Vehicle details: Audi A6
Year: 2006
Symptom: Engine stalling,
Poor starting

Audi A6 | Engine cutting out

The car came to us from a transmission repair center. The problem was an occasional engine cut–out during cold cranking. It is difficult for technicians to investigate this particular problem due to it occurring during cold cranking. During cold cranking the air/fuel ratio is not able to be controlled. We made a list of possible causes:

  1. Mechanical problem of the engine.
  2. Faulty exhaust gas recirculation (EGR) Solenoid.
  3. Faulty purge control solenoid valve (PCSV).
  4. Faulty air flow sensor (AFS).

Testing this problem using the “self–diagnosis” function in the serial diagnostic tool did not reveal a problem. Using the PicoDiagnostics cylinder balance test also did not work this time (as there was no cylinder showing low piston speed although there was a misfire in a specific cylinder).

We decided to use the PicoDiagnostics compression test to check for a mechanical problem in the engine.

PicoDiagnostics compression test

Figure 1: PicoDiagnostics compression test

In the above screen capture you will see that PicoDiagnostics showed that cylinders B, D, and E (which we will call Bank 2) were at 81%, 80%, and 80% compression, while cylinders A, C, and E (which we will call Bank 1) showed all cylinders at 100% compression. If the compression leakage had occurred from a valve or piston ring we would not see this level of compression from the Bank 2 cylinders on the engine.

Now we could narrow down the problem for the timing issue. We compared camshaft sensor of Bank 1 cylinders and Bank 2 cylinders, and the crank angle sensor.

 Using PicoScope to diagnose the timing issue

Figure 2: Using PicoScope to diagnose the timing issue

As you can see from figure 2 above, we have measured:

  • Channel A (Blue): Crank angle sensor
  • Channel B (Red): Bank 2 camshaft sensor
  • Channel C (Green): Bank 1 camshaft sensor

When we see the waveform above we can tell that Bank 2 camshaft sensor is 7 teeth behind the Bank 1 camshaft sensor.

It is now clear to us that somebody has completed incorrect timing work while repairing this vehicle. The ECU had adjusted excessively well which is why there seemed to be no acceleration problems. This car had been moving from workshop to workshop to find out the real problem, which we found with our PicoScope.

It may have been easier to detect this problem if the previous garage (transmission repair center) had explained to our workshop that the customer had also felt slow transmission change (due to a low engine output), but even without this information we were still able to successfully diagnose the problem on this vehicle, thanks to our PicoScope.

Download the PicoScope data files of the waveforms featured in this article:

psdata 10 MB


11 comments | Add comment

David Barzelay - Israel
September 27 2015

Bravo Pico scope the best scope in the world

David Barzelay-Israel
September 27 2015

I am thrilled every time by the efficiency of a charming Picoscope tools

Dwight G
October 13 2014

When the temp decreases to fall temp the vehicle will not start (elevtrical.  During warm weather car works perfectly The battery is new.  It feels like the motor uses power only from the battery and does not charge the battery or allow the battery to charge It was fine all summer but is faltering now in cooler weather as it did last year.

Prof. Guillén
August 04 2012

gracias por sus datos son muy interesantes, quisiera apoyar a este foro pero mi problema es que no hablo el idioma ingles, y eso me tiene detenido en mis comentarios un saludo y mis mejores deseos para todos ustedes.

August 02 2012

This is call positive criticism first test vacuum then running compression test. That problem was that the transmission shop turned engine backwards and jump timing belt

July 31 2012

brilliant diagnostics,well done and thanks for sharing.excellent use of a picoscope that tells no lies only what it is seeing and used by someone that is switched on

July 31 2012

great work ... can you please tell me if you did a vacuum test before and after and what the vacuum was before the repair just thinking if the very basic done like checking vacuum before getting out scan tool and pico scope because most times you can see lower vacuum when timing is out ..

July 30 2012

Nice demonstration thank you for sharing

July 30 2012

Well done,very good explains ,but I think if the timing were out that far you should get a fault code A cam sensor implausible signal or faulty ex. I have seen this fault many time .
All brand

David Paterson
July 30 2012

This is another great example of both the use of Pico, and an effort by someone to show the advantages of the scope. May I be slightly pedantic to start with?

There is a slight contadiction, or wrong choice of description if I may say so, as during cranking - hot or cold - the engine hasn’t actually started! In my book, “Cut-out” would be associated with an engine which has already stopped cranking; has started, and then stopped.

That out of the way, it was good to see that a list of possible causes was considered. This is most important and should if possible include the maximum mount of information from both the customer and anyone else who has had a play beforehand. The latter is always more difficult and often ‘fragile’ since since it often represents a failure of someone else to find the fault, meaning that they are not going to be too keen to admit the fact; nor to assist!

In this case, an important point noted early on was that a serial diagnosis with a scan tool revealed ‘sweet fanny Adams.’ The point here is that this is often the case; but more importantly, even if a scan tool does reveal a code - or several codes - it is often difficult if not impossible to be SURE of the problem, or the cause of the problem. It’s the latter we must always focus on, and for that we need some sound and logical proof. This basically what the scope allows us to do: to prove a suggested or suspected problem.

All too often we tend to rely on the scan tool, and that either through lack of understanding of how it communicates with the various systems, or even how a system or group of systems communicate. We need to understand strategies of systems; how and when they interrelate, as well how they don’t or can’t because of a lack of infoirmation; a lack of communication; wrong or suspect information, etc. Most of this information is coming from various sensors and often shared by numerous systems; themselves usually linked by a communciation Bus,which may or may not be working at the time of testing. Not always easy!

Often we are not all sure what the scan tool is trying to tell us, but the oscilloscope has the ability to ‘get into’ a component and to confirm whether or not it is performing as expected. In other words, we can’t interpret a fault indicated by a scan tool if it gives “wishy washy” information. We simply can’t prove that the information is correct. Often, we either gzet nothing - or we get a host of fault codes. The latter are often ‘bye-products” of a single problem.However, with or without codes present, the oscilloscope gives us the opportunity to look at what is happenning “now” - not a few milliseconds ago, which in todays vehicles is like reading an account of World War II story. Live Data with a scan tool is never ‘live.’ The oscilloscope DOES give LIVE data. The scan tool conveys information seen by the on board computer, and stored in it’s memory. This information is thrown ‘randomly’

Ricardo Leon Sandino Ll.
July 30 2012

Excellent diagnostic problem. Congratulations.

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Case study: Engine cutting out