fast efi problem

Grounding the ECU at the battery is a bad thing!!!! A "single point ground" on the engine is correct. That puts all sensing references, in the same boat. By now it is possible the ECU circuitry is damaged. The ECU harness could shunt the starting current, it impressed with the starting voltage drop, when grounded at battery. Most CMOS circuity can only tolerate a maximum of 0.3V below ground, or above (5V)Vcc without damage. ECU internal ground circuity on PCB is not designed to shunt starting currents. Some may get by with good ground and battery near engine, but grounding at trunk battery is as bad as it gets! Grounding some things at battery, while other components are grounded at engine is very bad.

Wire routing is done in a way to minimize mutual coupling. Sensor wiring is kept separate where possible from injector, coil wiring and other switched loads. Avoid wiring loops where ignition cables electromagnetically induce into sensor wiring.