I just bought a float switch to monitor the level in the expansion tank. Thinking about the failure modes of the K series, I wonder if I wouldn't get an earlier warning with a pressure transducer. Pressure will rise linearly with engine temperature so monitor engine temperature which is easy because there is a coolant temperature sensor to sniff and then once the temperature is at thermostat temperature, test the pressure in the expansion tank.
If the pressure rises suddenly over the blow off valve pressure due to a catastrophic HG failure, you should know about it and by then, all bets are off but there will normally be a fairly constant pressure in the system. In the event of a hose or radiator failure, a sudden pressure drop would be registered before the level in the tank fell. A one bar transducer and a bit of silicon sorcery would suffice and the whole could be rigged up for about £20. The hardest thing will be providing a display of low coolant pressure but I think I'll take a look at the oil temperature gauge and either put a red LED in it or replace the gauge completely with a small screen carrying both readouts which can change colour if out of limits or even give the effect of a big "game over" light. With a bit of guile, it could even be made to look like the factory dial, had I the patience to programme the graphics.
