[Signman]

With EFI ground reference for each sensor should be relative to it's location due to possible voltage flowing through the ground circuit from leakage of other systems same for data logging sensors they are looking for 0-5 volt input; slight variation can mean a lot..
Bad grounds show up as erratic sensor data also look for electrical noise which can influence wiring not shielded.
Shielded wiring should always be grounded to the chassis to battery. Don't depend on engine and chassis bolts without checking with a meter, gaskets and other products can act as an electrical insulator.