I'm not an EE but the whole capacitive probe setup seems sketchy to me.
You have a 5v square wave signal coming from the sender with limited or no shielding.  In some cases the sender is connected to a freq->voltage converter and then on to the engine monitor.  The converter is a circuit board in a plastic box with no shielding (and maybe no grounding) in my case.

I have the same behavior vis-a-vis the radio interference zeroing the readout.

EI says resistive probes wear out sooner but I wonder.   Seems like a more appropriate solution.

Anyone got experience with resistive probes?

Colyn