This time – a compelling case. Short history: N43 series engine, low power version; full functionality of the engine restored (NOx system repaired, Stratified charge restored).
Customer complaints – sometimes both driving or idle, and increased permanent vibration (“diesel” feeling) can be felt. Typically such symptoms indicate injector problems; the defect is quite common. I asked the owner of the car to send me diagnostics data. No error messages in the DME error message memory.
Here is the fuel mixture:
Correct. DME has not created two virtual banks (the car has only one wideband probe), but otherwise – no objections.
Here, the NOx sensor also confirms the correct fuel mixture. In addition, last time in Overrun mode, everything was in order – the injectors were not leaking.
But the last injector tests in Homogeneous mode indicate serious problems:
Injectors in this menu: 1/4/3/2 order. As we see, the injectors are divided as if by banks. For the injectors of the 1st bank, positive flowrate corrections are applied; for the injectors of the 2nd bank – negative.
Live data of the injectors:
Confirms that a huge flowrate correction is applied (even around 25 .. 30%)!
Something has happened to two injectors of one bank, and precisely with flowrate in the same direction? Unlikely!
In addition, the injector flowrate corrections in the Stratified charge are entirely correct!
This case is bizarre! Yes, the problematic issues happen when the injectors have problems with atomization, with beam – then increased corrections in Stratified charge are applied. But this time, everything is “opposite” – the injectors have issues precisely in Homogeneous mode!
In addition, the indications of the control probe look suspicious:
Problems with the probe? Test in the kick-down mode indicated that the Lambda probe works correctly.
Despite as if correct fuel mixture – it is thou lean? Additional air in the exhaust?
I asked the owner of the car to check this possible defect. It didn’t take looks – yes, the exhaust was not airtight (the additional air suction was detected close to the exhaust manifold). When the excess air suction problem was solved, all problems disappeared.
How did such a specific defect appear?
Despite the engine being equipped with only one wideband probe, DME “works” as if two independent banks: 1/4 and 2/3 cylinders. In the limits of each virtual bank, DME performs trimming of the wideband probe (both idle on the go), using the indications of the control probe.
If (as in this case) is excess air suction to cylinders No.2 or No.3, in these cylinders, the wideband probe is trimmed incorrectly – for a much more rich fuel mixture. Instead, when the chemical efficiency tests are performed, DME identifies/measures increased activity of both cylinders (which is only logical due to incorrectly trimmed probe) and reduces it, applying negative flowrate corrections to the injectors. As the mechanical defect (excess air suction) is unstable (due to engine vibration, temperature changes, etc.), a significantly different fuel mixture is created, and the disbalance of cylinder efficiency between banks appear. Feeling of “diesel” – natural outcome. And, of course, increased density of misfires due to incorrect individual fuel mixture of cylinders.
Note: the same defect can happen to N43 (also N53) with two wideband probes. In case of excess solid air suction, there will be error messages regarding trimming the wideband probes. Still, if DME manages to trim the probes, there will be no error messages regarding their performance!
