Firstly, the symptoms of a signal ground loop:

1) Inconsistent fuel mixtures, possibly leading to misfires, not being able to rev past a certain RPM
2) On closer inspection, you will see that the analog sensor readings, especially coolant temperature, will be jumping all over the place
3) The problem will be less severe at idle (you may not even notice it, except that the coolant temperature will jump around by 2-3 degrees), but when you get up to higher injector duty cycles (full throttle at higher RPM), the problem will become more severe

These are normally indicative of a ground loop. Looking at the wiring diagram for any of our ECUs (except plugins), you will see that all of the sensor grounds (CAS GND, TPS GND and MAP GND) DO NOT connect to the engine block or the chassis; they only go to the sensors. This is important; if you connect any of these signal grounds to the engine block then you are almost guaranteed to have the problems above.