I have used this compression tester on my w115.
12 Piece Diesel Engine Compression Tester
It is not a quality device, but is good enough for getting a general sense of overall engine health.

I would recommend you do the compression test sooner, rather than later. It is fairly cheap and provides key information to solving your problem.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).