Every repair of Valvetronic is time-consuming and expensive, so the question – what symptoms are characteristic exactly to the defects of Valvetronic – is very important.
As already mentioned in previous entries regarding Valvetronic, it’s impact and main problems could be characteristic exactly to idle and/or in range of low required torque.
This time I will tell more about symptoms exactly in these engine performance modes – idle and low torque.
Comprehension of what is uneven run, rough idle for every person will differ. There are perfectionists, which feel one misfire a day, and there are also people, who don’t feel, that the 4-cylinder engine works on 3 cylinders. So I will try to describe more precisely, what symptoms are important.
We have to divide two main symptoms:
a) permanent vibration in idle;
b) uneven idle due to misfires.
Permanent vibration in idle (a)
RPM in idle is stable, no shivering. Permanent vibration is present, which can be felt, when touching the wheel, can be felt as a vibration of the seat. In idle the engine works as a diesel engine (subjectively).
Uneven idle due to misfires (b)
The idle changes with “jumps”. The engine shivers not regularly. This shivering, exactly as in the first case, can be accompanied with permanent vibration (see the description above), but most important – irritable shivering can be felt.
I assume, that the engine has no problem with regulation of fuel mixture – no error messages regarding it (lean/rich mixture, error messages regarding Lambda probes, air mass measures, throttle etc.). I assume, that CCVV valve also is checked – it also can be a cause of incorrect fuel adaptations and problems with fuel mixture, when the engine switches from non-Valvetronic mode to with-Valvetronic mode. I assume, that, when the misfires are detected, the ability of the engine to maintain the Stoichiometric mixture (Lambda 1.00) has been checked at first. The main question – can the problems with Valvetronic be the cause of such (misfires/shivering) problem?
Short answer – within the limits of reason: no, Valvetronic is not the cause of this problem!
My experience confirms – even in situations, when the mechanical efficiency of any cylinder is reduced for 40% or even a little more, the engine (of course, with vibration, because the efficiency for at least one cylinder is very low) is able to work without shivering, it means, misfires are not observed.
Let’s see not the too extreme case, more close to a typical scenario.
N46 engine. The unevenness of the engine can be felt (we will not even specify, what kind of unevenness). Rough run menu confirms unevenness. For a 4th cylinder, averagely +1.5 units are displayed, for other cylinders around -0.4 units. Turning off the ignition coil of the cylinder No.1, Rough run displays +8.0 units. Accordingly, the efficiency of cylinder No.4 is reduced by around 15 .. 20%.
Simply assuming, that 15 .. 20 % of fuel of cylinder No.4 is not burned (due to problems of Valvetronic – decreased lift of intake valves), it’s Lambda is around 0.80 .. 0.85;
Lambda of the second cylinder of the same bank is around 1.15 .. 1.20 (because DME will try to maintain the average Lambda for both cylinders 1.00).
Both values: 0.80 and 1.20 – are suitable for burning the fuel. Anyhow, for the warm engine with ignition system in work order – no misfires are acceptable!
For the 6-cylinder and 8-cylinder engines, the potential problem with misfires is even less pronounced, because the damaged (with reduced fuel fulfillment) cylinder will be compensated by 2 or 3 cylinders of the same bank – their mixture is much more close to correct fuel mixture (it’s not so lean).
Note: as we know, a lean mixture is more “dangerous” for misfires, so “less lean” mixture is a good news. For a warm engine, the threshold for misfires is by Lambda around 1.4 .. 1.5.
So – unless the wear of the Valvetronic is not so significant, that the air fulfillment of the cylinder is for 50 .. 60% less than optimal – Valvetronic IS NOT the cause of misfire!
Causes of misfire (short summary):
a) mechanical damages of the engine (reduced compression);
b) wore out spark plugs (both with increased air gap and defects of insulation);
c) damaged ignition coils (leaks both outside and inside the coil, shortcuts of windings, short-circuit of diode in the secondary loop);
d) incorrect fuel mixture (very rich: Lambda below 0.70, or very lean: Lambda above 1.40);
e) specific (and rear) defects – the connection of the ignition coil disappears, damaged management transistor of DME.
So – if the compression is checked, the fuel mixture – correct (within the bank), extreme differences between efficiency of cylinders are not measured (at least 2 times differently): have to check spark plugs, ignition coils (typical defects).
Unfortunately, the problem is complicated with an obstacle, that misfire counter reports via OBD mode 6 are available only form MSD80 and MEVD17 series management units. Management units of older models don’t report misfires. If the misfire counters are not known, we can observe Rough run data. Unfortunately, if these Rough run data are too smoothed, it’s very hard to identify the misfiring cylinder – for the guaranteed result, all spark plugs and ignition coils have to be replaced.
Additionally, misfires can give permanent vibration (which are confirmed by Rough run data)!
Especially particular this problem is for Bosch management systems. I will comment on this problem a bit more detailed. I have to add, that, of course, there is zero information from the manufacturer (it is understandable – nobody will talk about their own problems), so everything, written below, is my experience and research.
At first, I have to mention misfire identification by changes of flywheel speed: patent of Siemens. Bosch, instead, has patented detection of misfires by accelerometer data. It is understandable, that the solution of Bosch will not work, for example, in idle, and also in modes, when the torque converter of the automatic transmission is not locked.
My observations confirm, that Bosh management systems are much more “slender” in detecting misfires. It look’s that Bosch however also uses flywheel data to identify misfires, but the results are quite sad.
Additionally to it, that the self-diagnostics don’t display damaged cylinders, there is one more side effect – if the engine has misfired, this misfire can be the cause for incorrect adaptations of cylinders, as a result – the engine will have permanent vibration.
Let’s calculate, how often has the misfire with efficiency 0% ( or -100% against nominal) happen, for the average efficiency of the cylinder (average of all cycles) would be changed for 5%: 100/5 = 20. So – if every 20th time there will be a misfire, the average efficiency of the cylinder will be distorted for 5%.
In idle (600 RPM) the engine performs 10 turns per second, or, for example, each cylinder of N46 performs a cycle 5 times in a second. Every 20th cycle – after every 4 seconds. Double misfire happens? If it will happen after each 8 .. 10 seconds, mathematically – the result will be similar. The real situation, unfortunately, is even worse. Directly after a misfire, the fluctuations of the flywheel speed can be observed – it makes the calculation of it’s turning speed more complicated; during first seconds after a misfire, the mathematical error is even larger (and gradually decreases, “moving away” from misfire).
I think this simple calculation shows very well, that if any cylinder has misfire and DME don’t recognize it (includes in the calculation), problems with the calculation of efficiency of this cylinder continues for 10 seconds or even longer.
Additionally, the cylinder adaptation (to avoid swift and unpredictable changes) are applied slowly (typically – period for at least 5 .. 10 seconds) – permanent unevenness is guaranteed for a long time!
We have to keep in mind one more nuance, which confuses many. Quite often the allegation is heard – problem with Valvetronic, because, if turning it off, all problems are reduced.
But we have to remember – turning off the Valvetronic, the engine works in limp mode: with turned-off individual adaptations of cylinders. Accordingly – the misfires don’t affect the balancing of cylinders. Yes, they don’t disappear, but the impact of them (misfires) to the idle significantly decreases – no permanent vibration is created.
Only after misfires are prevented:
a) delete old (all) adaptations;
c) perform an adaptation of the flywheel;
c) create new idle fuel adaptations.
Only after performing these procedures – Rough run data of cylinders can be evaluated.
If then stable and permanent vibration (which is confirmed by Rough run data after several driving sessions), possible cause – problems of Valvetronic. Regarding possible problems of Valvetronic read in related entries.