How to reset long-term fuel trim

[todor]

Is there any way to reset the long-term fuel trim? Some other makes' forums say to disconnect the battery for 15min... but that didn't seem to work on my '98 M3 - I disconnected the battery for more than 30min, but my OBD2 tool still showed a -8.6% long-term fuel trim adjustment on both banks.

Is there any way to reset it via the OBD2 protocol? Or do I need some BMW-specific tool?

[epj3]

Plug in an OBDII reader and use the reset feature (ie to reset a MIL)

[nickdrivesm3]

MIL reset will only reset OBD II monitors, it wont reset adaptations. You would need dealership diagnostic equipment to perform adaptation reset.

Or disconnect the battery completely and touch the posetive and negative battery cables together.

[todor]

Or disconnect the battery completely and touch the posetive and negative battery cables together.

Does that work? Isn't it dangerous?

Btw, I already tried just resetting the OBD2 fault codes (MIL), but that didn't do anything to the long-term fuel trim adaptation - it still shows -8.6%.

[whopwood]

I thought a power off reset was only for OBDI. OBDII needs a tool AFAIK.

autoenginuity.com sells one... I believe it can be done without the BMW add-on package.

[todor]

Oops, apparently there was already another thread on the exact same issue:
http://forums.bimmerforums.com/forum...d.php?t=675921
(I somehow missed it in my initial search)

That autoenginuity tool seems very cool, but is also expensive...
I'm a software engineer by profession (unrelated to cars) and I almost have a setup with which I can send commands through the OBD2 port. Do you know if it's possible to reset the LTFT through that, or do I need to be going through the round BMW-specific connector? I looked at the OBD2 specification, and there's nothing about resetting these values in there - the only thing you can reset is the CEL codes and some test results from O2 sensors.

[whopwood]

It is possible through the OBDII connector. I regularly connect the autoenginuity directly rather than use the BMW adapter.

Edit: Ok, I'm 99% sure you can.... The unsure 1% of me will check it out in about an hour and post the results...

[ABC12345]

Clearing the fault memory or reseting mixture will only fix the problem short term. It will most likely come back. The DME is reading -8 long term (mulitplicative) so it is pulling fuel under a load. Need to find the cause of the excessive fuel. A pluged air filter, broken fuel pressure regulator, bad o2 sensor or mass air some thing along that line.

[todor]

Clearing the fault memory or reseting mixture will only fix the problem short term. It will most likely come back. The DME is reading -8 long term (mulitplicative) so it is pulling fuel under a load. Need to find the cause of the excessive fuel. A pluged air filter, broken fuel pressure regulator, bad o2 sensor or mass air some thing along that line.

Yes, obviously, I wasn't planning to "fix" the car by resetting the LTFT. I want to use it as a way to measure how any mechanical fixes that I do to it affect the LTFT - like replacing the FPR, MAF, etc. The idea is to see how fast the LTFT goes back down to -8 when I reset it (this will be used as a baseline) and then see if it behaves differently when I start replacing mechanical parts one at a time, monitoring LTFT for a few days after each replacement.

[nickdrivesm3]

Yes, obviously, I wasn't planning to "fix" the car by resetting the LTFT. I want to use it as a way to measure how any mechanical fixes that I do to it affect the LTFT - like replacing the FPR, MAF, etc. The idea is to see how fast the LTFT goes back down to -8 when I reset it (this will be used as a baseline) and then see if it behaves differently when I start replacing mechanical parts one at a time, monitoring LTFT for a few days after each replacement.

Take your car in for proper diagnosis, don't try shot gunning your problem and waste time and money in the process.

[todor]

Take your car in for proper diagnosis, don't try shot gunning your problem and waste time and money in the process.

I've got a used FPR and MAF from a friend's part-out of a '96 M3, so I'm not gonna be spending any money there. The O2 sensors are relatively new (5k miles on them), so I doubt they'd be causing a problem.

What do you mean by "proper diagnosis"? If there were a way to get a definitive answer on what's wrong with my car, I'd gladly get it done, even if I have to pay a shop to do it. But as far as I know, they're not gonna do anything more than what I could do myself. Or am I wrong?

[nickdrivesm3]

I've got a used FPR and MAF from a friend's part-out of a '96 M3, so I'm not gonna be spending any money there. The O2 sensors are relatively new (5k miles on them), so I doubt they'd be causing a problem.

What do you mean by "proper diagnosis"? If there were a way to get a definitive answer on what's wrong with my car, I'd gladly get it done, even if I have to pay a shop to do it. But as far as I know, they're not gonna do anything more than what I could do myself. Or am I wrong?

All I'm saying is that you should have a professional diagnose the car's problems. I understand you want to save money and ask BfcFI for tips/advice, but when it comes down to it you should have a professional opinion as to what, why and how.

[328iJunkie]

Just because your a certified tech doesnt make your diagnosing skills any better than his. I worked at a VW dealership for a few months. Some of those techs were dumber than i am.

[nickdrivesm3]

Just because your a certified tech doesnt make your diagnosing skills any better than his. I worked at a VW dealership for a few months. Some of those techs were dumber than i am.

I'm giving him my opinion, not my "diagnostic skills".

[ABC12345]

When you install the tested part do not clear the fuel trim values. Drive the car 5 or 10 miles, if the values come way down then that was the issue. By clearing the values it will take longer to isolate the bad part. from what I have seen values that are stable areound -4 will not cause the light to come back on. It would be best if they are around -1 to -2.

[todor]

When you install the tested part do not clear the fuel trim values. Drive the car 5 or 10 miles, if the values come way down then that was the issue. By clearing the values it will take longer to isolate the bad part. from what I have seen values that are stable areound -4 will not cause the light to come back on. It would be best if they are around -1 to -2.

Yeah, I could do that, but I thought it would be cleaner to start out with a blank slate after the mechanical changes. I have a tool that can show me the current LTFT, so I can monitor it from there. I just need to know how to reset it.

[ABC12345]

Not sure how to reset it on you tool. If you reset them it will take some time for the values to stablize before you get an answer if it corrected the problem. by not clearing them you will see a change in a few miles. The numbers will start to go down quickly if it was found.

Good luck with the hunt

[todor]

Not sure how to reset it on you tool. If you reset them it will take some time for the values to stablize before you get an answer if it corrected the problem. by not clearing them you will see a change in a few miles. The numbers will start to go down quickly if it was found.

Good luck with the hunt

Are you saying that initially it'll fluctuate too much for me to see any clear trend? That's probably a valid point, I'm just asking. Do you know from experience? The way I was imagining it was that it would take roughly the same time to move the LTFT from 0 to -1 as it would to move it from -8 to -7 for example. Is that assumption wrong? Does it fluctuate more when it's just been reset?

[Shuasha]

Cause a misfire,then reset the CEL. Worked for me all the time when I had a stock ECU.

[todor]

Cause a misfire,then reset the CEL. Worked for me all the time when I had a stock ECU.

Huh? Is that safe? What do I need to do to cause a misfire? And how would a misfire be related to resetting the LTFT?

[Shuasha]

I always had misfiring problems with my TT kit when I first installed it (ended up being plug gap). I would reset the code and it would reset the adaption. MAYBE resetting the codes did it, but I recall hearing (not 100%) that the misfire reset adaption.

[paul e]

So far as I know, nickdrivesm3 is completely right. Apparently, it takes a specific function of the bmw diagnostic computer to reset OBDII fuel trims which just isnt available on the relatively inexpensive OBDII scantools that most of us use. For several thousand dollars I believe we too can have the equipment that provides that function, only most of us dont. BTW, I just took a reading of my trims 4 yrs after my current software was installed, and it couldnt be much better: bank 1 was +0.8 and bank two was -0.8.

[nickdrivesm3]

So far as I know, nickdrivesm3 is completely right. Apparently, it takes a specific function of the bmw diagnostic computer to reset OBDII fuel trims which just isnt available on the relatively inexpensive OBDII scantools that most of us use. For several thousand dollars I believe we too can have the equipment that provides that function, only most of us dont. BTW, I just took a reading of my trims 4 yrs after my current software was installed, and it couldnt be much better: bank 1 was +0.8 and bank two was -0.8.

thank you...

/thread

[todor]

Yeah, well, as it turns out, there's a tool that costs $480 that can reset the LTFT (autoenginuity, with the BMW-specific add-on). I can possibly buy it, use it, and resell it and not lose more than $100 in the process. If I were to go to a BMW dealership, they'd charge me more than $100 just to reset it once.

If I asked them to troubleshoot my problems in general, I doubt the bill would be less than $500 and could get up to, say, $2000 in no time.

Anyway, my question about resetting the LTFT has been answered (except that I don't have a conclusive answer on whether it's safe to touch the positive and negative terminals with the battery disconnected). So I'll just try the things that I can try for free, and if none of them works, I'll probably have to seek professional help... for the car, I mean

[UltimateM]

(except that I don't have a conclusive answer on whether it's safe to touch the positive and negative terminals with the battery disconnected). So I'll just try the things that I can try for free, and if none of them works, I'll probably have to seek professional help... for the car, I mean

Yes its safe to touch them together.