I have been trying to adjust my VE table, and I have been staring at LTFT histograms, and it looks like the LTFT changes fit almost perfectly where I need to make adjustments. Since LTFTs are a percentage, and the VE table is a percentage, are the LTRIMs a percentage adjustment added or subtracted from the VE table? I don't know if this has been covered before (I can't find it anywhere), but I just thought it was interesting.

 

ya knwo, to be honest i thought they did but today i was doing the same thing. My idle sits at 30 g/cyl in the 800 cell. I lowered the VE there and in pretty much all cells under 1200 RPMS to get my Ltrims down from -9.6 and it doesnt seem to be making any difference. I figured if the VE was lower, it'd add less fuel??

Dave

 

I am positive if the VE is lower it adds more fuel. I think you need to drive around for awhile. I bet your average changes.

 

If VE is lower your SD-computed mass air flow will
be lower and less fuel would be shot. No?

 

You are right..I knew it...I was just thinking about it backwards for some stupid reason. The higher the VE on the chart more fuel, lower less fuel. Which is why lowering VE in low rpms solves starting and stalling problems...brain fart.

So back to the important part. I will test tommorow and see if they are applied directly.

 

on my c5 vette they dont seem to be. read what i wrote above, maybe im missing something

dave

 

Did you get your ltrims with the maf unplugged? If not your ve could be right and your maf is wrong.

 

No, it's plugged in. But I think it is a moot point. The engine adjusts fueling with the MAF plugged in, so to get it right you should need to leave it plugged in. Even if my MAF is wrong, at least I will be tuning to where it will be more accurate with it plugged in. Otherwise we would all just go MAFless...

But I could be wrong...

 

I wouldnt use the ltrims to fool around with the ve table. The ltrims only represent how far off from the EPA mandated 14.7 AFR the car is. THe ve table calculated what the engine wants andnot some arbitrary fuel trim. I would go about adjusting the ltrims by using the IFR table. The only time the ltrims are + or - is when the AFR deviates from the target AFR of 14.7 and i am starting to think with a big cam this becomes a problem because it is in fact to lean, so most just force the car to run in open loop and figure out the fueling via a wideband.

 

I agree, it is lean...up top. But the bottom is pig rich. I tuned my IFR table to get me in the ballpark. I "borrowed" a VE table from somebody who posted it awhile back and compared it to what I had been calulating. It was pretty close, so I smoothed it and used it. Now I am just using the LTRIMs to clean it up. It seems that the LTRIMs are dead-on with what I need to adjust on the VE table. It would only make sense, otherwise where would the LTRIMs be applied?

 

The Ltrims are applied to closed loop fueling. LTrims are figured out between the o2 sensors and the maf. With these two sensors you get fueling that is supposed to equal 14.7 during closed loop. If the fuel deviates from this value any between the o2 sensors and the maf sensor the pcm will figure out how much more/less fuel is needed. If you force the computer to run in open loop you wouldnt need LTRIMS as a measurement for anything. All you would need is a wideband to figure out what fuel ratio the engine makes the best power rather than having it forced to run at 14.7

If you are lean up top you can adjust that two ways either the pe vs rpm table or the open loop f/a table. A well known tuner on the forum (NoGo) suggested tuning through LTrims doesn nothing but trick tune the pcm. Even more so when you have a cam with a lot of overlap since reversion skews the results/readings from the maf.

 

It also adjusts fueling without the maf plugged in to my understanding. Don't mix up speed density with open loop. My point was at low rpms the pcm mostly relies on the maf to calculate pulsewidth, so why would you want to change your ve table.

If you want to make changes to your low rpm ve table based on ltrims i think you should do it in speed density so you're only messing with ve, not ve and maf.

 

BTW, I'm getting my info from here

 

Once you dump the MAF you are Open Loop. The O2s will still report BUT the LTFTs don't adjust,in the PCM, as they are closed loop.FWIW.
joel

 

Oh ok, thanks for clearing that up. I still think he may be barking up the wrong tree by changing the ve table. You could still unplug the maf and read the o2's since the ltft's aren't updated, that would tell you if the problem is in the maf table or the ve table. Just my opinion, haven't tried it though.

Another thing to try would be to sort your ltft's by mass air flow and see if there's a correlation.

 

not true. Fuel learn (Closed loop) will continue even without MAF.

 

Is that via the o2 sensors?

 

Gameover, do you think it would be a good strategy to unplug the maf, let the ltrims learn, and then use VE tables to zero out the ltrims. Then plug the maf back an and if the ltrims change use the maf tables to zero them out again? I am just talking about part throttle here.

 

be aware unplugging the MAF will alter your timing (use the Low Octane spark table).

 

Gameover - I'll retry this as soon as I get my car back. They did not "Learn" when I went Open Loop. Is there is something I need to do to get them to learn?? Are my O2s stupid ?
Ohhhhhh wait, my bad. Run MAFLESS, CLOSED LOOP. I thought it would go into open loop immediately with MAFLESS. I get it!!!! Thanks.


joel