ta2av

I have been reading up on LTFT and it seems being close to 0 as possible is the best, but what happens when it is too far off from 0?

quicksilverado

iTrader: (8)

The closer they are to 0 the better your gas mileage will be.

AutoDr

iTrader: (5)

Wrong. The closer they are to 0, the less the pcm has to add or subtract fuel to achieve a 14.7 to 1 air fuel ratio. Most pcm's will add or subtract up to 25% fuel before a rich or lean code is set. If you see -7 it means the pcm is reducing fuel by 7% but it can still control the air/fuel mixture.

BlownChevy

iTrader: (20)

you can tune to Zero all you want, your fuel mileage will not be any better than mine. Leave your LTFT's alone and get on with your life.

quicksilverado

iTrader: (8)

Correct me if I'm wrong. If the computer is not jumping all over with the LTFT and is staying close to stoic then the fuel mileage will be bettter.

BlownChevy

iTrader: (20)

Just something to think about......

Has anyone ever looked / scanned a 2006 truck? How close to zero are the LTFT's in a new vehicle? GM calibrates these vehicles for fuel mileage, in this day and age fuel mileage is mandated by the government and demanded by the public. There is a margin for error +/- in the pcm to allow the computer to adjust for unforeseen issues down the road. What you tune to zero today might be +/- 10% in a week with climate changes, dirty air filter, potential vacuum leak, bad tank of gas, altitude, barometric pressure, driving habits, etc, etc, etc.

IF the LTFT's are at + 20% (lean) the vehicle is still running stoichiometric, if the vehicle is running -20% (rich) the vehicle is still running stoichiometric. Zero % is still Stoichiometric, the only difference is the computer does not need to do anything to correct for a rich/lean condition.

there is time better spent watching paint dry than try to nail down 0% on your LTFT's.

Wilde Racing

iTrader: (26)

OR....turn off the LTFT all together and run only STFT. that way the computer only adds or subtracts fuel at little throttle or in CL.

12secSS

iTrader: (30)

Well, I beg to differ on the LTFT not needing to be at zero and leaving them alone. If the vehicle is running +20 at part throttle, the ECM is working very hard to keep the O2s around 450mV (it couldn't careless about AFR). This will cause it to add fuel when you go WOT, about 20%. So if you were running 12.5 without the fuel trim correction at WOT, it would be 10.5 with the fuel trim correction. The computer will always add fuel (+ trims), but will not remove fuel (- trims), this is a safety feature. If your fuel trims are off by more then 10% either way, I would highly recommend you get it tuned, especially if you have no other mods planned. The ECM from the factory is not calibrated properly, it is only calibrated to pass certain mandated regulations, and even then most OEM calibrators do not follow a set manufacturers' protocol to accomplish those results in a similar fasion. Now if the ECM is maxed out or not calibrated properly, other conditions are also affected, like torque calculation which is used for torque management and shift pressures for autos. By calibrating the ECM to run around 5% fuel trims, you are assured that your vehicle is running at it best. Granted, running 16.7 is best for fuel economy, and this can be reached safetly with a proper tune. Just my thoughts on the subject.

BlownChevy

iTrader: (20)

so you are saying tune around a possible mechanical issue?

+/- 10% is acceptable, otherwise the PCM would set a code for being rich/lean. Do you know what the threshold is to set a SES light? Again, the PCM is always looking for 14.68:1 when cruising, the +/- readings only mean it is working to get there. I guess if you want to give that hard working PCM a break then yes, tune to 0%.

There are MILLIONS of GM vehicles running on the road today without any issues....then only time these cars have "problems" is when the user gets a scanner and does not understand the strategies used to calibrate a vehicle.

12secSS

iTrader: (30)

If you want to discuss mechanical issues, there is an appropriate forum for that, this is the tuning forum last I checked.

Let me step back and add this, since we are in the tuning forum and I suspect everyone has decent knowledge of mechanical workings, only after all mechanical issues have been resolved. When you are assured no mechanical issues, then proceed. But we are digressing here.

I am the type that likes my math to work out properly, rather then just seeking the result of the equation regardless how it is calculated. I never said try to tune to zero LTFTs, I said +/-5%. Again, the ECM doesn't care about AFR, it only cares about 450mv, or what ever you have the O2 switch points set to. You can set the O2 switchpoints to .300mV and the Stoichmetric AFR setting to 12.8. The ECM will try and maintain the .300mV, which comes out to about 15.1 or so. In fact, take a look at Airflow Mode Zero of the Switchpoint Tables (IIRC) on F-Bodies, it is set at .320-.350 or so. 10% is quite a bit from target, using the truck setting you posted, 10% rich would be 13.34 and 10% lean would be 16.15. If the ECM is closer to the target or O2 switchpoint, it has more room to adjust for weather conditions or other conditions that could cause a too lean/rich condition it may not be able to correct for. Also, should your O2 fail (or something else), and they do, you can be rest assured you will not all of a sudden run pig rich or dangerously lean when the ECM reverts to open loop.

Just because the answer is correct doesn't mean it was calculated properly. Most of those "MILLIONS of GM vehicles running on the road today without any issues" are in fact running to rich, but as a safety reason (better to be rich then lean). GM calibration engineers'/technicians' employ various methods to calibrate models. Some are by tinkering with the IFR, or the PE table or the MAF. I have seen quite a few tunes from quite a few Gen3s. The method of tuning the ECM varies platform to platform and sometimes year to year.

I am not looking to argue over tuning methods, surely you have your own and I have mine. This is the tuning forum where people want to learn about tuning so they do not screw up their vehicles. They want to understand how things work. I will gladly help those interested in learning any way I can. Take care!