If you had a hole in a Piston you would have extremely obvious poor runing Engine at all speeds.
With the Oil Fill Cap off every time that holed piston would come around you would get a burst of Air and Oil mist. The last Diesel Engine I saw with that condition and was about the same size as a Mercedes the Air-Oil-Mist chuffed out about 2 feet above the Engine.

The part of the Exhaust Manifold from that cylinder might be way colder than the others also.

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84 300D, 82 Volvo 244Gl Diesel