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LTFT / O2 Analysis & Tuning Guide

Purpose

This document and the associated downloadable files are meant as an aid to assist you in diagnosing how your Air/Fuel (AF) mixture delivery is performing, and how to use Autotap and a MAF Translator to adjust various parameters. This comes from recent practical experience, talking with a few folks who really know their stuff (THANKS… you know who you are!) and a bit of reading. I have applied this knowledge to tuning my recently ‘juiced’ car and am happy with the results.

If you find any errors, or have differing opinions on the content, lets discuss it. I want this information to be as accurate as possible; otherwise it will not do anyone much good.

Let's Get Started

This document and the associated downloadable files are meant as an aid to assist you in diagnosing how your Air/Fuel (AF) mixture delivery is performing, and how to use Autotap and a MAF Translator to adjust various parameters. This comes from recent practical experience, talking with a few folks who really know their stuff (THANKS… you know who you are!) and a bit of reading. I have applied this knowledge to tuning my recently ‘juiced’ car and am happy with the results.

If you find any errors, or have differing opinions on the content, lets discuss it. I want this information to be as accurate as possible; otherwise it will not do anyone much good.

In this write-up, I will refer to a few products and will use abbreviations, including:

• Autotap from B&B Electronics (Tap)
• MAF Translator from Modern Musclecar (MAF Trans)
• MAF Translator settings will be written as x/y for base setting/WOT setting (i.e.: +5/-2 would be 5% rich on the base setting and 2% lean on the WOT setting.)
• Long Term Fuel Trim / Short Term Fuel Trim (LTFT / STFT)
• Throttle Position Sensor (TPS)
• Knock Retard (KR)
• Fuel Trim Cell (FTC)

Even if you do not plan on using a tool such as the MAF Trans, this may help you extrapolate meaningful Tap data.

Enjoy,
Craige K12

[ Click here to download the Autotap config file and the MS Excel data analysis template in .zip (PC) format ]

What the heck are LTFT’s and STFT’s?

To start off, a technical description of Fuel Trims is in order. The following is from p6-1156 Vol 3, GMP/98-Y-3 of my Helms Manual for the 1998 C5.

• Short Term Fuel Trim Description
  The short term fuel trim is a PCM erasable memory register. The neutral value for the short term fuel trim is 0%. Any deviation from 0% indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 0%. The short term fuel trim is rich when the scan tool indicates -99%. The short term fuel trim is lean when the scan tool indicates 99%. The short term fuel trim changes the pulse width by varying the Closed Loop factor of the base pulse width equation. As the PCM monitors the oxygen sensors input, it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term mixture trends. THe correction of long term mixture trends is the function of long term fuel trim.
  
  When the PCM determines that the Short Term Fuel Trim is out of the operating range, the following DTCs will set:
  
  *DTC P0171 Bank 1 Too Lean.
  *DTC P0172 Bank 2 Too Rich.
  *DTC P0174 Bank 1 Too Lean.
  *DTC P0175 Bank 2 Too Rich.
  
  
• Long Term Fuel Trim Description
  The long termm fuel trim is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation.
  
  While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 0%, the PCM will change the long term fuel trim value. Once the LTFT value is changed, it should force the STFT back toward 0%. If the mixture is still not correct (as judged by the HO2S), the STFT will continue to have a large deviation from the ideal 0%. In this case, the LTFT will continue to change until the STFT becomes balanced. Both the STFT and LTFT have limits which vary by calibration. If the mixture is off enough so that LTFT reaches the limit of its control and still cannot correct the condition, the STFT would also go to its limit of control in the same direction. If the mixture is still not corrected by both STFT and LTFT at their extreme values, a Fuel Trim Diagnostic Trouble Code (DTC) will likely result. When the PCM determines that the LTFT is out of the operating range, the following DTCs will set:
  
  *DTC P0171 Bank 1 Too Lean.
  *DTC P0172 Bank 2 Too Rich.
  *DTC P0174 Bank 1 Too Lean.
  *DTC P0175 Bank 2 Too Rich.
  

Under the conditions of power enrichment, the PCM sets the short term fuel trim to 0% and freezes it there until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct fot the commanded richness of power enrichment.

How do I find out what’s going on with my car?

Get out your Autotap, and start doing some logging while doing some normal driving (which of course should include some spirited WOT runs). I prefer to use the DOS version for several reasons. Reliability, faster refresh rates, and it is easier to establish a connection to your car than with the Win version. Autotap has a wide variety of parameters you can monitor, but don’t go selecting everything all at once. I have created a Tap config (.cfg) that you can run which monitors the pertinent parameters for our purposes here. A link to the config is located below. As a side benefit, the Excel template (also located below) is built for logs compiled while running this config.

What you will end up with is a log file that will look similar to this once formatted:

Note: The headings will be different…this excerpt is copied from the excel template that has the log file imported. The raw log file can be viewed, but it is messy!

Once you’ve got a decent sized log file, it’s time to analyze it. You can import it into Excel and look at what is going on, sorting on different fields to arrange the data in a suitable manner, figuring up averages, etc. If you have used the config file provided here, you can ‘crunch’ the data using the provided Excel template for a basic analysis. If you wish to sort the date on different columns, open up the file in a different worksheet, not in the template.

To use the template:

1. Start Excel, and open up the template you’ve saved to your computer.
2. Select 'Data -> Get External Data -> Import Text File' from the menu.
3. Browse to your log file, select it, and 'Import.'
4. Import the file as a delimited file, starting at row 2 and go to the next screen.
5. Select the 'comma' box (everything else is unchecked) and press the 'Finish' button.
6. If asked, you want to put the data in the existing worksheet:=$A$2.

The log file will be imported and if you scroll to the right in the worksheet, you will see a table similar to this:

Great, What does this stuff mean?

Now that you’ve gotten the results, here’s an explanation on what you’re looking at, and why I chose to calculate the data as I did.

We are basically concerned with two values: Average LTFT’s while in Fuel Trim Cells 1-19 and your WOT O2 B1S1 and B2S1 averages.

LTFT averages:
The LTFT’s are an indication on whether your PCM is having to adjust for a rich condition, as indicated by a negative value, or a lean condition, indicated by a positive value. The calculated average of LTFT’s is based on the PCM using FTC 1-19. There are actually 22 FTCs to choose from. FTC 22 is for WOT and we are not concerned with the LTFT values for that cell because we will be using the WOT O2 averages for WOT A/F values. FTC 21 is used for deceleration, and gives whacked out readings that we are not concerned with. Finally, FTC 20 is your ‘idle’ FTC, again not important.

WOT O2 averages:
The WOT O2 average is calculated from the B1S1 and B2S1 O2 sensors while under WOT. I have defined WOT as a TPS (Throttle Position Sensor) value above 99.5. Typically, when I go WOT, Tap indicates TPS of 99.6 or 100. This reading, which indicates voltage, can be correlated to an A/F ratio. For MY car (as calibrated by comparing my O2 readings to a wideband O2 sensor) I achieve a 12.5:1 A/F ratio when my O2’s indicate 0.900. It is generally accepted that you want your naturally aspirated (NA) WOT O2’s to be between 0.875 – 0.910. Since I’ve calibrated my O2 readings to a wideband O2 sensor, I know that I want my WOT O2 average to be right at 0.89 to achieve the ideal A/F ratio of 12.5:1 – 12.7:1. A higher voltage (0.93…) indicates a rich condition and lower voltage readings (below 0.875) indicate a lean condition.

As a side note, when not running WOT, the computer will do it’s best to be at the ideal stoichiometric value of 14.7:1.

Knock Retard:
The table also shows the number of occurrences of KR when above 3.5 degrees and the average value of your KR as a reference without having to sort through the data. Small KR values (below 4) can be considered insignificant.

So, if your LTFT averages are indicating a high positive number, you know that the computer is adjusting to correct for a lean condition and vice-versa. My average LTFT is consistently near –1.6, which is about as close as I can get to the ideal zero.

My WOT O2’s are consistently near 0.89, which is where I want to be running NA.

Besides using the template, import the log file into a worksheet and take the time to look at it. It’s interesting to see how all of the parameters logged interact with each other. You can sort the data based on different fields for further analysis.

Using a MAF Translator to correct your MAF sensor signal

Do you need a MAF Translator? With aftermarket or ported MAF ends, or a modified air intake, the stock MAF sensor will give erroneous readings because it is made to work with the STOCK MAF ENDS and airbox. If you’ve done such mods, you are more than likely flowing more air than the sensor tells the PCM about, and a MAF Translator helps you out by altering the signal the MAF sensor sends to the PCM.

An indirect benefit of the MAF Trans is getting rid of KR… remember, what the MAF translator is really doing is telling the PCM that there is more air than it is "seeing". As a result, the PCM selects a different table that has less timing advance. The result of this reduced advance is reduced detonation, and as a result, reduced need for knock retard.

For me to get to my near ideal numbers, I chose to use a MAF Translator. This is a brief description on how to use this tuning aid. Install the MAF Trans and mount in a suitable location.

For this example, we’ll assume that your average LTFT values are indicating a lean condition, indicated by a positive number. Start out with the base switch at 5% rich and leave the WOT switch alone. You want to dial in your base setting before messing with the WOT, as the WOT is affected (although only slightly) by the base setting. With the translator set at +5/0, monitor your LTFTs again, taking the average of them while you are in fuel trim cells 1-19.

See what your average is, and make an adjustment to +10/0 if your LTFT's are still averaging a few points above 0. The key is to use the setting that gets you as close to zero as possible. You may have to go back and forth a few times... this is a time intensive process...you won't finish in 5 minutes.

Once you get the base setting dialed in as close to zero as possible, it’s time to start monitoring your O2 sensors, B1S1 & B2S1 while you are under WOT. The ideal range of 0.88 - 0.91 should get you close to an A/F of 12.5:1 to 12.7:1.... where you want to be under WOT. If you are getting WOT O2 averages above this range, adjust the WOT switch to -2 and proceed to gather more data, and make another change to the WOT switch. Repeat until you're getting as close to .89 - .91 as possible.

Note: The base setting goes in 5% increments (…10%lean, 5%lean, 0, 5%rich, 10%rich…) and the WOT setting goes in 2% increments.

As I increased my base setting to 5, 10, 15, and back to 10% rich to bring my LTFT averages close to zero, I also saw variations in my WOT O2 averages… so get your Base setting dialed in before moving on to the WOT setting. In the future if you find you need to tweak the base setting (as I did after more mods) you may have to readdress the WOT O2 setting as well.

Parting Words

As of today, I have ended up at +15/-2 to get my LTFT's consistently averaging about –1.6 and my WOT O2's averaging at .89. When I spray, I go a bit fat (rich) like to 0.93, which correlates to 11.8:1 - 12:1 A/F. This rich condition is part of the beauty of a wet n2o system, but that’s a whole other topic!

I constantly run Tap if I’m out driving hard and quite often when I’m just out day to day driving. This allows me to keep tabs on what is going on. Be warned, it can be come a bit of an obsession.

Good luck, and I hope this helps you rather than confuse you!

Craige K12 (ck98c5)