This is sounding more and more like a head/head gasket problem,
The high pressure in the radiator is the key.
In a couple of earlier cars, I had head gasket leakage. The symptoms of this aer that the coolant system gets pressurised and loses coolant, The coolant changes colour (mine started green and ended up almost black) and eventually, you lose the water pump and other aluminum due to the reaction with the combustion gases in the water. Eventually the head gasket deteriorated so much that I couldn't go 10 km on it.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.