This part of the article is regarding qualitative indicators of the VANOS system performance. You can assume this part is a scientific treatise and ignore it if you have no patience to go deepen the theme. Instead, if you are interested in justifications for my conclusions “from a side view” (unfortunately, I don’t know the manufacturer’s calculations), you are welcome! Then you can skip to the continuation of the article – the practical part.
First, a short review regarding reference positions of the camshafts. After reviewing a significant amount (around two thousand) of engine diagnostics data, data was compiled.
The setting of the camshafts in the BMW AG factory is done correctly – I think nobody should wonder about it. According to the “meaning” of the DME itself, the reference position differs from the ideal position by more than 4 .. 5 degrees in sporadic cases, even for quite used engines.
Initially (a long time ago), the allowed deviance of the reference positions (“green” zone) was +/-10 degrees. Siemens DME was very “swift” by recording the error message regarding reference position – already by deviation of +/-11 degrees from the ideal. Bosch DME was slightly more “patient” and allowed an additional 3 .. 4 degrees as a “reserve”.
When the DME software was updated, these allowed values were widened (I don’t have exact data, but practical observations say that an additional 3 .. 5 degrees were “given”). For the engines with more considerable mileage (due to stretching of the chain), the error messages regarding reference positions of the camshafts start. Obviously, the manufacturer received a significant amount of reclamations to appear.
A second possible reason why larger allowances are given – the employees of the dealer centers were not able to “correctly” install the camshafts after the repair of the engine. Why in quotes? Because as the repair was done correctly – the chain was tensioned, and the BMW tool for locking the camshafts in the positions was used, but – the repair was unsuccessful! How so? Here I mention open of the reasons. The manufacturer can not prevent the free movement (which, logically, increases when the hub wears out) or somehow compensate them automatically – so the “solution was – to increase the technological tolerances.
Accuracy of maintaining camshaft positions
To make the description of this theme technically correct, the distinction should be made between terms: measuring accuracy, the accuracy of measurement display, and positioning accuracy. Because of this, my conclusions do not claim absolute accuracy, but they will allow evaluation of the actual situation.
The measuring accuracy of the shaft position should be the highest. Discarding mechanical problems (free movement of the hubs, etc.), it is defined by the quality/precision of the sensor; “resolution” of MCU (resolution of internal timers/interrupts; frequency of the timers; quality of the code) and accuracy of the mathematical calculations (type of data registers and memory cells used). All this is defined by the developer of the hardware and software; we only can rely on that there are no harsh mistakes.
Shaft positioning (positioning accuracy) will be less accurate because, in addition to the measuring error, there will be other problems – free movement of the VANOS mechanism; short-term inaccuracies due to changes in the oil pressure; inaccuracies caused by the vibration; changes in the chain tension, uneven engine performance, etc.
The accuracy of the measurement data display will be the lowest. In addition to all previously mentioned errors, in this stage, the data rounding errors appear (both within one measurement and the display of all measurements).
Why such a long introduction? This time we are talking about data and math. In the further part of the article, I will justify when in my opinion, it is (should be) the correct accuracy of maintaining the shift position; I will also explain how I got to such conclusions.
For example, the data refresh rate (in the VANOS menu) for MSV70/80/MSD80 INPA (a very similar situation also with Bosch ME/MEV) is around 3 times per second. Data displaying accuracy:
a. for reference positions: +/- 0.1 degree;
b. required and actual positions of the camshafts +/- 0.4 degrees.
By observing the data, it looks like the data are not averaged; INPA displays the current measurement, which has been “read” when it should be sent via CAN to the diagnostics equipment. So, in idle, we see only each 7th .. 10th cycle of the camshaft (and each 28th .. 40th measurement of the positions, considering that the toothed ring has 4 “teeth”).
The situation with ISTA is different. In call-up functions, ISTA does not give information regarding reference positions of the camshafts; data of required and actual positions are displayed with 2 sighs after coma, which means a resolution of +/-0.01 degree. Refresh rate starts with 3 results per second (if only 1 .. 2 parameters are chosen) till 1 measurement per second (if 5 .. 10 parameters are selected). The results of the measurements are rounded (between displaying cycles). From this rounding, the high data ”accuracy”/”resolution” comes.
Due to the rounding of measurements and displayed data, it is hard to evaluate data resolution in the Expert mode. Still, one is clear – the software developers have not saved the resources of the computer, and all parameters are displayed with an resolution of tenths or even hundreds of degrees.
So, in both INPA and ISTA, we see only each 7th .. 10th cycle of the camshaft (in idle; when RPM increases, data become even more “rare”). Additionally, ISTA rounds up the data between data output. In the Expert mode, data density is higher, but you will not see each measurement (cycle).
The most significant – so, if the camshaft will “shiver” for a concise moment (tenths of the second),
a. in INPA, we will not see this problem at all, but, if we are lucky – will see it very well;
b. in ISTA call-up functions, we will see the significantly reduced “amount” of the problem;
c. in ISTA Expert mode logs, we will see the most correct situation.
For the above reasons, I suggest:
a. when using INPA, observe data (during transition processes) several times, take into account the worst readings;
b. in the case of ISTA, if the Expert mode is possible (available), use it! If Expert mode is not available, unfortunately, you will not be able to see short-term problems with the camshaft position. Good news – typically: if VANOS has issues with the transition processes, there are problems maintaining the average (long-term) position too.
At this moment, we reach the essence of the entry – what values are considered to be the correct ones?
INPA. In the case of INPA, the correctly performing VANOS hub in stationary conditions maintains the position of the valve in the amount of +/-1 “step” displayed, which means: in the range of +/-0.4 degrees. If we keep in mind the wear of the hub, technological tolerances, etc., for me, the acceptable accuracy of the VANOS hub is +/-2 “steps” or +/-0.8 degrees. During transition processes (required position changes swiftly), short-term inaccuracy, of course, is more significant, but the delta of displayed (required – actual) parameters can not be larger than several (2 .. 3) degrees.
ISTA. Call-up functions. Due to data rounding, you will not manage to see short-term problems. Even more – you will not even see the required positions of the camshafts! So in this section, we can only evaluate “swimming” or slow jitter of the actual position of the camshaft. For a correctly working VANOS, the “swimming” of the data (rounded values) should be not more than +/-0.5 .. 1.0 degree.
ISTA. Expert mode. Expert mode allows for the evaluation difference of momentous value from the required value. Correct readings (analog to INPA): not more than +/-1 degree. In this mode, we also see the VANOS hub’s reaction during transition processes. The actual position in the range of a tenth of a second should correspond to the required position in the field of several degrees.
The opinion of the BMW itself regarding an allowed error of the actual position can be seen only in the ISTA VANOS test menu, where, for example, the following allowed short-term inadequacy for Bx8 engines is as follows:
The range is quite wide but is enough for a brief moment with such non-compliance for the test to be declared as failed.