Arduino as uart ttl adapter?

I'm having trouble using my arduino pro micro just as simple uart ttl adapter. I want to connect to my LPG ECU as shown here. I connected GND to GND, RX to RX and TX to TX(also tried the other way around). It just won't work. Tried connecting reset to gnd as some other have done it but that causes the board to lose connection with the PC :confused: Not sure what to do

PS. Its setup as Leonardo in IDE

The linked project use an Arduino board with a USB2Serial adapter onboard (p.e. UNO, Mega2560, etc.) but the Micro has the USB built into the main processor (ATmega32U4). To use this Arduino for the purpose you need a simple sketch (you have to adapt the baud rate to your needs) on the board and you have to swap the RX/TX lines.

void setup() {
  while (! Serial);

void loop() {
  if (Serial.available()) {
  if (Serial1.available()) {

Eh, still doesnt work. Though when I connect TX to RX line directly and type something in serial monitor, it comes back so this makes me think that its working.