Strange condition today...

[BlackGS]

I saw something really strange in my scan today during a WOT run. I have had the same tune flashed for a few weeks now. I have done many WOT scans and every one showed LTFT's at 0 or -0.8 and AFR at 11.3-11.8. I haven't touched anything, and the weather has been relatively the same, but today the WOT run had LTFT's at +7.0, and AFR was 10.5-10.7! What happened?

BTW, Commanded AFR has been 10.8-10.9 for all my WOT runs for a long time. I know I need to changed PE to raise that a bit, but it never caused this type of problem before.

[Perforator]

Welcome to my world.

[BlackGS]

Welcome to my world.

Yeah, that's what I was thinking!

I made another run and LTFT's were again at +7.0, but this time AFR was back to normal at 11.5. The only thing I can figure is that there is a small exhaust leak, but I'm positive it is after the wideband sensor just before the cat. I scraped my exhaust on a driveway recently, and it broke a weld joint loose. I did make runs after that though, and no problems, so I don't know why that would just start today, but maybe it got a little worse or something.

[Perforator]

I fixed my problem by pulling the fuse for the pcm for 15 minutes.
For some reason the reset fuel trim button wasn't resetting all the fuel trims.

[BlackGS]

I fixed my problem by pulling the fuse for the pcm for 15 minutes.
For some reason the reset fuel trim button wasn't resetting all the fuel trims.

That's what makes my problem so strange. The trims were already learned, and everything was locked in very good. I just made a WOT run two days before, and it was right on. I can't figure out what made the trims jump so high so quickly.

I'm going to reset the trims after I get the exhaust fixed. Hopefully, it will go back to normal.

[Perforator]

Bill told me it might have something to do with the two sets of fuel trims we have. Something may have triggered the use of the second set.

[JerryH]

Humidity changes can be responsible for this.

The only 2 sets of fuel trims that I am aware of are Shrot Term Fuel Trims (STFT) and Long Term Fuel Trims (LTFT).

[BlackGS]

Humidity changes can be responsible for this.

Thanks Jerry. Do you see this high of a spike from just weather changes? Does it go back to 0 when the weather changes back? It was more humid today than a couple days ago, but earlier last week, the weather seemed about the same as today. I'll have to start taking note of temp, baro, and humidity before I leave the house. Might make for some interesting comparisons.

[Perforator]

Humidity changes can be responsible for this.

The only 2 sets of fuel trims that I am aware of are Shrot Term Fuel Trims (STFT) and Long Term Fuel Trims (LTFT).

There are two sets of fuel trim cells Ie: ftc0,1,2,3...
We only use one set but I guess there are certain circumstances where the other set may be used...

[Russ K]

This is how the fuel trim cells are laid out. The LTFT map & rpm boundary tables are not yet available for the V6 cars. Once they are added to the editor, they can be set up to match the idle speed & map kpa of an after market camshaft.

Russ Kemp

[JerryH]

Russ: You did mention this before and said that this was for V8s (with 16 fuel trim cells), I look forward to tuning a V8 one day... sounds challenging!

Perforator: The FTCs that we use currently are 4, but now we find out that we may have more? I hope Bill chimes in here to shed a little more light on this for us. I tend to think that becuase I've never heard of anything more than 4 FTCs that this is all we have to deal with (at least for the 97-03's). When you mentioned 2 trims, I thought you were talking about LTFT and STFT.

BTW, has no one taken a scan and referenced RPM, KPA and FTCs to map these out? This seems quite easy to do in excel and a few scans.

My car is down until the weather is warm enough that I can remove my exhaust and fix an exhaust leak and while there, install the WB O2 else I would do it myself. I surely cannot take more than 30-45 min of scanning and playing in various gears/loads to get a darn good idea of how they are mapped out.

BlackGS: Without knowing your humudity changes and other things, its hard for me to tell you yes or no for your car, however I can tell you that I can get spikes of + or - 0-2 points between driving in the rain and driving on a nice dry day. Does it come back when things dry up? For me they have, so I will say yes. The simple fact is that the MAF has no way to compensate for humidity and a spot on tune becomes "ballpark" for a period of time.

[Perforator]

Perforator: The FTCs that we use currently are 4, but now we find out that we may have more? I hope Bill chimes in here to shed a little more light on this for us. I tend to think that becuase I've never heard of anything more than 4 FTCs that this is all we have to deal with (at least for the 97-03's). When you mentioned 2 trims, I thought you were talking about LTFT and STFT.

Yes, I remember when I first started tuning, and was reading as much as I could on the subject, I read about the two sets of fuel trim cells. One was with AIR on, one was with AIR off. From what I remember we only ever use one (I forgot which), but when I had the problem with my LTFT's in PE mode, Bill talked to one of the engineers and they said I might be triggering the other fuel trim cells. Now Bill said it was EVAP, not AIR so I don't know and for the life of me, I can't remember where I read it.

[JerryH]

Air as in A/C or air as in the air pump?

If it was the air pump, it could be that they meant in the spark tables, the closed loop idle spark correction? There are 2 of those in some bins... air pump on and air pump off. Besides that, this is all new to me.

[Chris@HPTuners]

the V6 code has two sets of learned LTFT's, they are not numbered seperately unfortunately. One set is used when EVAP is closed (ie. not purging) and the other are used when it is open (purging). If you see LTFT's rapidly change but the cell# doesn't change then what you are seeing is the switch between the two. You should be able to log the EVAP duty cycle to see it changing.

Reset trims should reset all the trims.

Chris...

[Perforator]

Thanks Chris. That's exactly what Bill said lol.

[JerryH]

Makes sense... but that doesn't affect us in an unknown way. Evap purges happen upon car starts for a short time and after we refill the tank. Thats why trims go wonky for a few minutes after a refill. Very easy to see!

This is not something that will unexpectedly sour fuel trims in the middle of a WOT run on a nice summer day and likely the reason why no tuner company on the market felt the need to add it. Humidity and temp changes don't cause the evap to purge and that second set of trims is something we really need not worry about, IMHO.

Thanks Chris.

[JFDugal]

has no one taken a scan and referenced RPM, KPA and FTCs to map these out?

You mean something like this?

[JerryH]

JF, nice work!

I was thinking of something simpler like Russ showed, but this does the same thing and then some (in a steroids kinda way... lol).

I'm seeing a nice regoin of overlap in the FTC 0,1,2 and 4 (more than I expected)... FTC 3 seems to be in it's own little world (as expected).

What kind of mods did the car that these charts came from have?

[JFDugal]

Hmm. I missed your post before.

The mods on the car that was used to obtain the data in my pic above are : S1x cam, GenV blower w. N* throttle body, 3.2" SC pulley, 9" open cone intake, TOG headers, RT downpipe, Borla exhaust.