While looking at the water temp connection wire I accidentally pulled it off the connection on the engine so it must have been loose. Putting it back on and restarting, there were values for a while and then back to zero until I stopped the engine.
So I'm guessing it is engine vibration coupled with a loose connection. I have since crimped the spade terminals and the fit is much tighter but I wonder if there are other things I should be doing to make sure of a solid and reliable connection. Will test the engine again this weekend.
Cheers
Paul