This Porsche Cayenne 3.6 V6 is an older car, but still in nice condition, with the issue of: “engine vibration after morning start”. It’s useful to mention here that cold start means 28 or more degrees Celsius, as this is in southeast Asia tropical climate.
There was no DTC and no warning light, but the vibration was easy to feel, simply by sitting in the vehicle and keeping it idle after cold start. There was also noticeable clicking or knocking noise, probably coming from the cylinder head area (most likely from the valvetrain).
From monitoring the misfire count with a diagnostic tester, it became apparent that the cylinder 4 was having misfire counts which were slowly disappearing as the engine warmed up, along with the clicking noise which also almost disappeared. However, the idle was not perfectly smooth even at engine operation temperature. On higher RPM and under the load, there was nothing noticeable at either temperature.
Where do we start? So far, we have confirmed the knocking noise from cylinder head area and cylinder 4 misfire counter. Given there should be not any noise, the next logical step would be to remove the valve cover and visually inspect, if we do not have Picoscope.
So, lets confirm what cylinder is really misfiring, where the noise comes from and verify the in-cylinder compression pressure and valve timing and here we are.
The first step is to confirm which cylinder is misfiring by analysing the crankshaft speed from hall sensor signal with sync by cylinder 1 ignition coil. All is easy to reach, backpin on engine ECU and ignition coil Cyl 1.
Here is the result; the firing order is 1-5-3-6-2-4, so the cylinder 4 fires just before cylinder 1 fires. The misfires caused by cylinder 4 have been confirmed.
Next is to record the in-cylinder pressure waveform on affected cylinder 4 and any other good one, in our case cylinder 3 which does not fire close to misfiring cyl 4, avoiding double misfire.
Below is the result, with help of reference waveform function we can overlay both for easy comparison, focusing on valve timing or leaking signs.
The cylinder 4 has higher pressure rise around second TDC 180 degrees; this is likely due to both EX/IN valves being closed, or nearly closed, at the same time. It could be due to leaking hydraulic tappet or worn-out cam lobe. The exhaust valve opening and Intake valve closing time is not significantly different from good known cyl 3.
The next step is to bring in the Picoscope NVH to accurately locate the origin of the clicking noise. This is done by calculating the speed of sound in cylinder head material, and then, once we know the time difference (delay) between two accelerometers, calculating the location of the click/knock origin in reference to accelerometers. (I use this technique to locate squeaking or knocking on body/suspension of luxury cars as, where even tiny common click has to be located and where other techniques failed to locate it).
Now, we place the accelerometers outside of point of interest, on the front and back side of cylinder head. There are bolts which allow the magnets to be attached easily. We now make an artificial knock beyond front accelerometer using a screwdriver. The accelerometer closest to the knocking has picked up the vibration first, while the other accelerometer located at the end of the cylinder head picked the same signal with delay. Using the trigger, we have the knock recording where best to measure the time difference with Picoscope rulers.
The next step is to measure the distance between the two accelerometers with a tape measure. This will give us the speed of sound in meters per second.
Once we know the distance between the accelerometers and speed of the sound in the material, we just need start the engine and capture the click or knock, then measure the time difference between the two accelerometers signals and identify which accelerometer captured the “knock” signal first.
Distance between accelerometers: 51 cm
Time delay after screwdriver knock: 190 microseconds
Time delay when engine idle: 20 microseconds
Accelerometer B captured the idle “knock” first.
Results from the excel calculator:
22.8 cm from accelerometer B
28.2 cm from accelerometer A
Using the tape measure, we have marked the knock location related to our accelerometers; this is perfectly aligned with suspected cylinder 4.
Looking at the highest knock amplitude, when we measue the angle related to cylinder 4 and place the rulers to idle waveform, it corresponds with angle where the intake valve is closing.
I think this is a good demonstration of the powerful NVH tool combined with pressure transducer. Picoscope effectively functions as rulers, math channels and Excel. Of course, it is possible to calculate the “knock” source with pen and paper, but why not to utilise this when we already have laptop on the bench?
