Good afternoon.
The owner contacted me with a complaint about the unstable idling of the engine. Range Rover LG 405 - 4.4 L - V8 - Disel. When trying to fix the problem, the owner replaced the 8-cylinder fuel injector, but the problem persisted. Measurement of compression by the standard method is too time-consuming process, and I do not have a special device at the moment, so Picoscope was used. On one of the attached files, when the engine is idling, you can see how the crankshaft acceleration steadily decreases in the 8 cylinder. I assumed that the low compression in the 8th cylinder was to blame, but when measuring the compression, using a current clamp, it turned out that the compression in the 8th cylinder was in perfect order.
When cranking the crankshaft with the starter, the current clamp shows a decrease in compression in the 3 cylinder.
When the engine is idling, the math channel indicates a malfunction in cylinder number 8.
Let me remind you that fuel injector number 8 is new!
I suppose that the problem still lies in a malfunction of the mechanical part of the engine, but I am confused by the fact that the compression is low in the 3 cylinder, and idle failures are observed in the 8!
Maybe I misunderstand the measurements taken with Picoscope?

The picoscope relative compression has no means to know which cylinder has low pressure. Numbering is arbritrary.

Second, you’re confused about missing teeths at crankshaft sensor signal with a genuine speed drop of the engine; is not the case.