if you have pressurized air available, you might want to pump some into the spark plug hole so you can listen for where its going ( have that hole on compression stroke, use a piece of vacuum hose and poke it in the exhaust pipe to listen for leaky ex valve, poke it in the oil filler to listen for leak past rings or hole in piston, and in the intake ( with the breather hoses disconnected so as not to confuse wih crqancase leak ) to listen for intake valve leak.

This can help save some guesswork down the line
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale