There's one place I know that has more ES9J4's in residence then FCF... The 406 Coupe owners club.
It turns out there are a lot of guys over there who have suffered from dashboard reading the wrong temperatures and have posted temperatures exactly like mine. They've worked out over there that the temperature sensor that feeds the gauge is a common failure. Over there it's almost regarded as a service item!
I've got a spare one on the V6 engine I've got up here so I went to have a look at it. I've taken it off so at some point I'll whip the one off the Activa and warm them in a pan of hot water and check the resistances. Annoyingly though, the sensor is NFP from Citroen!! Argh! When it was last available it was less then £20, so not really an expensive fix either! I wish I'd have thought about it when the engine was on the stand last year though
Last week when I went to the Activa the low coolant alarm light was on, upon checking it, the level was low but the bottle wasn't empty. I've since done about 30 miles in it and again tonight the level was low. I was pretty annoyed about it so ended up not looking at it last week. Tonight when I went looking for that sensor I saw a blue puddle on the top of the gearbox and a jubilee clip that was covered in white furry stuff. I managed to get a couple of turns on the jubilee clip so will keep an eye on it for level. Last week was the first time since the engine swap I've had to put any coolant in it so it must have just been working itself loose.
When the winter is here I might take the inlet off it and check the cam boxes have stayed sealed properly and do the sensor and check that pipe again.
David.