I got a 87 GXL with a S5 block with all of my S4 stuff on it. At first I had no spark but I fixed all that. I got the car to start. But when it starts the rpms go to 3,000 then wont idle. I can catch the revs when it first starts and can keep it going but only over 3,000 rpms anything under that and it dies.

 

Since you haven't had a response yet I'll mention a couple of general things. If you can get an automotive stethoscope probe down there far enough have someone listen for the operating sound of the primary injectors. You may be only able to get to one of them. The 6 port actuators work differently on s4 and 5. You might want to check to see if they are in the open position. Don't know if this would cause it not to idle or not but it is an S4/5 difference.