Well I might as well jump in here since this is a very similar issue I am having.
Basically, I have a 2002 focus so blacktop zetec 1.8 and temperature gauge isn't working...well when I say not working I'll explain below. It's had a brand new temp sender by the way and it still does the same thing.
This is totally random behaviour by the way, no pattern or anything just has a mind of it's own.
SCENARIO 1:
Go into car, ignition on, wait for pump, lights etc and the gauge will not move. It's supposed to make it's way up towards the small white block on the focus temp gauge (cold) so move about 2mm or so from it's rest position. Most of the time it does this and will not read for the entire journey. HOWEVER, if I then am driving for say 10-15mins, if I turn ignition off and restart the car, it goes up to halfway, where it should be.
SCENARIO 2:
Go into car, wait for pump, lights etc, and the gauge moves, as it is supposed to the white block at the beginning of the gauge (cold). It will then either stick there and refuse to read for the entire journey OR will read up to a certain point which can be ANYWHERE in between cold and halfway. So will be reading and moving up as the car warms up but get stuck when it feels like it. Again if I turn ignition off and restart the car, it will read as it should and be at halfway.
SCENARIO 3:
Car is fully warmed up and I am driving around. I come to park, leave the car, do what I need to do and get back to it within say 30mins-1hr. So temperature of engine obviously isn't at normal operating temp yet. So when I get into the car it should show somewhere between halfway and cold, which it does and ALWAYS does. The temperature gauge never refuses to stay stuck on its rest position if the engine is lukewarm. Then it will either begin reading and get stuck like scenario 2 or will read until halfway or will remain where it was when I started the car i.e. whatever it read when the ignition was first turned on and will stay there. If I turn ignition off and restart it will read halfway.
So............WHAT THE FUCK IS UP WITH IT???
Even when it's working properly I assume it is lying because it has been playing up for so long I just have lost ALL faith in it. I now NEVER trust it. I know it isn't the most accurate way of measuring coolant/head temperature but I have no way via means of a gauge of an accurate reading unless I plumb an aftermarket one in which I really do not want to do.
Help much appreciated.