now i still use arduino pcb board, so still use the voltage regulator from arduino.
i tried to use battery as power supply and use usb to ttl to connect it with PC, I only connected tx, rx and ground pins only, but the problem is still there. does adding MOV in VIN solve the problem?
conecting usb normaly was never a problem for me...
By good ground they mean good electrical connection between the car chasies and arduino ground,
Capacitors usually don't need protection diodes because they only conduct AC at specific frequency, smaller the capacitance higher the conduction frequency, capacitor is a high pass filter.
You put the capacitors between +5 and gnd, or between signal lines and gnd to ground high frequency noise
You put protection diodes like zener to prevent overvoltage in combination with a fuse, you can put a reverse diode across your voltage regulator (bit overkill), but you pretty much seem to be covered concerning those...
If you can't figure it out, scan your 5V and signal lines with a scope, and catch any transients, then determine the frequency and put a cap between to ground the transient, if there are different harmonics you need a cap for each prominent one(i think)... EMI can be tough to deal with, maybe search for a book about automotive EMI filtration and protection