Long Term fuel trims always negative (-5 to -10, sometimes up to -15)

[jtrosky]

Since installing my mailorder tune and monitoring my fuel trims, I've noticed that my long-term fuel trims are ALWAYS in the negatives - usually pretty consistently between -5 and -10 - but sometimes as much as -15. WIth my stock tune, the LTFT seem to be dead on.

After reviewing the differences between the stock and mailorder tunes, the only thing that I can see that could cause negative LTFTs are changes to the "MAF Airflow vs Output Frequency" table (values are higher in my mailorder tune).

So I actually have two quetions:

1. Are consistently negative LTFT really something that I even need to worry about?
2. If so, is it safe to revert the "MAF Airflow vs. Output Frequecy" table back to stock values to see if that brings my LTFT's back to 0'ish? I'm just not sure if modiying this table by itself can cause any issues with other parts of the tune - or is this table truly "independant"?

Other than PE-related changes, I really don't see any other changes that would affect fuel ratios in the mailoder tune.

Thank you!

[justinjja]

I am curious to hear the answer to this, because my mail order tune seems to be the same.
Only had the tune for a day, but it was around -15 when I did my first test drive while scanning.

I'm not smart enough to answer your question, but I have a guess: (based off of watching a youtube video about fuel trim while half asleep...)

The tune intentionally runs your engine a little rich, to avoid knock with the advanced timing.

[5FDP]

I'd just lower the table below like 7,000hz to bring the LTFT back in order.

When I tuned my grand prix, I never touched the MAF table below 7,000hz and only did it above that mark because thats when I'd get into boost and only then would I adjust the amount of fuel I needed.

[Road]

Personally I think mail order tunes are a waste of money. What works for one doesn't always work for the other. Good tuning requires a lot of logging and trial and error. You didn't state if you are stock or have bolt on's etc. Maf tuning is what you need to tune to correct your fuel trims. I would use the stock calibration or do what 5FDP suggested. If you take the time and post you mods and you tunes and do some logging and post up your logs and take advise from members on this forum you will end up with a better tune.
Mike

[jtrosky]

Yeah, I QUICKLY learned that "what works for one doesn't always work for the other"! I didn't like the mailorder tune at all (although, others on the Impala Forums love it). The problem is that I knew absolutely NOTHING about car tuning when I purchased the tune - at the very least, I learned from it, by comparing the differences between the mailorder tune and my stock tune.

I'm very different from most tuners in that I'm really not looking to squeeze every bit of performance from my car - I'm tuning things to make the car more comfortable to drive, that's all (instead of being tuned for maxium gas mileage like it is from the factory). I am learnng a lot from these forums - been very helpful! What makes it a little hard to learn is that I can't post my tune becuase it has it's roots in the mailolder tune. Don't want to be posting a commercial tune in a public forum, that's not fair to the tuner I bought it from.

Anyway, thanks for the info about the MAF table - I will do some testing with the info provided - it's much appreciated!

[jtrosky]

Almost forgot - my car (2012 Impala, 3.6L V6, 6T70 trans) is completely stock - no engine mods whatsoever.

Also - The mainorder tune also modified the Intake/Exhaust camshaft tables - is there any issue with reducing the MAF table while leaving those tables modified? I just worry that putting one table back to stock whle leaving other tables modified might be "dangerous" - I know nothing about MAF or air/fuel tuning whatsoever, so bear with me!

The main reason that I got the mailorder tune was in an attempt to make the shifing in this car more "natural" - always hated the way the car shifted - mailorder tune actually made it worse (for my tastes). I got it shifting MUCH better now though... Disabling skip shifts seemed to make the shifting so much more "natural" (no more "abrupt" shifts).

[Road]

Wow would be interesting to see what else is screwed up. Might be best for you to post your tune so it can be looked over to see if it's even safe. -15 fuel trims doesn't mean you are running rich it means your computer is having to compensate 15%. you are close to setting a check engine light. Put your stock tune back in and watch your fuel trims if they return to normal then you know it's the tune.

[jtrosky]

I've already checked the stock tune and the LTFTs are dead-on with the stock tune, so my negative LTFTs are definitely related to the mailorder tune.

However, like I said, I can't really post the tune as it's a commercial tune - not fair to the tuner I purchased it from. However, he is a VERY well respected tuner that has been tuning GM V6's forever. I truly don't doubt his expertise one bit - it's just that I am unlike most other people who buy a performance tune being that I'm more concered with the "comfort' of driving the car over the all out "performance" of the car.

[jtrosky]

FYI - my LTFT are MUCH better after reverting my MAF table back to stock values. I fell much better now. Also adjusted my Driver Demand table (increased last two rows by 20%) and Peak Torque table (stock+20%) and I seem to be able to reach 100% ETC (84% throttle pos) a little quicker and more consistently now as well. So far I'm liking my current tune very much!

It's amazing how much of a difference driver demand changes can make in the overall feel of the throttle...

Thank you all very much for your help - it's much appreciated!