Total crash. What happened?

This time – about the situation, which becomes “popular” in warm weather. The engine shivers, idle is very unstable, error messages regarding fuel trim (lean mixture) are possible, also misfires appear.


Why exactly in warm weather? Most probably in the colder weather, these cars won’t be able to start and would be towed to the service center.


In this entry about the following situation:

As we see (the engine runs at idle):

a) not taking in account integrator values, which are reaching +30% (it means, then the fuel amount is increased by 30% against planned);

b) fuel mixture is lean – around 50% of air gets in the exhaust.

After a while, possibly, also offset type LTFT will be overwritten (to values of increased fuel amount – they will exceed +1,50mg/stk for each bank).


Let’s check, if the control probe confirms lean mixture (it’s hard to guess the reason, why wideband probes of both banks would give such incorrect, but similar readings):

As we see, narrow-band probes confirm lean mixture.


In the same time – only error messages, which are recorded, are regarding fuel mixture – it’s lean (yes, I agree).

If we follow repair plans, offered by ISTA D, we can replace at least one-half of engine sensors. The workers of the service center will do exactly that. Or even more efficient – they would offer to replace, for example, all injectors. For me, it’s not clear, how the leaking injector could create a lean mixture, but if you like to pay EUR 2000 or even more – you can change the injectors.


During last few weeks several scenarios (with lean mixture) appeared, but they also had some differences:

a) in one case – correct data of mechanical tests of the injectors and at Stratified/idle;

b) in another case – very incorrect data of mechanical test of the injectors (additionally – in total chaos: increased flow-rate for one injector, for other – decreased at small load and totally different test results for wider/longer openings).


Incorrect test results of the injectors didn’t look reliable – completely inadequate injectors and averagely twice decreased flow-rate at the same time? Don’t sound reliable. Rather it look’s like problems with fuel or air supply, which “damaged” the results.


Such defect is very unpleasant – there are no (visible or confirmed by self-diagnostics) damaged elements, at the same time – something is wrong with the fuel mixture fundamentally.

Basic things are clear – the lean mixture can be because of:

a) too less fuel;

b) too much air.


The situation is made unpleasant by the fact, that the amount of the injected fuel depends on measured air, the pressure in the rail system, several adaptations (for example, throttle, injectors). To make it even more complicated, INPA (don’t even mentioning ISTA D) don’t display total opening of the injectors, it means, if several injection impulses are made, only the length only of one of them is displayed. There is no precise documentation, how the length of the impulse is redistributed, for example, when affected by adaptations, when the rail pressure changes etc.

Also, the angle of the throttle opening is not exactly known. Unit or the position, displayed by INPA (../F5/F2/F1), is 1 degree, which by throttle opening in idle (2 degrees) is with very low precision. But we have to understand – initial throttle position is with high tolerance (in idle – with small opening angles), the possible position measuring error is compensated with throttle adaptations. What I wont to say with this: even, if the position of the throttle would be displayed more exact (for example ..F6/F1/Shift+F9), even then we could not trust these data, because the offset type error of the throttle opening sensors is relatively high.

Also several more – for example, air mass measuring sensor, rail pressure sensor – are not very reliable in this situation. Yes, if the air mass measuring sensor will display inadequate (several tenth times at least) measurement of the air mass, MSD self-diagnostics will identify it, but – not in this situation. Exactly the same is with rail pressure sensor – that could be a cause of the problem, and the MSD self-diagnostics could not display it.

If, for example, there would be problems with HPFP pressure valve, experienced diagnostics specialist will be able to identify it, using PWM data. Also, MSD self-diagnostics will notice, that something is wrong with HPFP, but in case of problems with pressure sensor, the changes of PWM are not so significant, that MSD would recognize it.


Why such long introduction? I just want to explain, how complicated is the situation. We cannot trust:

a) rail pressure sensor;

b) cannot use HPFP PWM data to control the rail pressure sensor;

c) air mass measuring sensor;

d) cannot use throttle data to control the air mass measuring sensor.


In such cases I recommend:

1. Check fuel supply:

a) evaluate jittering of LPFP sensor and stability of HPFP pressure ../F5/F2/F6

LPFP pressure has to be stable (5000 hPa +/-30 hPa), HPFP pressure has to be 150.000 .. 200.000 hPa;

b) control the rail pressure with a manometer (simultaneously to INPA data).

If we have confirmation, that rail pressure sensor displays correct data, we have at least one parameter, that we can trust.

If the rail pressure is normal, we can assume, that (in case of correctly measured air) the correct amount of fuel will be injected. Accordingly:

2. Solve the problem with air:

a) the amount of air has to be measured correctly (air mass measurer is responsible about it);

b) no additional air access is acceptable.


Let’s start with mechanical issues – additional air access to the inlet manifold. Possible causes:

a) ruptures in the pipe from air mass measuring sensor to throttle);

b) defects of “O” rings of the DISA valve(s) and the sealing ring of the throttle;

c) defects of crankcase ventilation, pipes of EGR valve;

d) rupture in the crankcase ventilation membrane;

e) any other loss of air-tightness.


In such case the smoke test is very efficient – ask your service mechanic to perform it. The most frequent locations of the defects – those, who are not visible, those, who have difficult access, for example – a pipe of the crankcase ventilation valve or CCVV membrane itself.


Attention! Even the very small problem of air-tightness can make fundamental problems! Even hole of few mm in the inlet manifold will paralyze the creation of adaptations (offset type adaptations for idle will be turned off, will be recorded error messages regarding fuel trim; the engine will shiver in idle, error messages regarding misfire will be recorded etc.).


When the confidence regarding that there is no additional air input in the inlet manifold is obtained, we can check the performance of the air mass measuring sensor.


Required preparations:

a) turn off the lights (low/high beam);

b) turn off the air condition;

c) turn of all other large consumers (saloon vent: till 23 .. 30%);

d) the battery has to be charged;

e) switch the engine in Homogeneous mode ../F9/F1, press F1*

* note: after repairs restore the management of DME ../F9/F1, pressing F5.

This table contains basic data, which correspond to the N53B30 engine.

2000 and 3000 RPM data were collected, because, if the engine runs very unevenly, the measurement of idle will be very unstable. In such situation is better to use data of increased RPM.


Accordingly: if the air mass measuring sensor shows as if correct (appropriate to exact measurement/table) air consumption, but the fuel mixture is still lean – obviously, the data of air mass measuring sensor is incorrect.



a) if the main problems are with offset type adaptations and integrators in idle, with high possibility – the additional air input is one to blame (when RPM and/or load increases, the total air flow increases and the effect of additional air suction drops – the defect disappears in higher loads);

b) if the problem with fuel mixture is in all range of RPM/torque (confirmed with incorrect injectors data: ../F5/Shift+F6/F3; ../F5/Shitf+F6/F5), most probable cause – problems with fuel pressure. Fuel mixture discrepancies will affect the creation of the fuel mixture regardless of RPM or required torque.