only runs when starter engaged
 

s5 t2 rebuild and I finally got it to start as long as the starter is turning. As soon as I let the key go, the car shuts off. I'm thinking ECU cause I can't figure out what's on when the key is in start except for the starter. Any ideas?

only runs when starter engaged
 

This is a long shot, but what are the chances the ignition harnesses are swapped? They're not dummy plugged. I would reference the FSM wiring at the ignition and verify with a multimeter.

I didn't do anything with the ignition harness during the rebuild. It was running (till it blew up) before the rebuild. I was looking at the FSM and noticed inside the main relay is a holding circuit, the output of which goes to the ignitors and the ECU. I'm wondering if the holding circuit failed, it won't hold position when the key goes back to accessory and the output circuit open. I think one way to check it would be to turn the key to the on position and spin the motor by hand and see if the plugs spark.

Jumper the fuel check connector and try to start the car in the normal fashion.

Tried that. Getting plenty of gas...to the point of flooding

Near the leading coil is a Green check connector with a few wires. One wire is Black/White which should have 12 volt power w/key to on. This wire powers the ECU. From a non-electrical viewpoint, a vacuum leak could be your issue.

take the ignition switch out and clean the contacts inside

I can't find the check connector your talking about. Is it the same one that you connect the lights to adjust the TPS. I've going over the vacuum lines again and all of them are fine. I'm digging through my box of parts to see if I have another main relay. If I'm reading the wiring diagram right, the voltage your talking about on the B/W wire should be coming from that relay. When you turn the key from on to start, it looks like it should energize a holding circuit inside that relay which will hold the voltage once the key goes from start back to on.

The TPS check connector is on the passenger side. The lead coil is on the driver's side. The TPS connector also has a B/W wire which is the same as the wire found in the Green check connector. And you are correct in that the relay keeps power to both the B/W and B/Y wires w/key to on, start and the on position after the key springs back from the start position when the car is started..

how about this, if you jumper the fuel pump test connector do you hear the pump running with the key in the on position? you never elaborated on this, and it will tell me if the car is actually in the on position in run.

I Haven't tried that yet, but I have had the key on the one position and rotated the engine by hand and can hear the injectors clicking followed by the pump cycling.

oh ok, then likely a vacuum leak.