I don't think you're gonna be able to pin point problems as precisely as you hope - nice experiment if you can - but with the information you already have it seems to me to be time to pull it to pieces and use the old MK1 eyeballs

For that generation of Mercedes engines I suspect corrosion around the head gasket area to be present

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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!