The first symptoms would be head gasket symptoms. After the belts are on and the cooling system is properly filled and bled, the things to watch for would be coolant loss with no apparent leaks, overpressurization of the cooling system, whitish "smoke" from the exhaust, possibly harder starting and poor idle, water in the oil (milky appearance on dipstick and inside filler cap), oil in the water, continuing overheating with no apparent cause... Stuff like that.

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1987 W201 190D