Cooling system problem

[trainman]

If it helps, I'm having exactly the same issue. So far cap has been replaced, thermostat got done a while ago, engine and radiator have had a good flush.
I find that removing the overflow, opening the vent and returning the excess to the radiator about every 2-300km is enough to keep it happy. Clearly not a good long term option though, especially as the weather warms up.
I can't say I'm convinced in the merits of horizontal flow radiators either.

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