Originally Posted by jeremy
Will start investigating, it's a tailor made smiths from Europa spares, so Defo 12 volt. I have access to a nifty temperature reading gun, which will read temp if block , radiator injectors and expansion bottle... So hope that the temp of 86 is correct and not 120. .
Why do you believe your gauge is 12 volt?
The original Smiths gauge was 10 volt, and the one I bought from Merlin Motorsport was 10 volt.
What sender are you using?
I was not happy with my temperature readings either (same M50B25 as yourself). I didn't like the Smiths sender having to be fitted into an adaptor collar to fit into the M50 head port: nor did I like the 10 volt requirement.
In the end I sent my standard BMW VDO sender to Caerbont Automotive (manufacturers of the current Smiths gauges) and had them make me a Smiths Classic faced gauge but with programmable logic in the back which they calibrated at every 2 degrees in a bath of oil to my BMW VDO sender.
I now have a gauge which tells me exactly what the temperature in the head is.
If you are interested they gave it a part number, so you may be able to simply order one off the shelf?
My gauge shows around 90C when driving steadily below 60mph, but rises to 95-100C when pushing along at 75-80mph for prolonged periods on a motorway - (remember water's boiling point is around 110C when the system is pressurised at 13lbs).
Out of interest, I fitted a Ford thermostatic fan switch in my radiator, which is set to come on at 92C and go off at 88C.
Whilst driving over to Wales recently on a 600 mile round trip the fan only cut in when the temp gauge showed 95-100. Initially I was worried by this but then rationalised that the radiator temp, mid way down , will be cooler than the head temperature. Secondly I don't want the fan to be coming on whilst driving at reasonable speeds, but only when in slow traffic or stationary................. or thrashing it on a motorway!).
That said I still nervously wait for the fan to kick in when the temp gauge starts to rise when sitting at traffic lights on a hot day - so far it has done so every time without any sign of boiling.
One last point - I did have a problem with losing water over long distances from the header tank over flow - until I replaced the pressure cap with a new one (13lb). Since then I've not lost any water, despite pushing on at 75-80mph continuously for around 2 hours along the A14/M6/M54 on a very hot day.