[Andy24601]

The expansion tank cap 'lets go' and squeezes coolant out after long, higher speed journeys. I have replaced the cap with a decent, BMW one (febi) and then replaced it again. I have also replaced the thermostat housing and radiator.
So - I fitted an aftermarket temp sensor in the top radiator hose because of (what I thought) was overheating issues.
Using the onboard diagnostics, the engine seems to run at a temperature of around 88 to 91 degrees, but the top hose aftermarket sensor shows a temperature of somewhere between 100 and 105 degrees.
So; should the top hose be hotter than the engine block? Why is the pressure in the expansion tank causing the cap to leak?
There is no evidence of head gasket failure (mayonnaise etc.) Could it be an airlock? Any thoughts or suggestions gratefully welcome