The fuel system monitor is an on-board strategy designed to monitor the fuel control system. The fuel control system uses fuel trim tables stored in the powertrain control module (PCM) keep alive memory (KAM) to compensate for the variability that occurs in fuel system components due to normal wear and aging. Fuel trim tables are based on engine RPM and engine load. During closed-loop fuel control, the fuel trim strategy learns the corrections needed to correct a biased rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim has 2 means of adapting: long term fuel trim and a short term fuel trim. Long term fuel trim relies on the fuel trim tables and short term fuel trim refers to the desired air/fuel ratio parameter called LAMBSE. LAMBSE is calculated by the PCM from the heated oxygen sensor (HO2S) inputs and helps maintain a 14.7:1 air/fuel ratio during closed-loop operation. Short term fuel trim and long term fuel trim work together. If the HO2S indicates the engine is running rich, the PCM corrects the rich condition by moving the short term fuel trim into the negative range, less fuel to correct for a rich combustion. If after a certain amount of time the short term fuel trim is still compensating for a rich condition, the PCM learns this and moves the long term fuel trim into the negative range to compensate and allow the short term fuel trim to return to a value near 0%. Inputs from the engine coolant temperature (ECT) or cylinder head temperature (CHT), intake air temperature (IAT), mass air flow (MAF) sensors are required to activate the fuel trim system, which in turn activates the fuel system monitor. Once activated, the fuel system monitor looks for the fuel trim tables to reach the adaptive clip (adaptive limit) and LAMBSE to exceed a calibrated limit.

Short Term Fuel Trim
If the oxygen sensors are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7:1 for gasoline), the PCM enters closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy continuously adjusts the desired air/fuel ratio between rich and lean causing the oxygen sensor to switch around the stoichiometric point. If the time between rich and lean switches are the same, then the system is actually operating at stoichiometric. The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometric is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%. Some calibrations have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometric. This practice is referred to as using bias. For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce oxides of nitrogen (NOx).

Long Term Fuel Trim
While the engine is operating in closed loop fuel control, the short term fuel trim corrections are learned by the PCM as long term fuel trim (LONGFT1 and 2) corrections. These corrections are stored in the keep alive memory (KAM) fuel trim tables. Fuel trim tables are based on engine speed and load and by bank for engines with 2 heated oxygen sensor (HO2S) forward of the catalyst. Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:

• Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.
• Long term fuel trim corrections can be used both while in open loop and closed loop modes.

Long term fuel trim is represented as a percentage, similar to the short term fuel trim, however it is not a single parameter. A separate long term fuel trim value is used for each RPM/load point of engine operation. Long term fuel trim corrections may change depending on the operating conditions of the engine (RPM and load), ambient air temperature, and fuel quality (% alcohol, oxygenates). When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different RPM and load points. The LONGFT1/2 PID(s) display the long term fuel trim correction that is currently being used at that RPM/load point.

 

Very nice write up!

 

I don't take credit, I directly copied it from the FORD manual so there was no discrepancies.

This is just for those that want to learn a little more.

 

Oh great!
Isn't that special! I gave you a compliment on copied copyrighted material.
You could have specified that in the first post.

 

Now, now, 88 - since I know you and others don't, they might think you were getting all upset! He's not, i.ride., he's just chagrined that he didn't know it was out of the manual.

But, I agree with his original post. It's good of you to post this information. The whole concept of fuel trims can be pretty confusing, especially to someone who doesn't understand the meaning of a "feedback loop" (not you 88!).

I recognized it from information you gave me before, i.ride. And, I have to say, it takes a few readings to understand.

What I think is interesting in all this is that the feedback mechanism is just a simple "too hot/too cold" input. So, as your post points out, the control on the fuel injectors is pretty much always "wrong", but the average is "right"! It's actually a pretty clever way to do things with simple sensors.

The other nice thing about it is that the system is self-correcting - compensating for age, minor malfunction, different fuel formulations and so on. Overall, a pretty slick, relatively low-cost, design!

- Jack

 

 

How do any of the tuners in use disable short and long term fuel trims from being stored? Or another way to pose the question.... what is the point of a tuner when the ECU is going to write over/trim out the fuel tables back to 14.57 to 1 A/F ratio anyway? All of the tuners I have looked at are 50 state legal, so I assume they don't disable or bypass the O2 sensors and they likely don't keep the ECU in open loop.

 

The placebo effect?

 

The adaptive strategy can be turned off, and usually is when doing dyno or track tuning. In such a tuning scenario, open loop fueling is forced so the O2s have no effect. Both functions would be reversed when the tuning is done.

Think of the adaptive strategy as a good thing. The pcm is always updating the tuning and adjusting for vehicle wear, how you drive, etc. The amount and type of adaptive strategy can be adjusted in the tuning with more than a dozen adjustments.

14.7 to 1 is the most efficient a/f ratio to burn gasoline. Other ratios for different fuels.

 

Please notice the OP's original discussion. The closed loop fuel trims can be "biased", either slightly rich or slightly lean. In fact, Bill Cohron, (Power Hungry Performance) says he biases the cruise settings on the lean side to save fuel. Running slightly lean when the engine is not under load is not harmful.

And the settings can be biased slightly rich under part throttle acceleration or mild load to improve torque.

It's not a "placebo" effect at all.

- Jack

 

Thanks i.ride for posting the fuel trims article. There was plenty of interesting information.

 

If the adaptives can't be turned off during normal use, what it the use of loading a performance tune? It is just going to be trimmed out away right?

Adaptive is good when being EPA compliant, which does always equal the best power. 14.75 to 1 is the most efficient ratio for combustion, but not for making hp, it's much too lean.

I have a 2001 F150 5.4L and have ordered a '10 F150. Today out of curiousity I hooked my ScanGuage up to my '01 and it stayed in closed loop the entire time I drove it, part throttle or full throttle. So what is the benefit of remapping if it is going to trimmed out anyway when closed loop? They can't claim 50 state legal and disable the O2 sensors.

 

They can be biased but won't they be trimmed when getting feedback from the O2 sensors? I wouldn't want to go any leaner than stock, 14.75 to 1 is pretty lean.

I am sure the gains posted by the tuner/programmer manfacturers are legit, but drive the truck for a week and let it go through several warm up and cool down cycles and all of those gains that were programmed in will be trimmed out when in closed loop. I am strickly speaking of gasoline engines not diesel.

I have checked most all of the popular tuner/programmer sites and none state that they prevent the ECU from trimming the programs and they all state they are 50 state legal. Just doesn't seem like the whole storey is there.

 

Sounds like there's something wrong with the tune in your truck. It should be in closed loop when warm and not at high load.

And, you do not have to disable the O2 sensors to bias the trim.

- Jack

 

Sorry I went back and reread my posts and I had it backward. It was closed loop the entire time.

My truck is stock.

If you don't disable the O2 sensors, how do you make permanent changes to fueling without the ECU trimming the fuel map back to stoich? The O2 sensors, are narrowband correct? If they read higher or lower than stoich, they are going to trim the fuel map either way. How do the tuner's overcome this and remain 50 state compliant?