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Importance of Reseting Fuel Trim (LTFT).

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tightmopedman9
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Re: Importance of Reseting Fuel Trim (LTFT).

Post by tightmopedman9 » Sat Nov 02, 2013 1:10 am

In regards to the original post, you stated that the LTFT will change throughout the RPM range. I don no believe this is correct.
First of all, LTFTs are be based on airflow/load, not RPM. You can be at multiple load values all within the same RPM range.
Second, I believe LTFT is a constant number. From datalogging on my car extensively LTFT stays set a constant, it changes periodically, but not throughout the load range.

donis222
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Re: Importance of Reseting Fuel Trim (LTFT).

Post by donis222 » Mon Sep 22, 2014 5:24 pm

Thank you!!
I tried to find when this adaptive fuel map (AFM) was introduced. I found that it was on 1991 940T.

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Re: Importance of Reseting Fuel Trim (LTFT).

Post by homerh » Wed Nov 12, 2014 10:00 pm

tightmopedman9 wrote:In regards to the original post, you stated that the LTFT will change throughout the RPM range. I don no believe this is correct.
First of all, LTFTs are be based on airflow/load, not RPM. You can be at multiple load values all within the same RPM range.
Second, I believe LTFT is a constant number. From datalogging on my car extensively LTFT stays set a constant, it changes periodically, but not throughout the load range.
Thanks, tightmopedman9 for answering the question that was on my mind the whole time I was reading this thread, namely whether there is a single LTFT constant stored for all points on the map or a unique value for each point on the map. I could not see how there could be one for every point because only one value can be read by my scanner and I would expect it to be changing constantly while driving if there was a separate value for each point on the map. On my car LTFT is generally steady as rpm and MAF values change although it does drift up and down a few points over time, depending on the type of driving I am doing.

I now have several questions about my 98 S70 GLT (221K) related to this topic:
I recently noticed a loss of power combined with what feels like a mild engine miss under light to medium throttle (20-30% absolute TPS value). The vibration is most noticeable when cruising in top gear at around 2,000 rpm and then giving more throttle to climb a hill. The interesting thing is that it still pulls pretty well at WOT and idles very smoothly. When I monitor the ECU parameters, what struck me most is the LTFT is consistently hanging around -11-12%, whereas it has always been around +2% in the past. I have tried resetting the LTFT by disconnecting the battery several times and was surprised by how quickly the LTFT would go way negative, dropping to -8% after driving less than 1/4 mi at 30 mph. On should know that the plugs, injectors, all PCV hoses and both O2 sensors were recently replaced. Compression reading on all cylinders are in normal range (185-195 psi). Fuel pressure looks good (30 psi at idle, 42 psi at WOT) and there is no evidence of vacuum leaks. MAF was cleaned and reads about 5 gm/s at idle. The only code is P0455 (evap sys leak - large), but pinching off the hose from the canister just before the manifold did not help.

The questions I have are:
1) I was under the impression that LTFT is only adjusted under steady state cruising conditions. Is it normal for LTFT to be adapted under these driving conditions and so quickly?
2) What could be causing that much negative LTFT, considering all that has been replaced or verified to be operating properly?
3) Are there any other diagnostic tests I could perform that could help pinpoint the cause of the negative LTFT?

All ideas and suggestions are welcome.

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Re: Importance of Reseting Fuel Trim (LTFT).

Post by northernlights » Fri Nov 14, 2014 3:50 pm

When I replaced a set of stock injectors with slightly larger ones, it took about 30 minutes for the LTFT to settle at a final value. The STFT was a different story - that was an instant response down to -20%. And FWIW, I didn't reset the LTFT, mostly by accident as I assumed pulling the ECU would cause it to loose its memory. It didn't and started at the same LTFT I had with the stock injectors, so I assume it started with all of the previous values. The Motronic system is very smart, and quite adaptable.

If your LTFT suddenly went that negative then I would look for something like a leaky fuel pressure regulator. I've seen a number of vehicles where these leak into the intake when they fail, which would explain the enrichment, plug a bad regulator can certainly mess up fueling.

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