STFT and LTFT. Q/A

It’s already a while after first entry about LTFT and LTFT, I got many questions, which I answered personally. Below is a compilation of frequently asked questions – nuances, which deserve more detailed outline.

 

Which scenario – positive or negative LTFT values – is better?

If LTFT (both offset and multiple) are in an acceptable range, there is no difference, they are with “plus” sign or “minus” sign. LTFT compensate technological differences from perfect values of the engine hubs. These differences are of casual nature – unique for the particular engine.

 

What is an acceptable range of LTFT?

  • -1.0 .. +1.0 mg/stk for offset type adaptations;
  • -20 .. +20% for multiplicative type adaptations.

These values I consider as “red line”, when performing diagnostics.

Of course, we have to pay attention, if LTFT approaches values, mentioned before – obviously, there are some conditions, which require significant correction of the initial situation.

Moreover, Siemens management systems try to minimize both offset and multiplicative adaptations (LTFT) – very slowly (in time of several hundreds of motor hours) they are reduced, “sharing” the differences from ideal between components, which parameters are different from ideal. Therefore it goes without saying that directly after re-adapting the engine, LTFT is significantly different from 0, but after several thousands of kilometers, the differences are significantly lower.

Note: more significant are differences of LTFT between banks!

 

Why are the differences of LTFT between banks more significant as absolute deviations of LTFT from 0?

The difference from 0 of LTFT is affected by components, common for both banks: air mass meter, fuel pressure sensor, throttle (it’s sensors), EGR valve and it’s adaptations, etc. In the worst scenario, differences between all these elements can be quite significant (they are summed), but it will not point to any technical problem.

In turn, differences between banks are created by unique components of each bank: injectors, Lambda probes. The large difference between the LTFT of banks can point to problems of elements, mentioned before. Defects/problems of injectors, which can cause large differences between LTFT of banks:

  • incorrect coding of injectors or inadequacy of it’s parameters to the coding;
  • leaking injectors, incorrect fuel beam.

The defect of Lambda probes, which can cause large differences between LTFT of banks:

  • clogging of wideband Lambda probes, problems with calibration resistor;
  • clogging of control probes, inability to generate required voltage.

Of course, there can be also mechanical damages (more common – for an engine with Valvetronic: for example, problems with HVA), air supply in the exhaust system, etc. It’s clear, that also such damages (no-existence of them) has to be checked.

 

Correct differences of LTFT between banks:

  • 0.5 mg/stk offset LTFT;
  • -10 .. +10% multiplicative LTFT.

Also, these values are determined by my experience. If I see larger differences, I will make the first steps to identify the problem:

  • check, if the control probe is able to generate the required voltage (above 0.8 V): perform a test of Lambda probes (N43/53: ../F9/F3) or check their max voltage in kick-down mode;
  • check corrections of injectors, for example: ../F5/Shift+F6/F5 (N43/N53, Stratified charge) or live data, for example: ../F5/Shift+F6/F1 (N43/N53).

 

How to evaluate the situation, if LTFT of one bank is with a positive sign (DME enriches fuel mixture), but LTFT of second bank is with a negative sign (DME makes fuel mixture leaner)?

This situation is normal and shows, that existing situation is close to perfect. If the difference between LTFT of banks is in correct values – perfect!

 

How to evaluate the situation – LTFT and integrators are with opposite “sign”? For example, LTFT is with minus sign, but integrators – with plus sign?

Also, this is a normal situation! LTFT compensates technological difference from the ideal of the engine. STFT (integrators) – short-term inadequacies. While STFT is an incorrect range (-10 .. +10%), there is no reason to worry. In turn, not depending on the polarity of LTFT – if STFT is significantly out of -10 .. +10%: obviously the engine has some problems – conditions of fuel mixture creation changes significantly.

Typical causes for a change of offset type STFT/LTFT (idle mode): rupture of CCVV valve membrane or air suction around its perimeter, other problems with airtightness of inlet manifold.

Typical causes for a change of multiple type STFT/LTFT (driving mode) – problems with fuel pressure (pressure regulator or pressure sensor).

 

What is the distribution of impact of offset/multiple LTFT?

Very approximate distribution of these LTFT can be seen in the drawing:

A: offset type LTFT

B: multiple LTFT

  • if RPM of the engine is below 1200 RPM AND required load – small: offset type LTFT are dominating;
  • if RPM of the engine is above 1200 RPM OR required load exceeds 10 .. 15%: multiple LTFT are dominating.

 

A sample of STFT and LTFT for Siemens and Bosch management units (INPA menu)

Siemens

A: Integrators (STFT)

B: offset type LTFT

C: multiplicative type LTFT

 

Bosch

A: Integrators (STFT)

B: offset type LTFT

C: multiplicative type LTFT

 

As we see, integrators and multiple LTFT for Siemens shows %, but for Bosch – as multipliers, the reference value of integrator and multiplicative LTFT is 1. Siemens offset type LTFT shows as the difference of mg/stk, but Bosch – as an additive with reference value 0.

 

Related entries:

STFT and LTFT