You need a compression gauge ($30 at Harbor Freight), not a leak-down tester. The leak-down tester is useful for finding out WHERE your compression loss is. If you do a relative compression test and all of your cylinders are low (say 325PSI), but relatively even, the leak-down tester will tell you where the compression loss is, but not why your operating state is so rough.

The compression tester will tell you which cylinders are low, if you have a pair that are low (indicating a blown head gasket between adjacent cylinders), or if the mechanical health of the engine is ok and you need to look at fuelling and timing instead.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)