Unstable engine

[Alexandr]

Hello, friends.
Car Range Rover car with a TDV8 4.4L engine has a loss of power. Compression measurement for this engine is too complicated! Therefore, we decided to use the Picoscope Diagnostics. Found a lack of compression in one of the 8 cylinders. Since we did not have a pressure sensor in the kit, we decided to measure the efficiency of the cylinders by the current of the starter, but we ran into a problem. When the PСV is turned off, the engine control unit does not provide pulses to the fuel injectors! Accordingly, there is no way to determine on the waveform where 1 cylinder.
The next step to determine the first cylinder was a connection to the camshaft position sensor, and it turned out that the signal from the camshaft position sensor has a constant signal similar to duty cycle, which also makes it impossible to identify 1 cylinder.
I had to partially disassemble the motor and measure the compression through the glow plugs. In cylinder 3, compression is completely absent!
On the attached files you can see how, when the engine is idling, the oscillogram greatly fails on the 3rd cylinder, and then the rise follows.
please tell me, how does this engine identify the operation of the cylinders by the camshaft sensor?
Thanks!

[ben.martins]

Hello Alexander,

Thank you for your post and I'm sure there will be others along to help you with this problem.

You've already done a lot of the hard work already and great to see the use of the math channel to reveal to the changes in crankshaft speed and as you've seen, there is a slow down related to cylinder 3.

Most modern camshaft sensors will have a 720 degree, repeating pattern in order to determine cylinder position. I might be checking my connections before determining that this is indeed the camshaft sensor. I've made mistakes in the past where I thought I was on the camshaft sensor but it was an oil control valve for a camshaft dephaser. One quick way, if you have a scan tool to hand, would be to disconnect the sensor you have connected to and see what fault code comes back. If not, I would be looking for another sensor with a different pattern.

I understand the you also were looking at relative compression testing with injection disabled. As you've found, because the injection signal is no longer present we don't have a reference to a cylinder then we can't determine the cylinders using the firing order. The way to do this is using the camshaft sensor. We made a video of this technique which can be found here - https://youtu.be/EsJBOFzETjQ?t=1582. This was the first Practical Pico session we ever did and we were still experimenting with how best to record them going forward. For this reason, the quality isn't the best but hopefully the content is useful.

I hope this helps and please let us know how you get on.

Kind regards

Ben

[Alexandr]

Hello Ben.
Thank you for the answer and for the link to the explanatory video!
Before connecting to the camshaft sensor, the electrical circuit for this vehicle was carefully studied. The green signal is exactly the signal from the camshaft.
After a little reflection, I came to the conclusion that to determine the first cylinder, the engine control unit relies on a low signal from the camshaft sensor and the subsequent signal from the crankshaft position sensor.
For the full working cycle of this engine (720 degrees), the camshaft signal has 4 low levels and 3 high.
It can be seen from the attached diagram that fuel injection into 1 cylinder always occurs when the signal from the camshaft is low. Thus, the top dead center 1 of the cylinder in the compression stroke will always be at a low signal level from the camshaft position sensor.
Primitive of course, but it seems to be true
Sincerely, Alexander.