Sometimes we hear – while performing the diagnostics, you say – here, there is a problem on the exact hub. But no error messages regarding it appear! Can I believe such diagnostics?
If we analyze this question logically, we have to conclude, that if there always would be exact and correct error messages recorded, there would be no need for additional diagnostics. No analyzing of live data would be necessary, no additional tests needed. It would be enough to read the error messages and replace the damaged element!
Unfortunately, sometimes (even very often) repair specialists without experience do exactly that. As a result – a significant amount of money spent without any result. Yes, in very simple cases simple reading of error messages helps, but mostly – it’s not so simple, and the error messages only identify the direction, where the damaged element could be found.
In this entry, I will explain, why and in what cases the error messages don’t appear, even if the diagnostics (live data, specific tests) identify the problem.
Even very simple situations can bring surprises. A simple example – the valve. It’s not even important, what’s this valve for – regulating the position of VANOS, oil or fuel pressure or any other parameter. DME usually controls the connection of the actuator, it means, if the connection will disappear (there will be a problem with valve winding, wires or connector) – DME will notice the problem.
But, if the connector would be oxidized? The valve will not perform normally, even the connection (bad quality) as if could be identified. And what would happen, if electrically the valve is OK, but mechanically – it’s damaged? The connection test will not see this problem. Possibly, the DME will see such problem via functionality problems of the engine (or hub). But, for example, in case of problems with VANOS position, there could be several reasons – damage to the valve, mechanical problems of the hub, problems with oil pressure. Example with VANOS – if additionally, for example, the signal of the sensor has also disappeared, it is almost impossible to identify the amount of damage remotely (without step-by-step repair/diagnostics).
One more problem, what DME has to face when evaluating the performance of each hub – what allowances are acceptable for each of them? How large (in absolute and relative values), how long, what are the exceptions (if such exist)?
If we continue with VANOS – it’s clear, that it has a mechanical inertia. It takes some time for the camshaft to turn to the necessary position. This time can vary significantly, depending on oil viscosity and pressure, friction, “distance” between existing and necessary position, turning speed of the camshaft and also changes in this speed (it drops or increases) and other parameters. In turn, the oil viscosity depends on its temperature, wear, label, fuel addition. Oil pressure – depends on required pressure, engine’s ability to maintain it, momentary oil consumption. This listing can be continued for a long time, but it’s clear – there are many parameters, many of them are swiftly changing in time, many of them – very dependent on external conditions.
The main conclusion – to ” identify” the parameters of functionality (requirements, which should be fulfilled for the error message to appear), the significant reserve should be foreseen. For example, if normally the VANOS hub is able to react to change of position for +/-10 degrees during 0.1 seconds (the engine with mileage 0, in work temperature), for specific conditions we have to foresee, for example, tolerance of 10 times (used engine, oil replaced 20’000 km ago etc.) Accordingly, the error message will be recorded only in case, if VANOS will not reach the position of +/-10 degrees during 1.0 second. To prevent DME turning in a too complicated monster, usually, no multi-dimensional maps of requirements for error message recordings are made. Usually, the situation is limited with “if in time of X seconds the condition Y is not fulfilled”. As parameter X is chosen max possible (the worst possible scenario), with previously mentioned significant reserve. So we don’t have to be surprised, if in case of short-term problems no error messages are recorded. In case of VANOS – the error message regarding position is recorded only then if the required position is not reached during several seconds, but the engine will not be able to perform normally, even if the VANOS will react several times faster!
One more common situation – “quality” of the parameter. Let’s continue with the same example – VANOS. How accurately does the VANOS hub have to maintain the required position? How large differences, also in time, from required position are acceptable? Even if the average value of the parameter exactly corresponds to the assigned value, in short-term it can be different (and usually also is different). Situation with this criteria is even more complicated – we have to take into account countless obstacles, mentioned before, short-term differences from ideal are also created by all kind of playback (axial, radial) of all kind if hubs, precision of position sensors itself, reaction time of DME (both measuring and managing the position).
Short-term differences from the ideal parameter are called jittering. Jittering is characterized by several parameters: max deviation from the ideal parameter, average weighted values difference from ideal, shape and time of these differences. How you understand, the number of evaluation criteria only grows!
In the case of jittering the error messages regarding the performance of the current hub will not be recorded! Jittering is a ” quality” parameter, which can be evaluated by analyzing live data.
To perform analysis of live data, you will need both bits of knowledge of engine construction (performance of it’s mechanical and electrical systems) and also experience.
Several examples:
observing several tenths of correctly performing engines, I have made a conclusion, that in even conditions VANOS of N43/N53 series engines is able to maintain the required position +/1 step (1 step = 0.4 degrees, “step” of INPA bar), PWM jittering of the valve +/3%; short-term inadequacy – to 5 .. 10 steps, time – to 1 second; oil pressure for N43/N53 series engine in even idle: 2600 hPa; +/-100 hPa, separate rare peaks to +/-200 hPa, PWM jittering of the valve +/-5 %.
These data are acquired, observing large enough to speak a number of engines – this experience is very useful in diagnostics. Accordingly – if live data of some engine display data, which are significantly different from averagely observed – obviously, this engine has some problems. Unfortunately, if you don’t have experience in diagnostics of the exact type of engine, these “plausible data” are not known to you, accordingly – you have no possibility to evaluate, does the exact hub performs “with quality” or not.
The opinion of every car owner, how (how evenly) the engine should work, is very different. I had diagnostic cases, when a customer says: ” I have no claims, just couldn’t pass the TI”, but I find, that the engine has continuous misfires, it shivers even in idle, it has reduced power. And also there are opposite cases – the owner complains, that “yesterday I drove for 2 hours and sensed 1 misfire – something is not OK, obviously”.
In my practice, I always point out to all incorrectly and/or poorly performing hubs. It is up to each owner of the car, repair and what to repair.
And finally – if you want to enjoy the perfect performance of the engine, ALL its systems have to work perfectly! Yes, the shivering of the engine can be because of unstable (low) pressure; because of unstable oil pressure; because of damaged crankcase ventilation; because of poor quality ignition coils, because of spark plugs, which are not replaced in time etc.
