Porsche Cayenne V6 | Porsche Cayenne misfire DTC and engine warning light mystery

There are situations where all diagnostic processes taught by mainstream training courses won’t help or can lead in the wrong direction. I have one such story to share, hopefully, it will be an inspirational diagnostic process.

The background:

The diagnostic test always reads the same DTC, one or more cylinders misfire - sometimes all of the cylinders misfire - and the DTC has a very high misfire counter. When previously repairing the vehicle, all related parts; coils, spark plugs, injectors, sensors, and fuel delivery were replaced. Each time the test drive was carried out, the vehicle passed and was returned to the customer. However, the vehicle would return a few days or weeks later with the same DTC issues.

First diagnostic approach with a different strategy:

Given the amount of repairs and installation of new parts, it was clear that a different strategy was needed, the question was where to start? As always, it is best to delete the history and information previously given, as it can be misleading. The previous technicians had good experience in the industry and this is a good enough reason to assume they were misled by some tricky information, most likely, unrelated to the newly installed parts.

The first step is to experience the warning light coming on during a test drive, easy to say. However, after a few test drives, the warning light did not appear and the DTC was not triggered (no wonder it always passed Quality Control). Whilst attempting to simulate the customers' driving style, the engine warning light appeared. At 1200 rpm, around 90 kph, I felt the engine misfire and the car began vibrating for roughly 3 seconds (While driving the car was fine, it felt like I was traveling along rumble strips on the road).

After reviewing the results from the test drive, I saw the same DTC issues arising. I cleared the fault memory and carried out another test drive, trying to replicate the conditions in which the fault initially appeared. The next step in the process would be to use the PicoScope NVH to diagnose the problem.

After repeating the process several times, using varying speeds and clearing the DTCs, I learnt that the vibrations were related to almost zero torque at around 1200 rpm from the second gear upwards. Meaning, that the PicoScope NVH would not reveal anything new.

New diagnostic plan:

I checked and found the Technical Product Information (TPI) for this vehicle, describing the vibration caused by the transfer box (additional gearbox responsible for 4x4 drive, central diff), but this was previously rectified and repaired.

Given this is a hybrid vehicle, anything on the powertrain can be responsible for the issue. The easiest way to find the fault is to monitor the rpm of the individual parts in the powertrain. There was also the question of whether it could be caused by the E-machine or the DC/AC converter, as the vehicle contains a Hybrid motor generator.

PicoScope connection and recording:

Let’s connect channel one to cylinder 4 ignition coil driver signal for sync (easiest to reach), then channel two to the engine rpm on the crankshaft sensor (G28) using pin S3 at the ECU socket. Channel three to one of the hall sensors of the E-machine (wire harness) and channel four to the HV current at orange cable from HV battery to DC/AC inverter using a current clamp.

Now I know how to replicate the malfunction, I need to carry out a test drive and record the data. When the probes are attached to the correct contact points, the PicoScope software will automatically detect the attachments and start producing data from the channels on the software screen. This process will run automatically, without any user input until the Stop lozenge is pressed. The user will have the opportunity to save the data to the memory bank and can access and review the data at a later date. Using the PicoScope software means that there is no need to have a second technician present to record the data when going for a test drive. 

First conclusion after data captured by Picoscope and analysed:

Fig.1 shows the point at which the vibration started.

In Fig.2, the vibration is visible after one engine revolution. The rpm of both the engine and E-machine rise and fall approximately two times between the spark command. This means that there is a double frequency concerning the engine cycle and the engine rpm. After reviewing the measurements Fig.3, we have achieved a frequency of around 15 Hz. 

This would explain the misfiring DTC, pointing to random cylinders or all of the cylinders with too many counts. The engine ECU identifies a misfire by analysing the crankshaft speed acceleration, which is caused by combustion pressure. This, in turn, accelerates the crankshaft. These sorts of vibrations confuse the ECU algorithm, and therefore the software will identify the actions as misfires.

I can replicate the misfire and record it as a waveform in PicoScope, and then all that is left to do is identify what part of the powertrain causes the 15 Hz frequency vibration.

Fig.4 is a simplified diagram of the powertrain, the parts are in order of the possible speed monitoring points.  

Engine (crank sensor), Hybrid clutch (no sensor), E-Machine (3x Hall sensor), Gearbox (1x input and 1x output sensor), Transfer box (not checked, unknown).

Recording additional data to support the deeper analysis:

The question is, where does the vibration come from? Is it the gearbox/transfer box side? or the engine? or is it the 3-phase electric motor/generator? or the DC/AC converter?  

To gain a better understanding of the issue, I carried out more test drives. I recorded the vibration, whilst also monitoring the gearbox input and output shaft speeds via corresponding sensors.

Determining the vibration origin:

Fig. 5 shows the engine speed frequency increasing and decreasing, around one engine revolution or 15 Hz. The E-machine displays the same frequency in addition to both transmission speed sensors.

However, whilst the engine speed increases when vibration is present, all of the other speed signals decrease. I determined the vibration differs by the opposite amplitude between the engine and the rest of the powertrain. The frequency is consistent in all four speed sensors, roughly one engine revolution, and at 15 Hz regardless of the vehicle speed or gear engaged.

In other words, the split or phase is between the engine and the E-machine, where the Hybrid clutch is located.

Let’s have a look on the Hybrid clutch:

Looking at Fig.6 from the parts catalogue, the clutch plate looks like a damper to me, something similar to a dual mass flywheel in terms of function.

So, there must be some kind of spring and shock-damping design in the clutch plate. With this in mind, assuming the damping is non-functional, the clutch plate would be just a spring and each spring has its own frequency, right? Previously, I observed the vibration is always in one range of engine 1200 rpm or 15 Hz frequency. It is not related to the gearbox output so therefore it is not caused by the gearbox or the transfer box. The logical diagnostic conclusion based on the data shown here, 15 Hz multiplied by 60 = 900, so the powertrain vibrates 900 times per minute and the rpm is 1200, it is close enough that the rpm could cause the clutch plate resonance, this would point to a faulty Hybrid clutch plate.

It means ordering a new hybrid clutch and handing the car over to the workshop for clutch removal.

Clutch plate physical inspection:

The conclusion was correct, the hybrid clutch plate damping was the fault. It was acting only as a spring. The new clutch rectified the complaint and the vehicle has been returned to the customer.

Thank you for reading this case study, I hope it inspires other Pico users to explore new PicoScope capabilities or help with decision-making to invest and add the PicoScope kits to your workshop tool list.

Thank you and credit to Roman Franta for providing this case study. For more information, check out the original post on the Pico Automotive forum.