Alright I need some help on this.
I did a rebuild and the engine doesn't run. It's a 1988 S4 T2

Video:

Full throtle or not it doesn't change anything.
The only mod i did during rebuild is a new Racingbeat aluminium flywheel. Everything else is stock.
Compression is even and above 100psi everywhere.

 

I assume you've gone thru the obvious things, like making sure the MAP and MAF are plugged in securely? Throttle positions sensor and CAS sensor are calibrated correctly? Fuel pressure is good?

 

Is the green connector on the back of the water pump housing plugged in?

Causes this more often than you'd expect.

 

I've had this problem before, turned out to be the MAF. On S4s, it has a switch in it that kills power to the fuel pump if it doesn't detect that the MAF is seeing any air pass through. I don't remember if in my case, it was my MAF or the connector falling apart, but I bypassed the switch (spliced the wires together at the connector) and never had troubles after that.

Borrowing the words of satch from when he helped me figure out my problems many moons ago:

So if I'm remembering this correctly, there should be a yellow check connector near the passenger side shock tower, 2 pin with a hood over it, that you can jumper to test if this is your problem or not.

 

Agree with the previous, looks like the MAF. When key is in start the fuel pump runs, but afterwards only the MAF signal keeps it on.