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22 Posts
Discussion Starter · #1 ·
Dear forum members,

I have some troubles with my 2006 E65 V12 N73 (TU) car. While driving moderately in dense traffic, the engine suddenly lost its power and the message "Engine fault, reduced power" was displayed on the bottom right of the dashboard (below the rpm gauge). Power was reduced to the point that the car became almost undriveable, consisting a significant traffic obstacle and danger. Since then, this error comes up and disappears regularly. Sometimes it goes away after a few kilometers, but some other times it stays for a hundred or so kilometers. It pops up far more frequently than tolerable.

I managed to catch the problem with ISTA+, and the following codes were found:
002CF8 DME2: Throttle-valve potentiometer, bank 2
002CFA DME2: Throttle-valve potentiometer 2, bank 2
and a few minutes later
002CF9 DME2: Throttle-valve potentiometer 1, bank 2
was added.
These were found after deleting all former codes and luckily spotting a full episode of the problem in ISTA+.

Earlier I have found some other, similar fault codes, including:
002CF0 DME2: Throttle-valve actuator, bank 2, control range
002D05 DME2: Throttle-valve actuator, bank 2, cancel with UMA re-learning
Occasionally some other codes come in (might be unrelated to the main problem):
002F0B DME2: Intake/air temperature sensor, bank 2, plausibility
002D29 DME2: Differential-pressure sensor, bank 2, intake manifold, plausibility

In one short episode the engine was in such a bad shape that I had to stop on the roadside and put the transmission into neutral. The engine was struggling to continue running, like being choked or misfiring very badly. However, in usual cases the engine runs rather smooth (albeit very weak), and after checking the lambda control parameters in ISTA+, everything seemed rather normal. Additive adaptation values were < 0.5 and multiplicative adaptation values between 0.97 and 1.03, from which I would conclude that combustion cycle is still properly maintained. (Am I right that these are excellent lambda adaptation values?)

I wonder if anyone more familiar with the topic than me (I'm an absolute beginner) have experienced such problems or have the knowledge to interpret these symptoms.

I would conclude from these codes that my throttle valve unit on the right hand side of the engine is worn out ans should be replaced. However, these units are rather heavily priced items, between 250 and 600 EUR, so I would like to be sure before ordering a new throttle valve.

Could it be some problem of the engine control unit, or maybe cable harness problem? Or any other ideas where to look at?

Thanks for your help in advance.

22 Posts
Discussion Starter · #2 ·
For the benefit of the community I would like to summarise my experiences.

I had been struggling with this problem for a while, and reconfirmed the problem in ISTA+ several times with carefully watching the re-occurrence of these fault codes. I understood from TIS/ISTA+ that there are two independent potentiometers and wires to monitor the position of the throttle and the ECU is very carefully cross checks information from these. The limp mode is result of ECU recognising that it receives self-contradicting information from these and this is an important safety feature.

I tried to follow some of the advanced diagnostic instructions (ABLs) very carefully, but these had fairly little value for me, as they had no options to directly study the details of the operation of the throttle body (TB), unlike some other engines (like S85, maybe). It seems that the relevant software routines weren't developed or they are well hidden in the dungeon of ISTA+ and I couldn't find them. There were some minor tools and hints available, like being able to check that the 5V power supply is present in the connector of the TB and some other things. It was valuable information that neither this wire is broken nor the supply unit is faulty, however, I couldn't check the analogous data wires (potentiometer 1&2) and the corresponding input stages of the ECU the same way. So after I carefully stuided the relevant wiring diagrams - actually following the advice of one of the ABLs - I disconnected the corresponfing connector of the ECU (DME2, in my case), and checked the data wires using an ohm meter. These seemed OK.

At this point I had seen no oppurtunity to get more confirmation, lost my patience, risked buying a brand new TB unit (OEM VDO) and installed it following the instructions from TIS/ISTA+. (Hint for the other victims fighting with the same problem: the connector of the TB units are very hard to unplug before unscrewing the TB from the intake plenum on cylinder bank 2 and absolutely impossible to disconnect on cylinder bank 1. Therefore it is much easier to go against the advice of TIS/ISTA+ and unscrew the TB first and unplug the connector second. Always replace the gaskets as they are very badly deformed/cooked after a few 100k of operation on the hot engine and they are available pretty cheap (~$5).)

For my great disappointment the car showed exactly the same symptoms after reassembling everything. I got upset and tried to reinitialise everything that seemed even remotely relevant in ISTA+, including lambda adaptations, etc. I have been driving around for an hour, stopping at several places and trying new and new magic clues found on various websites about resetting TB adaptation. After one of these experiments (something like stopping the engine, flooring the accelerator, switching ignition on, waiting 30sec, releasing accelerator, starting engine, idling further 30sec?) the problem slowly started to evaporate. The car (gradually?) regained its power, the limp mode message went away and a few minutes later the check engine light went off. The car seemed perfect. I'm not sure at all, that the improvement was the consequence of my experimentation, since I have seen the problem disappearing earlier several times.

However, in the coming days I have driven quite a few hundred kilometers in various traffic conditions, without seeing the problem. Then a week later - BIMM - the limp mode message showed up again. I wasn't happy, but it disappeared in a few minutes. Not much later (minutes of driving?) it came back again. Having no better idea I drove to our garage, and disconnected and cleaned both connectors (at the TB and at the ECU) thoroughly using an electric connection cleaner spray.

Luckily, after this the problem evaporated and I hope I will never see it again. Now I have drive more than 1000 kilometers and haven't encountered the issue again. Touch wood.

I investigated the old TB unit using a laboratory power supply and an oscilloscope. Having no breakout cable set or female connector for the TB, I had to resort to some rather rudimentary tests by applying 5V supply and connecting the pins of the potentiometers in the TB connector to a storage oscilloscope using insulated wire clips. Moving the throttle flap by hand* in the TB I could follow the output voltages of the voltage dividers of the potentiometers one by one, and tried to observe any glitches, spikes or other irregularities of the voltages. For my surprise, I found no such thing. However, I could neither check the linearity of the potentiometers nor that the sum of their output is constant. In some cases, I had the impression that the linearity of potentiometer 1 at the end of the range (wide open) wasn't OK, it seemed to slightly decrease at the very end. However, I'm not sure this isn't due to being out of in the operational range of the TB, or due to my rudimentary measurement setup. (Fighting by one hand to turn the flap while trying to keep on all connections in place by the other hand.)

My analysis of these events is therefore rather ambiguous.
(1) It might be and it is very likely that the TB unit was indeed defective and it needed to be replaced.
(2) It might be that some of the pins in the connectors were slightly corroded, etc, and needed the cleanup.
(3) The combination of (1) and (2).
The fact that replacing the TB vastly reduced the occurrence of the problem and finally, I hope, eliminated it, makes me inclined to beleive in (1).
Also I can imagine that the ECU actually needs some time to recover from the fault. For example, if there is only a small faulty part on the metallic path of the potentiometer, the ECU can spot it by chance and go into limp mode for some predetermined length of time before trying to recover. If it experiences such inconsistency again in this period of time, then its timer might restart. This makes drawing strong conclusions very hard regarding the health of the TB unit, if it is in the initial stage of decomposition of the metallic path of the potentiometer and the ECU only rarely can spot the problem.

I hope I won't have any issues with this again.

*Do not move the mechanism of the flap in a TB unit if you want to reuse it ever. The gearing in the TB unit's housing has very high ratio and you can easily destroy the plastic toothed gears. If you want to investigate a possibly operational TB unit, you can rotate the flap by applying voltage to its electric motor. You will need surprisingly high current power supply, expect several amps or more. My small 1.5amp lab PSU couldn't do the job.
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