The 75 1.8 on FI will start and runs, but when the idle drops when the auxiliary air valve closes the tick over revs drop straight to zero.
The tick over adjustment screw on the throttle body makes no difference.
The ECU has been change as well as the airflow meter and the Head temperature sensor II is new.
The filler cap two seals are new.

Any ideas on what is causing the 1.8 not to idle?

The 1.8 engine especially doesn’t like vacuum leaks. Check all your hoses. I even replaced my air intake manifold gaskets to solve my leak.

Also replace your injectiors seals too. Also check throttle body seal.

Good luck.

https://www.pelicanparts.com/More_Info/0221...&SVSVSI=979

https://shop.914rubber.com/18-Throttle-body...ke-914TBG18.htm