Engine temp drop, coolant issue

Lofidelity

Has anybody experienced an occasional coolant issue where the heater blows cold, engine temperature drops way down and a peek under the bonnet shows the coolant level in the reservoir has gone well above max? Most of the time there is no problem. Coolant was leaking around the reservoir before so it looks like the coolant pressure is too high. Two garages have tried and failed to fix it using coolant flush, checking coolant leaks, and even an EGR cooler replacement but it still happens. They said the thermostat is working ok too. Its a diesel, doing city driving. Thank you.

CoBo55

Is the radiator fan working? What car is it?

Lofidelity

https://www.boards.ie/discussion/comment/118666955#Comment_118666955

Not sure, i suppose i should hear it at idle. One thing it does though is rev slightly higher as a DPF regen happens. This happens most days which i think is more than usual.

Its a 10 year old Transit 2.2, 100bhp 6 speed, doing stop start city driving for about 20 hours a week.

CoBo55

Had a similar problem with a connect, fan switch was faulty, not hard to spot as it was falling to bits ☺️. Engine was boiling over and trying to push the coolant out the expansion bottle. When it's doing the regen is the fan running too?