In the previous entry (here), I described a situation that confirms that if the AC (and other related hubs – cooling fan, alternator, automatic transmission, etc.) is defective, the idle of the engine becomes of deficient quality. That time, I mentioned that after the repair of the defect, all symptoms disappeared. Even when switching off the AC (via a button on the IHKA panel), everything was ideal. 

I was wrong!

Diagnostics of another B58 helped to indicate the problem. The idle of this engine was of deficient quality. No “full-fledged” misfires were present, but short-term unevenness reached +/- 7 .. 10 units. Such unevenness indicates profound instability of performance: short-term mechanical efficiency of the cylinder reaches even +/- 10 .. 15 % difference from ideal. 

In addition – VANOS positions are correct. Valvetronic jitters slightly, but exactly enough to maintain the average idle RPM. What happened? Have the spark plugs worn out? 

I accidentally noticed that the owner of the car does not use AC. I asked to turn it on to exclude the possibility that this (turned off) AC has any impact on the data.

After a moment, when stopping on the roadside, both of us (me and the owner of the car) noticed that idle had become much more correct! All the problems have disappeared! Live data confirmed: yes, the situation is radically better. 

After seeing this phenomenon, I decided to test this once again on my own car – are there, however, some differences in idle depending on the AC to be on or off? 

Remark: In my blog, I have previously mentioned my observations that MSD80/81 (BMW E series) stops updating the individual adaptations of the injectors in idle/Stratified charge if the AC is turned off. I have no information regarding problems/switching off regarding other adaptations (but there is a significant possibility that previously mentioned injector adaptations are not the only ones that are turned off). 

To test the assumption that switching off the AC has a dramatic impact on idle for the F series B58 vehicle, I switched off the AC for a longer time. At the first moment, nothing changes. But during the next 3 .. 5 minutes, an increased short-term unevenness could be felt. No, it’s not a permanent vibration, but exactly a jerking idle. In addition, the RPM and the idle character changed when the load was changing (for example, when the IHKA fan RPM increased). 

Here is how the idle looks 5 minutes after switching off the AC:

Definitely – the short-term unevenness is above the norm. In addition, an increased average-term (one/two/three seconds) unevenness is observed. It happens that some cylinder has incorrect mechanical efficiency for even several seconds!

And here is the most interesting moment:

On the 145th second, I turn on the AC. See, how radically has the stability of idle changed/improved?

Here, 30 seconds later – a perfectly stable idle! It is not even possible to compare the first two images.

Here, we see the following mode: AC = ON, but the required air temperature in the vehicle: is + 30 C. We can assume that the power of the AC compressor is 0 because the required inside temperature is high. Idle – no problem at all.

Here: the required air temperature is set at +19 C. Yes, everything is fine; the idle is correct.

Here, is how the idle of the B58 engine looks when driving for a longer time with it turned off AC:

Where the essence of this problem is hidden?

My information may not be accurate because I base it only on my observations – the data from the manufacturer (as usual) is 0. 

If the user turns off the AC (by using the button on the IHKA panel), DME has no option but to make sure that the compressor of the AC works properly. Accordingly – there is no option to make sure that the reported mechanical load is correct/corresponds to the reality. Situation – similar to in case of damaged AC. This time, the consequences are not so harsh, and it looks like only the updating of the individual adaptations of the injectors is turned off. 

How do these individual adaptations work?

As I already mentioned in the previous entry (here), DME plans the mechanical load of the engine and, based on this data, calculates the amount of additional fuel to be injected to keep the idle perfect. As we can guess, this does not always make this prognosis turn out ideal. One of the reasons – is injectors and nuances of their performance for these engines.

When working in this full functionality mode (with turned-on AC), DME analyses the performance of each cycle of each cylinder and the fact, how successfully it has managed to prognose the mechanical efficiency of each cylinder. If the prognosis is incorrect, the adaptation map of the current injector is modified immediately. 

For example, DME finds out that the mechanical load will increase to 10Nm or 20%. When calculating the energy, DME concludes that the amount of injected fuel should be increased by 1mg/stk. When performing the injection, DME detected that the result was not perfect – the mechanical efficiency of the cylinder increased by 15% (instead of the planned 20%). Obviously, the injector is slightly “lazy”. DME modifies the adaptation of the current injector in such a way that the efficiency in the next cycle will correspond to the ideal. During the next cycle, DME again evaluates if the perfect mechanical efficiency of the cylinder has been reached. Is there any need to make some corrections in the adaptation map? This is the way DME tries to reach the perfect prognosis of the idle. 

If the AC is turned off, the creation of these adaptations is switched off, too. Everything is fine for some moments, but even several minutes is enough for the quality of the idle to drop. Why so?

The B58 series engine works with a high Rail pressure (100 bar in idle, till 300 bar on the go), and the opening times of the injectors are very, very short. In idle, the injector is in so-called “fly” mode for 30 % to even 50% of the time. It means – at this time, the injector is in the middle position between “closed” and “open”. This intermediate state is very unstable, and DME regularly has to correct the voltage/current, supplied to the injector (parameters of the injectors change depending on temperature, Rail pressure, etc.). Exactly due to this long and unstable intermediate state, Bosch had to introduce these additional adaptations. 

Remark: a certain time ago, Bosch was forced to admit (the information can be found in TIS and is mentioned here) that these new-generation injectors have more significant problems than planned. For example, for N20/26 engines, there was an injector upgrade (from EU5 to EU6, changing the size of the injection beam). The electrical wastegate was introduced, but the most important – the major software upgrade followed. This upgrade is intended to introduce the fly-time adaptations for these injectors. 

In my opinion, this is certainly a failure – it is not okay that the idle becomes of low quality if the user turns off the AC! But I don’t think that BMW AG will listen to it…