I recall reading somewhere what value STFT had to exceed to modify LTFTs. Sometimes I notice in a given Trim Cell that LTFT is neg and STFT is positive. Its quite odd and im trying to grasp how exactly STFT modifies LTFT. I know STFT is reset every ignition cycle so please help me out here

Thanks
Dave

 

As you have pointed out already, STFT are updated rapidly in response to feedback from the O2 sensors. Once the STFTs reach a certain preset upper or lower bound they are reset back to zero and the LTFT is increased or decreased accordingly. Once the LTFT reaches a preset upper or lower bound then a rich or lean DTC is set.

Disclaimer: I am not an engine tuner. This explanation is from observation only and is just my interpretation of those obdervations. I could be wrong - I have been before It is not a definitive guide to LTFT and STFT operation.

Each LTFT cell (from 0 to 15) covers multiple engine operating conditions (i.e. multiple MAF/RPM table lookups). The LTFT correction is an average correction for that entire range of conditions and is based on the average STFT for all operating conditons within that cell. In pre-OBDII vehicles the LTFT was know as the BLM (BLock Learn Mode) because it is the learned fuel trim for a "block of operating conditions". The "block" refers to a rectangular set of lookup values (i.e. multiple MAF/RPM lookups) in the PCM's fuel tables.

The STFT is a more precise, almost instantaneous correction for the engine's current MAF/RPM and is based directly on O2 feedback. In pre-OBDII vehicles the STFT was know as the Integrator because the PCM integrates the O2 feedback into the current (i.e. short term) fuel trim calculations.

That means you could have a large operating area that, overall needs a negative LTFT but one or more smaller areas inside it that (due to current operating conditions) need a less negative adjustment.
So you will sometimes see the STFT as positive while the overall LTFT is negative (or vice-versa).

Bear in mind that depending on the frequency and magnitude that the STFT is positive when the engine is operating in a negative LTFT cell that the negative LTFT cell will eventually average back (i.e. learn) to zero or even continue through to positive.

It is interesting to reset your LTFTs to zero and then idle the engine so that only one idle cell is in use. Then, assuming your LTFT cell at idle is not 0, watch the STFT and LTFT as they "learn" back to their pre-cleared states. In most cases it only takes about 10 minutes to learn the current idle fuel trim cell value.
You can actually watch the STFT values step up/down every few seconds and when it reaches a limit it resets and the LTFT is adjusted (learned).

Regards
Paul

 

I always thought of short term fuel trim as the **** , acts on every whim & going with the flow.
Then long term fuel trim as the responsible carfully made decisions under tight well thought revue.

 

Yes and no
Actually the closed loop (STFT or Integrator) fuel is being driven by another value called the Proportional Fuel.

Rather than reposting some of the information here, I will just post the link to the site I am a member of. Just scroll down a bit and you will see the information. I'm off fishing this week, but I will do a complete writeup on how to tune closed loop fuel. Heavily modified engines need some tweaking in this area.

There are a few guys on thr 454SS board who are trying to see what the limits of the TBI system are. They are using custom PCM algo I wrote so they can use a variable fuel pressure regulator on a TBI system with a 900 CFM throttle body custom made by Jim D.



http://www.454ss.com/cgi-bin/2004for...num=1081825382

What can lead to rich LTFT and lean STFT? A lot of things. VE table being out is only one of them. Over/under shooting your proportional gains is another. Seeing as the engine dynamics are always changing, it's not uncommon to see a slight shift in the % fuel change (even on stock motors).

I consider a motor well tuned if the final fuel values are +-4% on either side. You have to add/subtract the STFT value from the LTFT values to see where the engine fuel really is. Although not all motors can acheive this.

For this reason alone, I now disable the LTFT learning while tuning and use only the STFT for gettings things tuned.

We already have the closed loop fuel tables in our editor.
I will be adding these LTFT/STFT constants in our HP tuners software on our next update (not this one).


Have fun guys, I'm off to catch some fish.


Ken
HP Tuners

 

Any additional tables for open loop fueling??????????????

 

Yes there are Bink.


I was mainly referring to closed loop fuel.
There are separate idle proportional tables that are used
based on an option bit.

If you are talking about modifiers for the open loop AFR, then yes.
Two of them are minor tables for cold/warm startup AFR.

There are also few other tables used in open loop mode. One of them is
trans related. Unless you are trying to get your car/truck out from
being stuck in the mud or snow, it won't be of much use to you.

As the Final AFR routine chooses the AFR value that will provide the
most fuel, there is no need to modify every table that exists in the VCM.

There are however tables that deal with the changing base mass fuel rates during
the changing AFR rates.


Parameters like the Transient Fuel Calculation tables. They affect the fuel delta rate
to the engine by limiting the fuel mass/cylinder to keep it from collecting on the cylinder
walls. Fuel that does not remain suspended in the airflow will not burn causing excess
emissions and increased cylinder wear.

Here is an example of some of the tables in the LT1 code.

;--------------------------------------
; Fuel Boiling Time vs. Coolant vs. Map
; table size 5 x 15
;
; TBL = 100 * SEC'S
;--------------------------------------
ORG $26F2 ;

FCB 0 ; Min COOL Val
FCB 0 ; Min MAP Value
FCB 5 ; COL'S/ROW

;--------------------------------------------------
; -40 Deg C COOL
;
; Seconds Kpa MAP
;--------------------------------------------------
FCB 69 ; 0.690 20.0
FCB 86 ; 0.860 40.0
FCB 92 ; 0.920 60.0
FCB 98 ; 0.980 80.0
FCB 104 ; 1.040 100.0


;---------------------------------------
; Fuel on Cylinder Wall Multiplier Vs Cylinder time/DegK
; Table Lines = 255
;
; TBL = 256 * Mult
;---------------------------------------
ORG $2740

L2740 FCB 255 ; 0.996
L2741 FCB 255 ; 0.996
L2742 FCB 254 ; 0.992
L2743 FCB 253 ; 0.988


;--------------------------------------------------
; Puddle Limit vs. Filtered Airflow
; Table Lines = 17
;
; Table = .4096 * mgrams
;--------------------------------------------------
ORG $2893

mgrams gms/sec Airflow
;----------------------------------
L2893 FCB 255 ; 623 0.0
L2894 FCB 255 ; 623 8.0
L2895 FCB 255 ; 623 16.0
L2896 FCB 255 ; 623 24.0
L2897 FCB 255 ; 623 32.0


While some people like to play with these tables on the LT1, others just wound
up disabling the option bit...

L2028
; b7 Not Used
; b6 1 = Transient Fuel Enable
: b5 Not Used
: b4 Not Used
;
; b3 1 = VATS
: b2 1 = Crank Air
: b1 1 = Speed Density Select
: b0 1 = Traction Control Enable


While we are adding a lot of tables that the average user can benefit from,
we are going to leave certain tables for the pro version only.
There are certain tables that will get users into trouble.


For those of you interested in this, Delphi has some excellent documentation on their website
explaining how they calculate intake valve temperature to predict what fuel entering
the cylinder will remain unburned (1999-01-0553).


Ken