Ive got and 86 Fc and an 88 Fc.88 not running or it will start but not stay running.I have an 86 that runs great but is ugly as shat lol.Easy swap or is anything that will cause me problems.The 86 motor is to go into the 88.

 

Easy swap. Motors are the same.

 

Yes, It is a straightforward swap. 86-88 were all S4. The only difference in FC engines from year to year is when you go to the S5 (89-91). Putting an 86 engine into an 89-91 is a bit more involved, but can be done easy enough too.

 

You will need to use the injectors from the 88. In mid 87 they went to high impeedence injectors. The injector clips and ecu/engine harness have slight variations due to that.