I'm not entirely sure, but I think after an extended period of "no battery" the ECU will completely reset including all emissions/trim data, and you'll just have to run it through a complete OBDII drive cycle for it to relearn, (kinda a pain in the but) but free. Atleast, I don't THINK it keeps the fuel trim etc. when it requires a drive cycle reset? I'm sure someone will chime in.

 

[quote=john65b;261573]Ok. I have a question. I watch my Precat and postcat O2 sensors numbers and with new O2 sensors, they vacillate a little - watched them closely on idle and accelerations, and my sensors show the respective fuel trims now are within + / -10% (mostly around a couple %). The long term fuel trim remains at 25%.

I think it has to "learn down" the Long Term Fuel trim before the truck knows there is no lean condition.

Does this sound correct? If so, how long does it take for long term fuel trim to drop to within acceptable limits where it doesn't throw P0171 / P0174 codes?

I have not driven this truck, as I thought I could be doing some harm, but the more I think about it, I should drive it to learn down the fault....and check the LTFT after a week...
[quote]


first and foremost, stop everything you are doing with this vehicle. Based on the information you are giving, you could have one of a few things. Both exhaust manifolds are leaking or an leak in your intake system AFTER the MAF. how many miles and what year is your truck. and how much vacuum are you generating.

 

The truck has 118K Miles. Its a 2002 Disco II. How much Vacuum am I generating? I do not know - do you want me to put a vacuum gauge on it? I don't think this is typical sensor data.

I have looked for a vacuum leak and cannot find one (from MAF to Throttle body). I pulled the three vacuum connections on the pass side and plugged vacuum inlets and did not affect the Short term or Long term trend.

I would think I would be easily be able to see and hear a Vacuum leak to send the LTFT readings to 25%

Let me know.

 

ok...buddy...trust in the "sensor data". you'll never fix your truck that way. what your not getting is that sensor data is influeneced by everything that happens before it. so if one iota of air is gettin in after the Maf, guess what, your engine is now running lean.
Sensor data is not everything. It isnt the end all and be all of all problems. All sensors and codes do is give you a general area in which to look.
you said you checked maf to throttle. have you checked after throttle? The actual intake itself?

and fyi, long trim fuel trim will never change at idle. only while the vehicle is in motion above certain speeds.

 

I have cleaned the throttle body, and the IAC (it was caked with crud). I also ran seafoam through the air intake. I have looked around at the exhaust last night and saw / noticed nothing. The fact that the LTFT is same (high) on both banks, could we rule out exhaust since the leak would have to appear on both exhaust pipes? It would have to be air intake (vacuum), fuel quality or pressure, or something else upstream, as you indicate.

OK, please do not be offended at this next question (I really appreciate your assistance)

If the Lean Condition (P0171 / P0174) cannot be mitigated / fixed while the Long Term Fuel Trim remains high (because the computer is using the LTFT + STFT to determine how much gas to send), and the LTFT cannot be drawn down until the car has been driven over XX MPH for XX amount of time, I can throw all sorts of new parts, fix vacuum and exhaust leaks, try any and all fixes out there, and it still won't stop the lean condition, until the LTFT is reduced / corrected, correct?

Unless I drive on the expressway a while (which I haven't done in months - as I live 13 miles from work, and hardly go over 40mph), or get adaptive reset at dealership, can this problem be corrected?

I am sorry to ask this - I am NOT "challenging" your expertise...

 

Vacuum gauge reads 16 inches Hg at idle. Should I get readings at 2000 and 3000 RPM?

 

Nope, dont even bother. That just about elimates a vacuum leak. You could test for an intake leak one of two ways. The extrememly dangerous carb cleaners method or the safe but will cost you some $$ smoke machine method....
I'd rather not see you burn your truck to the ground, so go get a smoke test on your intake. They should be able to find the leak pretty quickly.

 

and for the love of sweet baby jesus, ignore LTFT please. For the last time, it has nothing to do with your problem. you are talking about something you dont understand.

 

And for whatever reason, you choose not to explain. I am quite sure this very complex Fuel Trim concept is within my understanding (25 years as a mechanical engineer working in the Oil Refining industry no less) , but it appears easier to say I am an idiot. Either way, its OK. Its cool. Fair enough. No more discussions on LTFT. We are good.

Now, back to the crux of the problem...I found two bolts (one on each muffler flange connecting the Cat and exhaust manifold) that are missing. I will replace these missing bolts and tighten all the remaining four, and kill the P0174 / P0171 codes.

Fingers crossed.

 

Yep, because thats exactly what I said. Please, moderators let the record show that I called this gentleman for all intents and purposes, a grade A idiot.

Since you wanted to be a dbag about it. Long term fuel trim is a numerical value that is calculated by the ECU using the values of engine and vehicle speed, ambient, engine and coolant temperature and vehicle load. It is used, along with O2 sensor readings (of how much air is in the exhaust stream) to vary pulse width injection (i.e. how long injectors fire) when the vehicle has warmed up and is running in Closed loop operation.

Closed Loop operation - See Wikipedia bc I really dont feel like writing the definationfor closed loop right now.

So exactly what have we learned today class.
your problem P0174/0171 means the system is too lean. Therefore we can deduce that since you have no vacuum leaks, it must either be an intake or an exhaust leak that is letting air in either before cats or after the MAF. Another possibility is that your injectors are clogging up and not spraying that amount of fuel they should. There are multiple potential causes to the system runnig lean but it all boils down to system has a leak somewhere or your not getting enough fuel. You said you found missing bolts on the exhaust, good. you may have just found your lean condition.

We have also learned that you, good sir, are a bit of a dbag when it comes to people helping you because your 25 years of experience of working in mechanical engineering should clearly grant you the knowledge to be able to understand and fix anything. Well, my paltry 4 years of automotive technician's experience pales in comparison to yours, but, its what I do. 5 days a week, sometimes on weekends and latenights too.
So the next time you wanna wave around what you do for a living, just remember, the guy your talking down at, may know just a little more than you.

Goodnight and have fun fixing your truck......