# Total Fuel Trim Calculations - What's Your Formula?

Scott Manager California Posted   Latest   Edited

As per Randy‘s request, I'm starting a thread on Total Fuel Trim and how it is calculated. I'd like to get multiple perspectives on this by hearing your thoughts.

It seems fair that if you were going to calculate total fuel trim you would simply add STFT and LTFT together, right? I posted the other day about a vehicle I was road testing as a follow up on a repair we performed. The graphed data presented (TFT), didn't line up with the 3D table (TP% x RPM) (Note: Table data is averaged). According to TIS, LTFT + STFT = Smoothed Fuel Trim.

So where are you on Total Fuel Trim?

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Chris Diagnostician
New York
Chris

I'm kinda with you on this. This is how I've based much of my diagnostic knowledge on the fact (myth?). I would love to learn how to interpret it differently.

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Geoff Diagnostician
Hawaii
Geoff

My immediate thought is "semantics". If every single instructor calls it "total" and now an article you found in TIS calls it "smoothed" I'm not worrying about it.

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Hans Diagnostician
Utah
Hans

I've see it where it wants you to add the two together. But I did notice Randy said additive and multiplicative, which happens to be my favorite type of fuel trim (VW/Audi uses it). It gives me three numbers to work with. Current o2 sensor reading (standard STFT) and a LTFT at idle and part throttle. It gives me a lot better insight to the fueling instead of just one averaged LTFT number.

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Albin Diagnostician
Washington
Albin

Total, means just that, the sum of all the fuel trims, which is long term and short term, but how about rear oxygen sensor trims? Not all manufacturers list this, but over the years, I have seen some rather heathy trim come from the rear sensors.

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Brin Diagnostician
Florida
Brin

Albin, the way that you describe it is the way that I've come to understand it. STFT+LTFT+Rear FT. What terms and how many terms you'll see depends on what vehicle you're working on, what model year, whether you are using a aftermarket or O.E. scan tool, or whether you are monitoring global or enhanced data. Regardless of what data there is, modern vehicle fuel control strategies use all

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Albin Diagnostician
Washington
Albin

Bryan, You make some great points. When working on a fuel trim issue, many times the generic side of OBD II will get the job done, and some times that is the place to go, although as you pointed out, this changes from year to year, model to model. This is one reason why "a quick look at fuel trims" can head a person down a rabbit hole. From time to time, it is a MUST that we study the data on

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Tim Curriculum Developer
California
Tim

I've been formulating a similar question that is related. Does Rear Fuel Trim get added to Total Fuel Trim? If we count all three it might add up! I assume that each manufacturer actually does this, but that they vary in whether or not the values are added (or multiplied) in. I think this is what Albin was referring to, and it's something I'm eager to 'catch' the next time I see a downstream

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Brin Diagnostician
Florida
Brin

Tim, I would have to assume that all manufacturers use fuel control strategies that use all feedback sensors for fuel control, adaption and correction. How they react to different kinds of faults depends on each manufacture's strategy for that vehicle. One single manufacturer can change the strategy from one year to the next in a way that the affect that that same fault, would cause a different

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Randy Analyst
Randy

So it says "Fuel trim is related to the feedback compensation value, not to the basic basic injection time." What does that mean? If you track Toyota enhanced PID for TFT (total fuel trim) it doesn't track with ST + LT especially when a fault occurs. Even if you add rear fuel trim, it still doesn't track so how do you explain the way Toyota handles TFT? Toyota did a presentation on their FFV

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Scott Manager
California
Scott

Q1: “What does that mean?” A1: It Depends® 😀 From my experience, injector on-time does not transfer linearly with the mass of fuel delivered. Once the BPW is established, there are other modifiers such as time (small PW), voltage, temperature, etc) applied (and more due to rear FT) that affect the final value. In the service environment, we as the service technician look at fuel trim as an

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Scott Manager
California
Scott

Randy, Here are a few screen caps from a 17 Tacoma with the 3.5L Direct Injection V6. Idle FT Data Cruise FT Data Cruise TFT Data Graph w/ Front Lambda Sensors Cruise STFT w/ Front Lambda Sensors I honestly don't believe that I learned anything from looking at this data and I haven't had time to dig into SI for TFT explanations but if you'd like to drop a few clues that would be nice…

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Randy Analyst
Randy

A coworker gave me a great line today and I plan on using it so consider it copyright protected.😀 “When the football player crossed the goal line, did he score a safety or a touchdown?” That‘s an 8 point swing depending on the direction you are going. You switched directions on me. This Toyota is very different than the Toyota that started this discussion. You will notice the TFT pid has

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Scott Manager
California
Scott

But I did use “multiple perspectives” in my second sentence... I‘ll see what I can do to bring in more input this subject.

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Albin Diagnostician
Washington
Albin

I have been following this discussion closely and to say the least, I am close to overwhelmed with the strategies of these late model vehicles. After looking at scan data from several late model vehicles, and looking closely at the captures you just put up, I can see no way to reverse engineer these systems. I think Randy said that way back early in this discussion.

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Jim Curriculum Developer
Maryland
Jim

Scott, Do you have access to this vehicle easily? If so, I would like to see the the A/F learning values both standard and dual and did you graph the TFT with the LTFT? The only thing I will say here is that the approach to this vehicle would be different based on design and available PIDs and Active Tests.

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Scott Manager
California
Scott

Hi Jim, I plan on taking a deeper dive on this vehicle as soon as I’m able. I’ll update you accordingly. Thanks!

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Scott Manager
California
Scott

I just finished reading SAE Paper … (thanks Randy) which is titled “Development of PZEV Exhaust Emission Control System” I’ve re-read several sections multiple times and I learned quite a bit. One big takeaway for me (related to this discussion) was the sub-O2 sensor construction mods to deal with the abundance of Hydrogen exiting the close-coupled catalyst (not engine out) when

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Randy Analyst
Randy

That is a great one, especially about the sub O2 hydrogen control. I always think of Jon Riggle when I read that part.:) When you want to go deeper, look for sliding mode controler and more specifically, PRediction and Identification type Sliding Mode Control (PRISM). Make sure you understand POQ as it relates to OSC and then you are off into the land of understanding modern fuel control. If

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Scott Manager
California
Scott

Yeah I bet Riggle gets a kick out of that. When he brought up that topic years ago I had no idea when he was asking about! As for your other suggestion, I’m on it and thanks!

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Paul Engineer
Michigan
Paul

Hi Scott, Current fuel control system are pretty complicated. As an example, Ford uses feedback from the upstream UEGO sensor (A/F Sensor) and a PI controller to determine the feedback trim needed to achieve the desired A/F ratio in the engine exhaust. This is called the inner control loop. A "PI" controller has a proportional term (short term trim) and an integral term (long term trim). The

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Randy Analyst
Randy

Paul, Can you answer Rob's post, why did you show rear O2 fuel trim that way on those systems? That's not what is really happening. Do you agree with me, we can't be using fuel trim the way we did 20 years ago? As I have already said, we see these systems fail IM240 with no DTCs and they are extremely difficult to diagnose. The last one saw a 20% increase in FE after repair. I have another

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Paul Engineer
Michigan
Paul

Hi Randy, Are you asking about the switcher and follower graphs below? The switcher is the one I described in my post. The outer loop is modulating the A/F through the catalyst. I'm not sure what model year the follower is but as I said, the rear fuel trim control strategy has changed over the years. In the 2000s, the idea was to try to control the A/F into the catalyst using the inner loop

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Scott Manager
California
Scott

Hi Paul, I believe he's referring to Robby's post within this thread where he's listed the YMME.

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Randy Analyst
Randy

We will probably have to discuss this over a beverage someday but we are on the same page. I am not disagreeing with anything you said, I was trying to help the readers with my rear fuel trim question. I have said there are multiple iterations of this control and, as you said, it is impossible to reverse engineer with all the underlying inputs to modern fuel control. Between Rob and my uploads…

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Paul Engineer
Michigan
Paul

Hi Randy, Yes, I agree. Looking at STFT and LTFT is useful but adding in rear fuel trim is not. Rear fuel trim is not directly feeding back to the injector pulsewidth.

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Robby Mobile Technician
Alabama
Robby

Hi Paul! "Note that when you use rear fuel trim, the gains you need for optimum control are affected by catalyst oxygen storage. Aftermarket cats affect those gains." Is this an example of what you're talking about? That is from a 2014 Ford F550 6.8L V10 that had a weird surge under load and setting a P0420. Come to find out, the Catalytic Converter had stopped up and the customer gutted the

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Paul Engineer
Michigan
Paul

Hi Robby, Your V10 data looks like what I am commenting about. There is no oxygen storage in the outer loop because the catalyst is gone so the control system gains are too high and it's bouncing off the stops. If it's a FFV, the software tries to infer the percentage of ethanol after a KAM clear of a refuel event. If it's not an FFV, the software assumes a 10% ethanol content. Normal adaptive

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Scott Manager
California
Scott

Thanks Paul, I appreciate your chiming in here. Yes I can see from what you wrote and the number of SAE papers Randy has referenced that these systems have come a long way and no longer work exactly how they did years ago. This definitely shed some light on the situation. Thank you.

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Scott Manager
California
Scott

Thank you for the extremely enlightening descriptions here Paul, I appreciate your insight!

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Robby Mobile Technician
Alabama
Robby

Hiya Scott, I'm a little late to the party, but would like to share a few thoughts. As far as adding LTFT and STFT, generally speaking, I feel it's pretty accurate to do that. I do it all the time when diagnosing fuel system issues. Plus, there are several documents from the OE (like you shared) that state to do that. However, there are times when that does not seem accurate. BTW, when I say

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Randy Analyst
Randy

Robby! Where to start................. You say you use it, then go on to prove it wrong so that begs the question: Do you know when to use it and when not? What is the true total fuel trim for your Expedition? I assume both before and after had exhaust gas readings of lambda 1.000? The examples you have given use different fuel control strategies and each must be discussed in isolation. Let's

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Robby Mobile Technician
Alabama
Robby

Hi Randy! Yes - where to start. This stuff is so hard to get across in captures and text. The example from my Expedition is just a classic/textbook case of a vacuum leak. High trims at idle, lower trims off idle - high LTFT and high STFT. I'll will use total trims knowing it's not totally accurate, but it gives the info I need to get a direction. From what I've seen, the inaccuracies really

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Randy Analyst
Randy

You can answer my question, look again. Is EQ reacting to STFT or is STFT reacting to EQ? The key to understanding this is 1/2 POQ. BTW, I work with hundreds of technicians and trainers and you have been the only other person (non-engineer) to ever send me files displaying this system even though it is fairly common now. I will look closer at your last capture later, I have to go now and I see

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Robby Mobile Technician
Alabama
Robby

EQ is reacting to STFT. You're capture doesn't show EQ desired, but I feel that STFT is reacting to the desired EQ. Take a look at this capture from a 2013 Ford Fusion 2.5L: RO2FT seems to be driving EQRAT_DSD. This shift in desired EQ is driving STFT which, in turn, causes the shifts the in actual EQ. Here's the same capture where I've copied the STFT trace and overlay-ed it with the EQRAT_DSD.

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Randy Analyst
Randy

Your rear o2 fuel trim pid is a happy coincidence, not reality. It’s messing with your brain. Move over to generic and ignore rear o2 ft. It appears to me your pid refresh rate is too slow as well. Having said that, you are so close, so close. I’m not being a jerk, it’s just that explaining the rest won’t be clear unless I upload more images and explain each one and I am exhausted right now and

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Robby Mobile Technician
Alabama
Robby

"Your rear o2 fuel trim pid is a happy coincidence, not reality. It’s messing with your brain. Move over to generic and ignore rear o2 ft. It appears to me your pid refresh rate is too slow as well." I'm still trying to wrap my head all that. I do have some data from Mode 1, but it's much slower than the OE IDS data. "Having said that, you are so close, so close. I’m not being a jerk, it’s

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Randy Analyst
Randy

Here are 2 different vehicles, both broken, both are 2007 Mazda CX7 with 2.3L. No DTCs, failed IM240 for CO. The second one had an increase of 20% on measured fuel economy after repair. I don't have that data for the first one. Using taught fuel trim procedures, diagnose these 2 vehicles. The fix was different for each one. The ones titles 35 mph and 58 mph are from the first vehicle. The one

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Jim Curriculum Developer
Maryland
Jim

Since I do not see VSS or APP or TPS I have questions. The captures look the same for the drive trace portion, only the Gas bench values change a little. 1. Where were the speed ranges in this capture? 2. What caused to bump to OL 3/4 through the capture? Decel? and finally, if this is steady state as you indicate by saying 35 mph and 50 mph why is load so variable? Or is this actually a trace

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Randy Analyst
Randy

It's an IM240 trace. Make sure you open in new tab so you see the whole image. Yes on decel fuel cut OL.

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Randy Analyst
Randy

I had to check my notes and papers but the sawtooth pattern is used during warm up of the catalyst so if Ford is following this principle then this capture was taken when the cat temp model prediction was low. I can't say if Ford does this though. There are 7 variations of the pattern depending on multiple factors. Pretty simple, Huh?

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Steve Instructor
California
Steve

Hi Randy, My understanding is EQ reacts to the final O2 based off the combustion event if this is a torque based fuel strategy. As explained to me by an Engineer, if the post Cat O2 voltage is where they want it to be, then fuel correction is near 0. This is manufacturer specific and could vary by manufacturer of engine management. This explains why there is a difference between the fuel

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Paul Engineer
Michigan
Paul

Hi Steve, In current control systems, there is a target rear O2 voltage that can change at different speeds and loads to produce the desired emissions. There is also a modulation around that point to get better catalyst efficiency. There is proportional feedback to maintain the target rear O2 voltage set point and there is an integral term that is used to adjust the upstream UEGO stoich point

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Jim Curriculum Developer
Maryland
Jim

STFT surely looks to be ahead of the game to me. If I understand the point you are trying to make...... the common teaching of fuel control is that STFT is mainly a response to a S1 sensor. Here your captures show an episode of "trading places" as an indicator that times have changed. This is no longer dad's Olds

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