X300 Long term fuel trim mysteries
 

Hi gang,

Rather than bore everyone with my LTFT tribulations, which makes for long and boring reading, let me ask this: have any of you experienced...and hopefully solved...chronic (and seemingly "fixed") LTFT readings? I don't know how wide spread this condition is, but I know I'm not entirely alone :-)

If anyone wants the long version, I'll post it.

Thanks
DD

Doug,
my 95 xjr has the same issues, long term fuel trims stuck at 35.1 no matter engine rpm. short terms are fine at idle but are rich when driving, -1 thru -25, strangely the only time it is at a solid and steady 0.00 is when im really stepping on the accelerator hard. what do you think?

Long term version might shed more light.

But ... a few months ago, I posted the same "problem". LTFT persistently at +10 on both banks of a V8 with negative STFT as if the ECM was fighting itself. No real answers other than the usual vacuum leak suggestion.

That was on 87 octane with "up to 10 percent ethanol".

On a hunch, just before a long highway trip I switched to 91 octane with "no ethanol" and did a reset.

After the trip, always refueling with the same brand 91 octane with "no ethanol", LTFT was at +2 percent. Actually, it was there by the end of the first tankful.

If you think about it, the lesser energy content and efficiency was being made up by the ECM in closed loop by edging up the fuel feed on a constant basis at a rate of +10 which is just about the effect of the ethanol.

The LTFT has not budged since changing to 91 octane with no ethanol. The STFT hovers either side of zero as it should. Needless to say, I stick to that one brand and grade of fuel. The actual cost is mitigated when you calculate milage on a miles per dollar basis as opposed to the usual miles per gallon. That +10 LTFT has a cost.

Europeans don't see this as much because they see less or no ethanol.

No comment :-)

Heh heh, not being a wise ass but I've given up on the situation. My car runs great, has good power, and the same or better fuel economy than any other XJR/6 out there.

I still get the totally random P0430 code (can be days or months between events) and freeze frame data always shows LTFT at "100" when the code sets....which I think is technically impossible. Otherwise my LTFT stays in the 70s....er, at least the last time I checked, which was ages ago. With a hard reset I can start from scratch but the reading is high again within a couple days.

Interesting commentary from Plums on ethanol! Wish I could find some E-zero fuel to experiment with.

Hopefully Ross will chime in as well. He very experienced in this area and has tried helping me to no avail (no reflection on Ross, mind you!) . I think he's had sucess with re-orienting the ECU which requires dealer level or equivalent equipment

A few years ago there was some speculation about an ECU anomaly with the 95 MY cars but I don't think that got beyond the speculation stage. I was gonna try to find a 96 ECU but Ross dissuaded me, as I recall. Not sure if it would even be doable.

Good luck
Cheers

I use the Ultragauge on both of my cars. The XJ6 gives LTFT of 53.9, but the XK8 readings are usually +5 to -5 range. So I just ignore the readings on the XJ6. I wonder if a '97 XJ6 would have the same problem.

Try post #15 here:

https://www.jaguarforums.com/forum/x...uestion-58622/

he seems to be related to the makers of the obdkey dongle.

Olsons on 99E Milwaukee, OR has pure 92 octane fuel. If I ever get the old girl back from the painter I am going to try it

What Doug and I have experienced is LTFT reading up in the 90s. That would be well outside the possible control range for LTFT. And both of us had normal looking STFT, that cycle about zero as they should and closed loop control.

I like your theory about e-10 gas plums, but I am sure it was not the cause for our trims being so far out. A few percent would seem to be attributable to ethanol's lower heat content, at most.

I have two ECUs, my original and a MY'96 spare. Both of my ECUs had the high LTFT reading. At the time, I did not yet own IDS or WDS, but AutoEnginuity and Proscan both showed the same high values.

The second ECU had a different "orientation" for the lmbda sensors, so when I swapped it in, I had to switch the sensor leads. (See archives for what orientation is). After I got WDS, I did reorientation on the MY 96 ecu while the lambda sensors were connected as for the MY 95 ECU in order to make the modules interchangable. I also ran TPS calibration to fix a high idle problem I had with either module. The MY 96 module settled out the idle, so I put the MY 95 module in and did the re-orientation just for the heck of it, then the TPS recalibrate. Now, both modules idles correctly and were oriented the same.

Here is the point of this story... Some time later, I was checking the fuel trims and I noticed that the LTFT and STFT were both looking normal - hovering near 0. I put the other ECU in and it was normal too. So, since I was not looking at the LTFT immediately after I recalbrated, I am not sure it was the reorient or the TPS, but it seems like one is what probably what "unwound" the LTFT. Unfortunately, I did not do a controlled experiment, so I am not sure of cause and effect.

Now that DOug has a seatbelt fault that I know will annoy him, I am hoping he will spring for a reorientation and TPS calibration while he has the seatbelt fault reset by somebody with the dealer software and he can prove or disprove my theory that one of those was the cure.

Or, Doug, we can wait until the next time I go out to Tacoma (we have a customer there) and I will bring my conputer and VCM with me and we can meet halfway.

I'm calling the nearest dealer on Monday to set up a visit :-)

Cheers
DD

My LTFT has now gone from 53.9 to -100 on both banks measured by the Ultrgauge.
Next week I'm going to get the Autoenginuity out, because that doesn't make sense.

Welcome to the club :-)


Cheers
DD

And I am betting you will get the same on AE- Let me know if you want to come by College Park to re-orient it.

Ross, I would drive a lot further than College Park to get this resolved. I will PM my email address.

Contacted dealer and set things up for Friday but my schedule changed.

Might be just as well as the dealer wants about $400 to recalibrate the TPS, reorient the O2 sensors, and pull/clear the air bag code. More than I wanted to spend right now, to be honest.

I might see what I can do on my own regarding the air bag light and continuing living with the LTFT as I have been for the last few years!

Cheers
DD

Doug,
I am having problems with Long Term Fuel Trims but instead of the usual high positive number in the LTFT and normal readings on the STFT I now have very high negative numbers, LTFT -80 STFT +35. I checked everything, o2 sensors, MAF, Coils, plugs, intake and exhaust leaks etc. I believed my maf to be bad because when unplugged car stopped shaking and idles great. Imagine my surprise when in installed the new maf and everything the same, all values stayed the same. But take a look at the attachment I found and I think this will explain LTFT mysteries.

Thanks for thinking of me!

Actually, I've seen that bulletin before.

I'm apparently a bit dense at the moment as I am not seeing the LTFT mystery explained. Are you saying that the not re-adapting AMFR and FMFR is what causes the abnormally high LTFT readings?

Cheers
DD

my understanding is that LTFT will remain the same unless an adaption is done with the new MAF or after the fault is corrected. it seems that the LTFT/computer does not recognize the correction and it goes on as business as usual despite the repair. taking my car tomorrow and will let you know if this was the magic bullet. Thanxs, David

Doug
While it may not specifically say that the high LTFT resets with adaption reset, it sure implies it. And, that is consistent with what happened in my case, AND it matches the old reports people made about swapped o2 sensors " locking up" the ecu. Thanks for the document, David!

I really must bite the bullet to have this done on my car, as you've so often suggested, Ross.

It's running SO well now, though, that I'm hesistant to even lift the hood :)

Cheers
DD

Yeah, I know what you mean. And it cannot possibly be that the LTFT is actually being used by the control algorithm that high, or the STFT could not bring it back to stochiometric. Also, we did the orientation on rj's car, and got no improvement. I do admit, though, now that I think about it, I used IDS on his car and not WDS, as I did on mine, and MAYBE ids uses a different operation for adaption reset, or I just did not get to the right menu since we were a little rushed for time. Maybe someone can comment which ids menu /item does the adaption.