't needed), replaced an obviously bad hose, replaced the IAC, the MAS sensor and a clogged fuel filter.

I've sprayed carb cleaner all over the engine but couldn't find anything.

I'm not much of a mechanic (In fact, I know very little) and I tried to do the rev test to make sure it's a vac leak (look at the stft at idle, then at 15k, then at 25k) but I'm confused by the results. Sometimes it seems like a vac leak, sometimes not.

I got an obd reader and used the torque android app to record the rpms, fuel trim and maf readings on my trip home from work hoping that maybe someone who knows more could look at it and have an idea of what's going on.

it's a comma separated database so excel or calc (from open office) can view it in a nice spreadsheet.
https://drive.google.com/file/d/0Byo...ew?usp=sharing

You've got an air leak on 1 side. A vacuum line break or MAF or IAC would affect both banks at once (p0171 & p0174). Sometimes (eg, mine) it's caused by a leaking gasket, such as the fuel rail gasket, the upper or lower intake manifold gasket, or fuel injector O-rings, or (rarely) the exhaust manifold gasket. Most likely an intake manifold gasket. It's a common problem. Good luck finding it.

__________________
1997 4x4 Mazda B4000 SE, Ext.Cab

http://troubleshootmyvehicle.com/for...d-P0174-page-1

__________________
1998 Ranger XLT Super Cab 4.0 ohv v6 - 5r55E automatic Trans - 8.8 with 3.55gears - Toreador Red Clear Coat Metallic -Explorer Sport Trac White Faced Gage Cluster -2006 Mirror Swap-16" Mustang wheels with 215-65-16" Falken Tires

On my sons silverado the most common p0171 code would be the fuel regulator. But in his case ended up being the fuel pump. Not enough fuel pressure.

__________________
Myrtle: & Danna
1999 Ranger 4x4 3.0 Stepside Ext Cab XLT
2000 Mustang 4.6 Roush Supercharged ( Show Car )
1984 Elcamino 350 330 HP ( Nick's Show Car )
1959 Wife ( Arm Candy )
Danna passed away 5/26/2016

I was hoping my problem was coming from fuel pressure, although not the fuel pump since that part is a couple hundred.
I was hoping the fuel filter replacement would have fixed that but I'm still getting mixed readings from stft which according to the site linked above (which I had already read a few times before) sometimes it's leading me to believe fuel pressure and other times it's air leak. Sometimes the readings are normal.

I was thinking that maybe because I was getting such whacked out readings, that perhaps it was a sticky valve somewhere.

*edit*
I don't know if this has anything to do with it, or if this is even still a problem as I've replaced some parts since winter, but over the winter when it was cold, the truck would have a hard time starting sometimes. It seemed the colder it got, the longer I would have to crank to get it going.

The only reason I remembered this and thought to bring it up was after reading this while googling the fuel regulator

"only fords use the schraeder valve. also, because of the check valve in the pump, if the pressure diminishes after shutting it off, that could be the cause of a cold start problem."

When my truck had a leaky intake manifold gasket, the engine would stall & die in colder weather. Esp. at traffic lights. Hmm.

__________________
1997 4x4 Mazda B4000 SE, Ext.Cab

Mine's never stalled in the cold, just took longer to initially start.
There were 2 times recently (before the MASS and fuel filter were replaced and in the warm weather) it stalled on the first slow turn after starting but then drove fine, but that's all.

One thing to note is that the O2 sensors are dumb. They will report a "Lean" condition whenever there is oxygen in the exhaust stream. If the engine misfires, then the fuel did not get burned, and there will be oxygen in that exhaust, triggering a 'lean' report.
Before following the 'lean' problem set, check that the engine is firing on all cylinders. You really need to figure out which engine you have to help avoid confusion.
tom

Wouldn't a misfire set misfire error codes?

well, I just bought myself 2 more years to figure this out. I cleared the codes with my blutooth obd2 reader as I was pulling in so they sent me to the full service e-check because the computer wasn't in ready status. Passed the tailpipe test easy

It was close though because the light came back on just as I was pulling out.