- Vehicle: Audi A6
- Year: 2006
- Sympton: Occasional engine cut–out problem during cold cranking.
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:
- Mechanical problem of the engine.
- Faulty exhaust gas recirculation (EGR) Solenoid.
- Faulty purge control solenoid valve (PCSV).
- 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.
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.
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 ECUhad 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.
|
