Fuel Trim Basics

Just curious about fuel trims.

If they are showing negative, what's going on and what does the engine need more or less of?

Same question for positive high numbers.

Where would these changes be made?

Of course, I'm talking here that it's been verified no exhaust or intake leaks.

I was running my 5.3 with what I discovered to be a PCM from a 4.8 liter. I changed all settings to 5.3 stock and it's running worse. A bit of stalling and negative SFT's.

Sorry, at work now, don't have a log file.
Just curious about basics and where to begin to correct/tune.

negative fuel trims = pulling fuel
positive fuel trims = adding fuel

tune the MAF to get them close to 0

Both the MAF curve and VE need to be re-calibrated if the trims are either way positive or way negative.

Of course things like checking fuel pressure for 58psi and doing the intake vacuum leaks and exhaust leak check like you did should be done first.

Using a wideband o2 sensor is the fastest way to go about this. With that sensor, an error graph/chart can be made to log the error between the commanded AFR and the actual AFR. Either the MAF or the primary VE can be done first and both would be done in open loop with no fuel trims and closed loop disabled. The error that shows up in the graph/chart can be copied, then using the special paste by half be applied to the MAF or VE.

Thanks guys.
Looking at all these numbers and stuff is dizzying.....wow.
Thank gawd, I can always put it back afterwards. I saved the last tune before changing it.

NO WB. I called HP Tuners before ordering and was told I only need it for WOT tuning. Apparently not, correct?

One more thing, I get what appears to be a missed/quick 3-4 shift under moderate acceleration.
Tune and logs attached.

Mods to the 5.3 are bored .020" over, flat top 4.8 pistons, a 6.0 liter cam and 243 heads.

If the engine is modded, fuel trim tuning is more work and takes longer. You can do it but the wideband is faster and more accurate. Then you'd need it for wide open throttle anyway.

Even though .020 over isn't that big, still calculate the cylinder volume and change that to match in the calibration.

Disable COT, that can swing fueling if it activates on you are random.

Typically engines that have increased flow will be lean all over. The logs show it's lean under some throttle but rich off throttle. Is fuel pressure getting higher than ~58psi at idle or coasting?

All in all it won't take forever to dial it all in, the setup is nothing crazy. If all the other things are working good and nothing is making you run in circle, getting the fueling under control shouldn't take long.

Thanks...a lot to swallow. Don't even know what COT is, let alone how to disable it. I keep googlin everything posted here.
So, it was stalling after modding the tune to 5.3 specs. I put it back to the 4.8 specs which were running pretty good, never stalling. Well, it was stalling after putting it back.. WT?

Then, I get to the cruise and somewhere abouts my canister, is leaking fuel. No DTC's. Tank had pressure after turning off even though the vent solenoid at canister opens on key off.


I will change the cylinder volume, set everything back to 5.3 settings and find out if the purge solenoid/vac canister has been causing vac leaks/fuel delivery to intake, causing some of my stalling issues, etc.

Cat over temp.

If you removed the cats and rear o2 sensor, you want that disabled.

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Originally Posted by 5FDP

Cat over temp.

If you removed the cats and rear o2 sensor, you want that disabled.

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Thanks. Cats in place, rear O2's not.

So, I had fuel in the canister, so intake issues. Got that cleaned out, went back to the 5.3 tune and logged this.

The trims go to red for brief times. I'm assuming this is nothing to worry about as long as they are OK during cruising.
Correct?

You still have to go through the airflow model and re-do it all. I tried to explain that in one of my previous posts.

The work still has to be done to correct some of your issues.

If the engine is running good and not stalling or doing anything weird, start adjusting the MAF and VE tables to bring the fueling back in line. The cruising trims are still either adding in +20% or removing a decent amount of fuel at times. When you get further into the throttle your long term trims max out at +25%, showing that it's still needing a lot of work to fix the lean condition.

Should probably add your throttle position channel and injector information so you can watch that too.

Thanks.

Huge learning curve to go with this stuff.
Appreciate it.

I find it important to note you should be disabling the mass air flow sensor when tuning the VE table because the dynamic air calculations bounce between VE and MAF depending on operating conditions.

I would suggest starting with the VE table and doing MAF last but thats just me.

Start by disabling the MAF sensor under engine diag -> airflow -> mass air high fail freq = 0.
This will disable the mass airflow sensor and all dynamic air calculations and run as purely speed density based off the VE table.

You then log LTFT in the scanner histogram with a layout that matches your VE table so you can copy you LTFT errors and paste into your VE table by right click -> paste special -> multiply by percent half.

You also want to disable deceleration fuel cut off (DFCO) and anything else that can affect the commanded airfuel ratio in closed loop (as thats what you'll be stuck to tuning in with narrow band O2 sensors. I.e. no WOT)

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Originally Posted by 3strokeEngine

I find it important to note you should be disabling the mass air flow sensor when tuning the VE table because the dynamic air calculations bounce between VE and MAF depending on operating conditions.

I would suggest starting with the VE table and doing MAF last but thats just me.

Start by disabling the MAF sensor under engine diag -> airflow -> mass air high fail freq = 0.
This will disable the mass airflow sensor and all dynamic air calculations and run as purely speed density based off the VE table.

You then log LTFT in the scanner histogram with a layout that matches your VE table so you can copy you LTFT errors and paste into your VE table by right click -> paste special -> multiply by percent half.

You also want to disable deceleration fuel cut off (DFCO) and anything else that can affect the commanded airfuel ratio in closed loop (as thats what you'll be stuck to tuning in with narrow band O2 sensors. I.e. no WOT)

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Thanks for this.
I need to research about pretty much everything mentioned here including histograms and errors, pasting....total newb.
But, I will start this morning.

I found the MAF and DFCO.
For DFCO, there are several settings.
Change them all to 0?

Then, flash like this to PCM to do tuning?

If you want to turn off DFCO, make the Enable temperature about 350 degrees and that will keep it from cutting fuel.
Yes, and then write calibration.

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Originally Posted by Lakegoat

If you want to turn off DFCO, make the Enable temperature about 350 degrees and that will keep it from cutting fuel.
Yes, and then write calibration.

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Thanks.

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Originally Posted by 123pugsy

I found the MAF and DFCO.
For DFCO, there are several settings.
Change them all to 0?

Then, flash like this to PCM to do tuning?

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Perhaps I should have said flash the PCM like this so I could then do the logging?
Then, use that info to do the tuning?

OK, I did that and got a log. No idea if I did it 100% correctly.

Edit, added before log. Big difference.

What to do with the info?

OK, I made some adjustments to the VE table twice.
On the left of the pics is just disabling everything for the logging, the middle is the first adjustment and the right is the second adjustment.

How close do the numbers need to be?
Do I use the average as my guideline? Or the plus or the minus?

Thanks.

Quote:

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Originally Posted by 123pugsy

OK, I made some adjustments to the VE table twice.
On the left of the pics is just disabling everything for the logging, the middle is the first adjustment and the right is the second adjustment.

How close do the numbers need to be?
Do I use the average as my guideline? Or the plus or the minus?

Thanks.

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Assuming you drove around for a good while to get those numbers the fueling looks alright ish, needs less fuel at idle more everywhere else it seems. Everytime you make a VE adjustment you should be resetting your LTFTs in the scanner so it doesn't skew the data. If you put bad data in you'll get bad data out.

A lot of this is trial and error at first but you're on the right track.

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Originally Posted by 3strokeEngine

Assuming you drove around for a good while to get those numbers the fueling looks alright ish, needs less fuel at idle more everywhere else it seems. Everytime you make a VE adjustment you should be resetting your LTFTs in the scanner so it doesn't skew the data. If you put bad data in you'll get bad data out.

A lot of this is trial and error at first but you're on the right track.

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Thanks.

Each run was 16 minutes and change, engine warmed up well.
Tables were empty each run and repopulated as I drove.

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Originally Posted by 3strokeEngine

You then log LTFT in the scanner histogram with a layout that matches your VE table so you can copy you LTFT errors and paste into your VE table by right click -> paste special -> multiply by percent half.

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Damn....I read this and was trying to change all the numbers manually by 1/2%. Didn't read carefully as I was still in the flustrated learning and nervous state. Feeling better about things now.
The paste special cut and paste works real easy.

So, I imagine there will never come a time when LTFT's will be at zero average when scanning.
Is it the average errors I'm supposed to copy/paste?
When to stop this and do the next step?

What is the next step?

Thanks.

the environment is constantly changing. given this fact, use the copy paste method until your error values being recorded are within +/- 5% of 0.
you should be monitoring average values when performing these steps. make sure you smooth your changes using the editor table smooth functions.

you can feed in some ignition after working the fuel tables into a desired level of error. motors close to stock typically dont need much more than the oem ignition map on standard pup gas. these motors have fast burn head chambers so ignition around 19-23* is pretty common depending on engine behavior and installed components.
this assumes the motor in question is N/A.

Quote:

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Originally Posted by cobaltssoverbooster

the environment is constantly changing. given this fact, use the copy paste method until your error values being recorded are within +/- 5% of 0.
you should be monitoring average values when performing these steps. make sure you smooth your changes using the editor table smooth functions.

you can feed in some ignition after working the fuel tables into a desired level of error. motors close to stock typically dont need much more than the oem ignition map on standard pup gas. these motors have fast burn head chambers so ignition around 19-23* is pretty common depending on engine behavior and installed components.
this assumes the motor in question is N/A.

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Thanks.

I'll tweak it a bit more and call it good.

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Originally Posted by 3strokeEngine

Everytime you make a VE adjustment you should be resetting your LTFTs in the scanner so it doesn't skew the data.

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How does one reset the LTFTs?

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Originally Posted by carzan

How does one reset the LTFTs?

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They don't actually reset. They are constantly running to give fueling info.

To clear them from your scanner, just click Log File > Close Log File > Start Scanning
I usually close the scanner and restart it.

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Originally Posted by 123pugsy

They don't actually reset. They are constantly running to give fueling info.

To clear them from your scanner, just click Log File > Close Log File > Start Scanning
I usually close the scanner and restart it.

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The LTFTs DO reset and you MUST reset them everytime you flash a change to your VE table or it will skew your data and youll be chasing your tail. The goal here is to try to get your fuel trims +/- 2%

Open your scanner and start scanning and look for the little green power button on the top of the scanner. Click that and it brings you into the scanner bidirectional controls.

Navigate to engine -> fuel then click "LTFT Reset".


On a side note LTFT works for doing these adjustments but it takes a long time for your LTFTs to average out once you reset them. I prefer logging LTFT + STFT as it seems to populate with accurate data much quicker than waiting on the LTFTs. Of course a wide band O2 is the best way to go about doing this but you can get danm close with narrowbands if you can spot trends and adjust accordingly

Quote:

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Originally Posted by 3strokeEngine

The LTFTs DO reset and you MUST reset them everytime you flash a change to your VE table or it will skew your data and youll be chasing your tail. The goal here is to try to get your fuel trims +/- 2%

Open your scanner and start scanning and look for the little green power button on the top of the scanner. Click that and it brings you into the scanner bidirectional controls.

Navigate to engine -> fuel then click "LTFT Reset".


On a side note LTFT works for doing these adjustments but it takes a long time for your LTFTs to average out once you reset them. I prefer logging LTFT + STFT as it seems to populate with accurate data much quicker than waiting on the LTFTs. Of course a wide band O2 is the best way to go about doing this but you can get danm close with narrowbands if you can spot trends and adjust accordingly

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Thanks, I hate to throw out bad info.
It just appeared to give me a fresh blank chart that started to populate when I restarted the scanner.

I'll have to check out the reset later.

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Originally Posted by 123pugsy

Thanks, I hate to throw out bad info.
It just appeared to give me a fresh blank chart that started to populate when I restarted the scanner.

I'll have to check out the reset later.

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It doesnt reset your charts fuel trim it resets the LTFTs in the pcm that it learned over time

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Originally Posted by anniversaryss

It doesnt reset your charts fuel trim it resets the LTFTs in the pcm that it learned over time

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Thanks.

OK.

Changes made and trims reset (properly this time) before logging.

I think it looks good, but I'm no expert as you all know.
Tell me what you all think, please.

No comments? What's next?
Will these numbers change after turning the MAF back on?

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Originally Posted by 123pugsy

No comments? What's next?
Will these numbers change after turning the MAF back on?

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I answered my own question.
Reset the MAF and proceeded to log huge LTFT's.
Apparently, my intake hose is leaking. The numbers told me to search for the problem.

I wasn't happy with the leaking intake tube, so I made up a solid aluminum tube with zero leaks and same results with MAF reinstated.
High L & STFT's and no intake leak.
Should I be looking at a bad MAF sensor or do I now need to do some different tuning for the MAF? :doh:

I'm running a similar setup in a 99 camaro with T56. 5.3 with flat tops, 243 heads, ls1 intake and injectors. LS2 cam from a corvette. Started with the 99 camaro ECM stock program and have tuned VE and MAF to 0 to -5 LTFT in most cells, runs really well. If you think my VE or MAF info will help let me know.

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Originally Posted by KDS

I'm running a similar setup in a 99 camaro with T56. 5.3 with flat tops, 243 heads, ls1 intake and injectors. LS2 cam from a corvette. Started with the 99 camaro ECM stock program and have tuned VE and MAF to 0 to -5 LTFT in most cells, runs really well. If you think my VE or MAF info will help let me know.

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Thanks.

I wouldn't mind seeing the tune.

good stuff here - trying to get my trims in check as well

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Originally Posted by K Rob

good stuff here - trying to get my trims in check as well

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How you enjoying the learning curve?

Be careful with spark advance if you change the tune. I developed piston slap after going a bit too extreme.
Or perhaps the 5.3 gets piston slap commonly? Don't know.

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Originally Posted by 123pugsy

How you enjoying the learning curve? .

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Quote:

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Originally Posted by 123pugsy

Should I be looking at a bad MAF sensor

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what did you come up with with the high lt/fts after maf hooked up?