Just acquired a nice '05 FX-4 4 liter with only 60k miles used as a weekend driver (never hauled or towed anything).

Runs GREAT, smooth at idle, nic power, etc.

About every week I get a MIL (Check Eng) Light and it's always "Bank 1 Too Lean," and "Bank 2 Too Lean."

I hooked up a good scanner to it and used Live Data to monitor what's going on.

Both pre-cat O2 sensors bounce between about .1v to .7v and graphing them, the pattern looks perfect to me. The voltage is supposed to oscillate as the computer goes rich/lean/rich/lean to bracket the "perfect" mixture.

The Short Term Fuel Trim Bank 1 and Bank 2 vary from -2% to +2% as you drive around at various speeds. I observed it for a 20 mi drive on secondary roads and Interstate and the Short Trim stayed within these bountaries. Looked good to me.

The LONG TERM FUEL TRIM, however, which to my understanding is the "average" of the short term trim over a long time (I don't know HOW long), shows 20% - 24% meaning that over the longer term, the computer is adding 20-24% EXTRA fuel to make up for some condition (like an air leak of unmeasured air) or something, which I guess is setting the MIL light.

Even at idle, however the engine is perfectly smooth, with Short Term Trim under +3%, and spraying flammable brake cleaner everywhere around the intake manifold doesn't make the engine change at all, and the Trim stays rock solid.

So if monitoring the Short Term Trim NEVER produces a reading of more than +2% while driving (Bank 1 and 2), HOW can the Long Term Trim possibly show +23/+24%??

Am I totally misunderstanding the theory of how ST and LT Trim works, and despite great O2 voltages, and ST Trim, is my truck running dangerously lean, risking burned valves or pistons?

Thanks,
Bob

How does the mass air flow look?
Jim

To be perfectly honest, I saw the MAF reading when paging thru the Live Data, but didn't make note of it because I got wrapped up around LT vs. ST Fuel Trim.

I just Googled and it said 2-7 g/sec would be a normal idle MAF reading, and roughly 25 g/sec at mid RPM. I will try to take the truck out tomorrow and record some readings.

If the MAF readings are normal, I guess the probable cause for a high LTFT (P0171 Too Lean) is a vacuum leak?

I would guess a too-low MAF reading (in the absence of a leak) would result in too lean a mixture (and the O2/ECU would use + fuel trim to richen it?), and a too-high reading would make it too rich (because the MAf is telling the engine it "needs" more fuel, but the O2/ECU would use - fuel trip to reduce it?).

Is that close?

Thanks!

I just recently had a bank 1 & 2 lean codes on my wife's 99 Explorer sport. I did some research on it & it turns out the DPFE sensor could cause the codes if it is failing & alowing the egr valve to stay open & dump extra air into the engine. I pulled the DPFE off & did the tests & the vacuum part of the sensor failed the test, so I ordered a new one & installed it & it has been 3 weeks & so far so good.

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2000 Ranger Extd Cab XLT, 4X4 Off Road, 4.0L, Auto, 8.8 - 4.10's, 01 grille & lights swap.
69 Mustang, modified 5.0, & 5spd.(project)
99 Explorer Sport, 4.0 SOHC, Auto, 8.8 - 4.10's, PW, PL, PM, Cruise, Tilt. (Wife's DD)
05 F-150 2WD Regular Cab. ( My DD)

Thanks! One more thing I can check, and I guess you had no DTC related to EGR or anything other than a Lean code.

I looked and there are YouTubes on troubleshooting the DPFE sensor.

Did it also show a very high positive LTFT in case you remember? I guess any perceived unmetered air would probably do that.

Thanks!