Finally I filled up the tank and got record FE, unfortunately it was the wrong end of the spectrum (9.0 lhkm or 26.1 MPG)
On my defence, the scrapcento took part in a rally and had 2 long visits at the mechanics' afterwards, with lots of idling. During the second visit it spit out a core plug again, which had been fitted incorrectly (probably the same plug the first mechanic replaced, but I'm not 100% sure).
As for ecomods, I replaced the coolant temperature sensor (it had most probably failed, leading to the head gasket problems) to one with different characteristics, so that I can use it with a temp gauge (previous one was for overheating warning light). I also got a temp gauge. Only minor issue is that the needle barely shows anything when engine is at operating temp (shows 40 deg C...) so I'm going back to the mechanic once more
The other "ecomod" is a short-shift mechanism and moving the gear lever higher, so that it's near the steering wheel for more convenient P&G
