Quote:
is it possible for a mechanical temp gauge to go wacky?
Yes. The mechanical gauge is a Bourdon tube which is a flattened tube with one end sealed but free to move and the other end fixed to the gauge. It is formed to a shape looking like approx 3/4 of a circle. The sensor bulb is connected to the fixed end so when the heat causes the ether to expand the tube sees more pressure and tries to straighten out. Higher temp = higher pressure = straighter tube. The free end of the tube is connected to the needle via linkage and as the tube straightens it pulls the needle.
If the engine has overheated the pressure could have been high enough to permanently deform the tube and cause the needle to read high. Usually if that has happened the needle also reads wrong at the other end and permanently sits somewhere above the low mark. Conversely, if any of the ether has leaked out there will never be enough pressure to straighten the tube to the calibrated point and the gauge will read low.
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