The Factory Temp Gauge on my third gen is acting 'weird', it always used to run at the low end (below a quarter mark) I changed my manifold/carby earlier this year and it appeared to run a little warmer. I then put in a 160 degree Hi-flo thermostat and the gauge still showed a bit higher. A couple of weeks ago I came home from work (the temp here at this time of year is quite cool) and the gauge was showing very hot and the engine was missing, panic !, thought I'd done a head gasket. Checked compression, all good, changed one spark plug, miss is gone but temp still showing high. To be safe I pulled the radiator out to get it checked, while it was out I decided to do the heater core as it had started to leak. This was more difficult than normal due to the RHD conversion. Anyway heater fixed, dash re-fitted, new radiator fitted after waiting a week for a core. Car is running at a good temp as I can place my hands on both ends of the radiator without getting burnt, gauge still showing high temp.
This is the weird bit, noticed that with ignition 'off' needle is near the top of gauge (I'm sure it used to stay at the bottom) then if you put on the headlights the needle jumps up serveral mm. I disconnected the wire from the sender and it still sits at the high end, today I stopped to get fuel and the needle was around the 240 mark, when I restarted the car it was 'Maxxed' out (still not connected). Do I have some sort of earth or power leakage problem to the gauge cluster ?, evey other gauge is working perfectly ! I thought of changing the sender but even that is tricky, 40 year old motor in a 25 year old car, how do I know the sender is the right one for this gauge ?
Besides it's still not connected.
Thanks, confused Aussie.

 

Check the sensor with an ohm meter. The resistance vs. temperature is fairly universal.

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