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

 

check your PCV elbows. Common spot to crack on the 4.0, and i had that problem with my 2003. The attached pics should help you find it. Look at all of these lines closely, if one is cracked that will cause a vacuum leak and allow unmetered air into the engine causing your extremely high LTFT

The 6758B is what went bad on my truck(have a picture attached of that as well)

 

So you are saying that the STFT is essentially meaningless and the fact that it stays at an average of Zero idle-high throttle really isn't telling me much?

The REAL measure then you are saying is the LTFT which would indicate a sizable air leak, but one the ECU can compensate for since even at idle, the engine is butter smooth.

I sprayed the heck out of all the lines with brake cleaner hoping if I HAD a leak, enough would be sucked in to notice a change of running smoothness, but found nothing.

I guess I was thinking the LTFT is kind of the longer term averaged STFT values, but the two are evidently unrelated to each other then?
You can have a HIGH LTFT but NEVER see a STFT reading I guess (since this is what I am seeing)?

 

So you are saying that the STFT is essentially meaningless and the fact that it stays at an average of Zero idle-high throttle really isn't telling me much?

The REAL measure then you are saying is the LTFT which would indicate a sizable air leak, but one the ECU can compensate for since even at idle, the engine is butter smooth.

I sprayed the heck out of all the lines with flammable brake cleaner hoping if I HAD a leak, enough would be sucked in to notice a change of running smoothness, but found nothing.

I guess I was thinking the LTFT is kind of the longer term averaged STFT values, but the two are evidently unrelated to each other then?
You can have a HIGH LTFT but NEVER ever see a STFT reading much different than Zero I guess (since this is what I am seeing)?

And the fact that the O2 voltage readings both are oscillating perfectly between roughly 0.1v and 0.7v are just telling me the O2 sensors are working with the ECU able to compensate for an air leak by adjusting the LTFT far enough for smooth running.

What does STFT really tell us then if I may have a large air leak yet the STFT stays at Zero +/- 2%?

Tying to learn the theory stuff to better understand what the scan tool is reading.

Thanks for the .pdfs!