If you were really measuring a true zero then you should not be able to feel anything sucking / flowing past your finger / hand whilst pressed up against the glow plug hole. (Er with all the glow plugs removed from the engine to help the starter and battery along)

I'd do this simple test first before zipping off for a leak down tester or pulling stuff to bits.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!