Long Term Fuel Trim

Hey, Riv'ers. I've been tracking some LTFT numbers and seeing that I almost always in the +'s. Just a guestimate but I think im averaging ~+8 on the LTFT. Any pointers for me? Sorry if this graph/data is illegible.

charlieRobinson wrote:

Hey, Riv'ers. I've been tracking some LTFT numbers and seeing that I almost always in the +'s. Just a guestimate but I think im averaging ~+8 on the LTFT. Any pointers for me?

That's pretty typical from what I've seen - today's fuel has less 'gas' in it so the system is always enriching to meet the new A/F ratio. One way to make it go away is to change the PCM's overall target A/F ratio from 14.7 to 14.2.

In other words, it's pretty normal and don't worry about it too much unless you actually are tuning.

The ideal is ~0 though, right? And definitely 0 at WOT? Would I see a difference if for example, my riv was tuned to be around 0 LTFT? Or is my +8 just fine?

Generally single digit LTFT is ok. A +8 would tell us you may be running a bit lean. Check for vacuum leaks or an air inlet leak. Could also just be a lazy O2 sensor. Can you monitor bank 1 ltft and bank 2 ltft separately? I don't know if S2 Rivs have 1 or 2 O2 sensors before the cat.

Derek, this is a little out of my area, but wouldn't a 14.2:1 afr make his LTFT go higher +? This is all pretty new to me so I sometimes get the + and - reversed.

First, confirm your scan tool is reading +/- correctly. "+" means lean, or fuel is being added. "-" means rich, or fuel is being trimmed. It's been reported some scanners get spark advance reversed, so it's possible here.

+8 isn't that bad in itself, but it is a significant step in the wrong direction. +16 is the limit, then the PCM can no longer trim or add fuel. That's when things can get ugly. You're aboute half way there.

If you're not locking in at 0 LTFT at WOT, imo that means you're getting too lean. deekster might be right about the fuel changing over the years, but I have to think your recent modding trend has a lot more to do with it.

I like to think of it this way: if you're close to 0 most the time, that's a lot less fuel adjustment demanded from the PCM, so you'll tend to have the optimum A/F ratio, rather than wait for the PCM to counter it (which likely happens very quickly). In cases where the PCM needs to adjust quickly to counter KR, I feel being closer to zero helps resolve the situation in less time.

What kind of runs do I need to scan to get a better picture for you all? WOT 0-90? Something like that?

Also, is it safe to say that my fuel economy will be lower with a LTFT of +8? Once I am sure I am running lean, what ways can I remedy this without tuning.. don't quite have the capabilities for that yet.

Also, is running lean a source for KR?

During WOT your engine works in open-loop mode, so the PCM doesn't use fuel trim. Instead it uses the fuel maps and MAFF values, so WOT scanning won't tell you much about your fuel trims.

Whether or not your MPG suffers depends on what's causing the lean condition. If your O2 sensor is reporting incorrectly that you're lean, even if you really aren't, the PCM doesn't know any better. It will adjust for the lean condition, burning extra fuel. Yes, MPG will suffer in this case.

Actually running lean breeds KR, but realize your lean condition is being corrected by fuel trim, so you are no longer running lean with the extra fuel added. You're fine until you reach the fuel trim limits.

It's important to understand LTFT is more like a fuel trim history or running average than a real indicator of what the PCM is doing real time. STFT is a better way to see fuel trim in action.

LTFT is a measure of long term corrections in fueling. 14.7 is the 'old gas' ideal air fuel ratio. To see the proper crosscounts on the O2 sensor, the computer needs to make corrections to fueling (STFT). Over time it develops a trend (LTFT) and bases current air/fuel calculations starting with the expected LTFT correction. STFT is the immediate correction.

If you are retuning your entire fueling tables you would start by using the correct air fuel ratio as your target (14.2). A 14.2 AF is slightly richer than a 14.7 AF, because the E10 fuel (usually 7-8% Ethanol) requires a little extra fuel to meet ideal combustion. The O2 sensor directly impacts STFT. STFT builds the LTFT table. Untuned cars tend to see about 8% across the board on LTFT.

Forget WOT - it's a completely different thing - WOT is using only the MAF and timing readings, as well as the last usable LTFT reading before you went WOT. A standard O2 sensor cannot measure at WOT, so the PCM ignores it. This is true for anything above I think it's 50% throttle or so. When the PCM reaches that point it will take your last LTFT and use that as the base, but has no input on what's coming out of the exhaust. This is the point where tuners will use a wideband O2 sensor that can still read the exhaust gasses.

When you go to a dyno shop and they insert a tailpipe probe, it's also not accurate, because the unburned fuel has been burned by the cat. A useful O2 reading needs to be taken pre-cat.

Derek, my question was: If Charlie is seeing a LTFT of +8 Wouldn't that mean he is compensating for an AFR leaner than 14.7:1? Wouldn't tuning for 14.2:1 magnify the lean condition that the PCM is compensating for creating a larger + LTFT? I'm not questioning your expertice. I'm just trying to understand this.

I've been reading up on this stuff to understand it so we can do our own tunig in the future. If you know of any sources I can get access to it would be greatly appreciated.

Thanks,
Dave

Dave,

If you are running lean when you are shooting for a calculated 14.7, you will see + LTFTs, meaning LTFT is enriching the mixture. A positive fuel trim means it is adding fuel. Because of the new fuel with less 'gas', you need more of it, and your target AF ratio should be more like 14.2. If you were to reprogram the PCM with a target AF of 14.2, your LTFTs would drop significantly.

I'd be happy to send you a copy of my PCM program, which runs with less than 2% LTFT and STFT most of the time. I changed my target AF to 14.2 as well as reprogrammed the entire MAF and VE tables.

Okay, I think I got it. Tell me if I'm wrong. The O2 sensor is reading lean because of todays fuels. Therefore the 14.2:1 would adjust for the fuel,or lack tere of, more or less fooling the O2 sensor. Do I got it? Be honest you wont hurt my feelings.

One way to think of it, the A/F ratio is the target for fuel trim to shoot for, based on information from the O2. A target of 14.7:1 is slightly leaner than 14.2:1, so in order to hit the new target, the O2's feedback translates into a slightly richer mix from the PCM. The O2 sensor isn't really being tricked. It's reporting the same information, but the PCM is now shooting (calculating) for a different target.

AA is right. I'm trying to think of a different way to put it -

The O2 sensor reports 'crosscounts' or how frequently the exhaust pulses blow burned gasses by it, and a measure of voltage for how much energy is in those pulses. the results will tell the PCM if the burn is ideal or too rich or too lean. The O2 sensor doesn't know what kind of fuel, only if the exhaust shows too rich or too lean, or just right. If you put in 'pure' gas, it will report a clean burn for a 14.7:1 mixture. If you put in E10, it will report a clean burn if the mixture is 14.2:1. If you put in E15, it will report a clean burn for 13.8 (I think? Could be wrong on that number) :1, etc. The PCM also has no idea what kind of fuel you use, only that the O2 sensor is reporting too much, too little, or just right, and compensating for it. The great thing about this system is that it's always automatically compensating for differences in fuel, atmospheric pressure, etc, without even needing to know any of those parameters! It just works.

One of the concerns about the push for E15 fuel is that at some point older systems will not be able to make enough of a correction without reprogramming, and start throwing lean codes because LTFTs will be too high. I think these GM OBDII systems can handle it but some OBDI systems will start choking, or at least that's the concern.

If you re-tune your car to run on E85, it's all a matter of tuning for the new fuel. The O2 sensor still works, and still reports a clean, lean or rich burn, but you need to give the PCM a new target (amongst other changes) or it will get all whacked. That's why 'flex-fuel' cars rely so heavily on the sensors in the gas tank to determine what level of ethanol is in the tank - anything from 'pure gas' to a blend anywhere in between to 'pure' E85 - the computer needs to know what it's burning and adjust accordingly.

My LTFT is always about -10% which is removing fuel, but I'm not 100% sure why, the car runs fine no idiling issues or when accelerating, I do have one code which is Catalytic Coverter Low Efficiency, (this is because the valve seals are worn and burning oil, in a long term destroying the catalytic converter).

Anyway, I hooked a Fuel Pressure Tester and the reading with KOEO was fine but at idle it fluctuates a lot! I would say around +/- 8psi which I believe the fuel pressure regulator is not working properly, I did apply about 15 in hg and it hold it pretty well.

I would like to know if you have any suggestions as to why it would have (-) LTFT.

Thanks,

rk0ehn wrote:

My LTFT is always about -10% which is removing fuel, but I'm not 100% sure why, the car runs fine no idiling issues or when accelerating, I do have one code which is Catalytic Coverter Low Efficiency, (this is because the valve seals are worn and burning oil, in a long term destroying the catalytic converter).

Anyway, I hooked a Fuel Pressure Tester and the reading with KOEO was fine but at idle it fluctuates a lot! I would say around +/- 8psi which I believe the fuel pressure regulator is not working properly, I did apply about 15 in hg and it hold it pretty well. I would like to know if you have any suggestions as to why it would have (-) LTFT.

Thanks,

Hold pretty well, or hold? There is a difference when talking about vacuum. The FPR shouldn't leak vacuum at all. If it's bleeding off vacuum, it's bad and probably dumping fuel into the engine, hence your -10% LTFT.

I'm sure blowing oil out the exhaust doesn't help either....

I wonder if the O2 is still reading accurately, being coated with burning oil for so long. I don't know for sure if that would affect it, but it's possible.

Abaddon wrote:

Hold pretty well, or hold? There is a difference when talking about vacuum. The FPR shouldn't leak vacuum at all. If it's bleeding off vacuum, it's bad and probably dumping fuel into the engine, hence your -10% LTFT.

well I was doing a quick test with a friend of mine to show him how it works and I used a "not to tight" adapter with the vacuum pump and the FPR inlet vacuum hose, so it was not an accurate test but is my primary suspect, the second one would be leaky fuel injectors which could be inspected by taking the spark plugs out.

AA wrote:

I wonder if the O2 is still reading accurately, being coated with burning oil for so long. I don't know for sure if that would affect it, but it's possible.

It is possible, but O2 signal looks correct reach/lean constantly and STFT reacting to it and steady in 0% +/- 3% and LTFT -10%

Will make correct testing on this FPR with vacuum pump and fuel pressure tester readings.