Fuel System Chaulk Talk - Long Term Fuel Trim

[SUMOTOY]

I'm more convinced than ever that approaching this from a TUNER point of view is coming at it from the wrong end.

I've had several offline discussions, working towards getting some understanding of how fuel systems work. I'm more convinced than ever that approaching this from a TUNER point of view is coming at it from the wrong end. I believe that's because EFI fuel systems are not understood well here. To that end, let's try a different perspective. Let's try using the OBDII to understand the fueling needs of the truck just from the FTU, not from any ascribed expert.

A couple of simple things first. On 96> trucks, if you put in forced induction and you didn't pop a CEL for Long Term Fuel Trim (LTFT), the fueling needs of your truck are within the factory spec of -0- +/- 15%. That simple. What do you need to do to ck exactly what the LTFT is?

In the measuring block values of your OBDII reader, find LTFT (bank 1 and bank 2) and datalog their values. Whatever you do to the engine, it will result in a change or adaptation to long term fuel trim. If you turn up the fuel pressure regulator for instance, your long term fuel trim values will start to go negative, up to -15% without a check engine light. What this means is that the baseline fuel is being trimmed back. Once you have exceeded -15%, your light will kick. Conversely, if you turn down the regulator pressure, your trim values will go positive which means that the baseline fuel (fuel injector duration or duty cycle) is being increased, again up to 15% before CEL.

Again, if anyone is running OBDII with forced induction and there is no CEL, fuel is within the factory spec of 0 +/- 15%. Those with the ability to get LTFT values, should post up that data. I urge you folks to get this data before you change hardware or softare of any type.

My intent on these threads was never to flame, only put forth that the accepted theory and practice of EFI is well documented. Regarding OBDII, just about every system runs the same way in terms of FTU INDICATION of the realities of your mods. Once this data is out there, there are ways to use it to solve some of the fueling system issues that may or may not be present.

SUPRA Fuel Pump Concern
In the meantime, I encourage someone that has a Supra fuel pump to measure the return line pressure of the Fuel Pressure Regulator. I've thought a lot about that mod, and it concerns me that folks are seeing benefit from it without the FPR. That's an indication there is a backup of pressure in the fuel pressure return line. Since that line is very small and usually contains a lot of plastic line, it can't take a lot of pressure...

Next up for discussion might be the MAF, and what it's values mean in the A/F mix.

Hope this helps from a different perspective

ST

 

[landtank]

On 96> trucks, if you put in forced induction and you didn't pop a CEL for Long Term Fuel Trim (LTFT), the fueling needs of your truck are within the factory spec of -0- +/- 15%. That simple.

Again, please explain this 15% that gives us a CEL. Where does that come in? Could you give me a senario of how it would be triggered.

 

[cruiserdan]

<snip> as a SC provides boost even at idle.

That is incorrect.

 

[Romer]

That is incorrect.

Rick, What Dan means to say is that the supercharger does not always provide boost and does not provide boost at idle. It will start to provide boost at low RPM as you step on the gas.

Remember Dan is a Hunter and not a Farmer

Scott, that is a well written post, appreciate the different persepective and the tone of the post.

 

[sleeoffroad]

SUPRA Fuel Pump Concern
In the meantime, I encourage someone that has a Supra fuel pump to measure the return line pressure of the Fuel Pressure Regulator. I've thought a lot about that mod, and it concerns me that folks are seeing benefit from it without the FPR. That's an indication there is a backup of pressure in the fuel pressure return line. Since that line is very small and usually contains a lot of plastic line, it can't take a lot of pressure...

ST

Why is it automatic that it indicates a rise in pressure due to insufficient return line pressure. Why can it not be that the old stock pump can't maintain 43 psi under high load requirements?

I would suggest that someone with a truck with SC and stock pump also measure the fuel pressure at the fuel rail under high pressure.

Other than the above, I agree with what ST posted, however I would also like to see a discussion on what you do for more fuel in open loop situations where a wideband O2 sensor indicates lean conditions.

 

[SUMOTOY]

You'll have to explain this is more detail. Because by adding a SC you are increasing HP by 31% (212 - 278). As I understand it that would mean an increase of 31% in fuel as well, which would be across the board as a SC provides boost even at idle.

Again, the calculations on the RCE site indicate that you can put enough fuel into the engine to support a low boost forced inductino system. You can add even more fuel by using a fuel pressure regulator. Run the calculations in Corky Bell's book, or find one on line. With 29lb injectors, the trucks don't appear to be out of fuel using the stock injectors. If this was true, the check engine light should have fires. If it didn't, the fuel compensation in the stock system, can meet the demands of the forced inducted truck.


SC do not provide boost at idle. SC only provide boost when the bypas valve is closed. It operates the same as a turbocharger in vacuum. Therefore both a supercharger and a turbocharger will have the exact same fuel needs as a stock truck during vacuum operation.


ST

 

[SUMOTOY]

Why is it automatic that it indicates a rise in pressure due to insufficient return line pressure. Why can it not be that the old stock pump can't maintain 43 psi under high load requirements?

Anything is possible. Before replacing a fuel pump, one would need to ck resistance across it, and pressure in the line. And you have it backwards, the rise in pressure is most likely the result of too HIGH a fuel return line pressure. That line was never designed for high pressure, hence my concern that without measure, just dropping in a supra turbo fuel pump is a dangerous assumption.

I would suggest that someone with a truck with SC and stock pump also measure the fuel pressure at the fuel rail under high pressure.

What pressure are you targetting this test be made at?

Other than the above, I agree with what ST posted, however I would also like to see a discussion on what you do for more fuel in open loop situations where a wideband O2 sensor indicates lean conditions.

Baby steps Christo. I think we start closed loop in data and discussion, and it can eventually seguay over to open loop operation.

ST

 

[landtank]

On 96> trucks, if you put in forced induction and you didn't pop a CEL for Long Term Fuel Trim (LTFT), the fueling needs of your truck are within the factory spec of -0- +/- 15%. That simple.

Again, please explain this 15% that gives us a CEL. Where does that come in? Could you give me a senario of how it would be triggered.

 

[landtank]

That is incorrect.

I guess I stand corrected. I assumed that as long as the SC was turning it was boosting manifold pressure. So while at idle it would still be negative pressure but not as negative as without the SC.

 

[Whipp]

Again, please explain this 15% that gives us a CEL. Where does that come in? Could you give me a senario of how it would be triggered.

I think that this 15% Long Term Trim comes into play when the ECU has noticed that it takes 15% more (or less) fuel for a given amount of air, as detected by the MAF sensor, to reach the stoichimetric ratio than what the ECU expects based on the internal tables.

Even though an engine with an SC may use 30% or more fuel (and air), the ratio is the same, so no MIL/CELs are illuminated.



Swapping in a differently calibrated MAF *or* injectors would throw off the expected ratio, triggering an error in the ECU. If the MAF *and* injectors were swapped with even calibration (ie. both had an increased capacity of 20%), the ratio, as read by the ECU should be fine and shouldn't trigger an error. The effect would be that all the fuel injection values would be pushed "towards the origin" of the table.

One of the problems with this is that while the engine is running with moderate to high boost, the ECU is still maintaining the 14.7:1 air:fuel ratio, which can cause knocking/detonation.

 

[sleeoffroad]

Originally Posted by sleeoffroad View Post
Why is it automatic that it indicates a rise in pressure due to insufficient return line pressure.

Sorry should have been "insufficient return line size"

I can see the pressure rising if all the fluid does not have a big enough path to return, however if I recall correctly from a couple of years ago when we did a SC with FPR and Supra Pump, we did not have a problem. We measured pressures at that time and did not notice pressures above the spec at idle.

Ken could verify for us if he can hook a fuel pressure regulator up to the truck.

I would suggest that someone with a truck with SC and stock pump also measure the fuel pressure at the fuel rail under high pressure.

What pressure are you targetting this test be made at?

Damn, must be early morning typing. "Under high load", ie WOT and full boost.

Sorry for the typo's.

 

[sleeoffroad]

Again, please explain this 15% that gives us a CEL. Where does that come in? Could you give me a senario of how it would be triggered.

Rick, the long term fuel trim will trip the CEL if out of range. I can not recall if it is exactly 15%. The value is documented in the manual.

 

[jerseyfzj80]

http://www.motor.com/MAGAZINE/Articles/092006_11.html

for the more technically oriented check this link out for a meduim explanation of ltft and stft and what things may have some influence on it.

Scott (i'm in now)

C

 

[landtank]

I think that this 15% Long Term Trim comes into play when the ECU has noticed that it takes 15% more (or less) fuel for a given amount of air, as detected by the MAF sensor, to reach the stoichimetric ratio than what the ECU expects based on the internal tables.

Even though an engine with an SC may use 30% or more fuel (and air), the ratio is the same, so no MIL/CELs are illuminated.



Swapping in a differently calibrated MAF *or* injectors would throw off the expected ratio, triggering an error in the ECU. If the MAF *and* injectors were swapped with even calibration (ie. both had an increased capacity of 20%), the ratio, as read by the ECU should be fine and shouldn't trigger an error. The effect would be that all the fuel injection values would be pushed "towards the origin" of the table.

One of the problems with this is that while the engine is running with moderate to high boost, the ECU is still maintaining the 14.7:1 air:fuel ratio, which can cause knocking/detonation.

You spoiled my fun. Yes that is exactly how I understand it and how CDan can correct his WOT issue.

Since the margine is 15% he could install the NON Turbo Supra Injectors that are 6% larger. While in closed loop the ECU would drop the DC to compensate, in open loop at WOT the slightly larger injectors would make up in gas that amount he is adding in with Water/Methanol. And this still would be within exceptable parameters of the programming.

I actually have a set of these coming in this week that are already cleaned. The EBay seller listed them at 380cc but I've confirmed that they are 325cc from a chart that calls out the connector color code for flow size.

 

[Whipp]

You spoiled my fun. Yes that is exactly how I understand it and how CDan can correct his WOT issue.

Since the margine is 15% he could install the NON Turbo Supra Injectors that are 6% larger. While in closed loop the ECU would drop the DC to compensate, in open loop at WOT the slightly larger injectors would make up in gas that amount he is adding in with Water/Methanol. And this still would be within exceptable parameters of the programming.

I actually have a set of these coming in this week that are already cleaned. The EBay seller listed them at 380cc but I've confirmed that they are 325cc from a chart that calls out the connector color code for flow size.

My concern with this is that the ECU still pays attention to the long term fuel Trim while in open loop, so the DC would still be backed off while in Open Loop. It just doesn't pay attention to the O2 sensor any more because ECUs usually richen the mixture a little at WOT and the narrowband O2 sensor is basically useless then.

There are wideband O2 sensor kits that have an output that simulates a narrowband sensor. These kits can change the fuel/air ratio at which they report the correct ratio, so that with increasing boost, they report a richer mixture as the "correct" mixture. But I think that too much of an adjustment may cause the 15% LTFT to trigger a MIL/CEL.

 

[cruiserdan]

You spoiled my fun. Yes that is exactly how I understand it and how CDan can correct his WOT issue.

Since the margine is 15% he could install the NON Turbo Supra Injectors that are 6% larger. While in closed loop the ECU would drop the DC to compensate, in open loop at WOT the slightly larger injectors would make up in gas that amount he is adding in with Water/Methanol. And this still would be within exceptable parameters of the programming.

I actually have a set of these coming in this week that are already cleaned. The EBay seller listed them at 380cc but I've confirmed that they are 325cc from a chart that calls out the connector color code for flow size.

So,

You would like me to try them for you?

 

[cruiserdan]

My concern with this is that the ECU still pays attention to the long term fuel Trim while in open loop, so the DC would still be backed off while in Open Loop. It just doesn't pay attention to the O2 sensor any more because ECUs usually richen the mixture a little at WOT and the narrowband O2 sensor is basically useless then.

There are wideband O2 sensor kits that have an output that simulates a narrowband sensor. These kits can change the fuel/air ratio at which they report the correct ratio, so that with increasing boost, they report a richer mixture as the "correct" mixture. But I think that too much of an adjustment may cause the 15% LTFT to trigger a MIL/CEL.

The 93-94 ECU is kinda stupid compared to the 95-97 piece. Maybe it's not smart enough to care?

 

[jerseyfzj80]

Soory for the long tech article in this link but I think it explains the situ. for most folks clearly enough, In relating this to S/C equipped cars. Here is my take I worked for a company that designed and built Eaton M60 superchargers for 2.8liter audi v6 30 valve motors. fueling needs were handled in three ways, new larger RC injectors (similar in size to a stock fraud (sorry i meant ford) mustang GT 4.6 mod motor) The ECU program was rewritten to increase injector pulse width/duty cycle, and a larger FPR. There was some debate from our customers if this was actually usefull or not. I would generally tend to want to make the mistake of to much fuel rather than to little! this yielded in top dog form approx 330hp on a 2000 audi A4 2.8 Q (if you are an Audi guy you will never see this nunber published cause "our car" has some extra fun things goin on with it!!!! things that were not available to the public. So in combination with those things we had a car that was very fast and ran like a top with the exception of a one time scenario of massive carbon build up that kept the valves from closing and the car from actually starting. That occured after approx 60,000 miles of beating the everliving life out of this car (magazine editors test drives 4 wheel burnouts/donuts etc you name and we abused it that manner trust me.
I think that basically the toyota world is trying to compensate the S/C system by throwing more fuel in the same duty cycle, and possibly even with the same injectors? i could be wrong But that is my undestanding and please allow me some lee-way as I have never even entertained the idea of an S/C on my cruiser ( iam to cheap to pay for the premium fuel!) So I don't even know what the kits come with. Also the "seveth injector" deal that alot of guys use on the 3.4l v6 in the tacoma is a real croack of s*** as well essentially they are taking a "Dry" manifold that is designed to just flow air and make it flow fluid as well last time I checked they both had there own seperate and different flow characteristics. Basically if you adhere to the theory in the previously posted article that will increase the LTFT far beyond a normal level as a result of the fact that there will be 1-2 wet cylinder with 4-5 not so wet ones if you understand. I watched a company try to convinve the porsche world that just tapping the intake manifold and slapping in an extra injector or two was the proper way to fuel your hopped up $120,000 996 twin turbo was the way to go, needless to say they are not really doing to much business @ this point. what an interesting theory on how to trash a $40,000 motor eh? (the porsche, not a cruiser motor)

Just my .02

C

 

[Whipp]

The 93-94 ECU is kinda stupid compared to the 95-97 piece. Maybe it's not smart enough to care?

That could be. I'm basing what I said off of the toyota information referenced in the previous Supercharger Fuel Map thread. That datasheet was a bit generic and didn't specifically mention the 1FZ-FE engine. But I still think that information is correct. In other ECUs I've read about, including the MegaSquirt DIY ECU, LTFT is used to account for gradual changes in the EFI system, such as sensors that creep out of calibration and fuel injectors that have collected varnish and don't flow as much as they used to. This trim would still be appropriate when operating in open loop.

 

[jerseyfzj80]

whipp
Right on the money the ltft is designed to do exactly that to allow a little wiggle room for the consumer and the manufacturer to not have to replace parts early on in the equation and to let them start to degrade to a certain level (15%) and then let you know that hey I'm gonna turn this light on overhere and you need to call C-Dan and get me some new chit so I run better