When I purchased my NA S4, it started but ran very poorly. The compression is consistent on all faces, though slightly low. I decided to replace all of the intake gaskets, all vacuum lines & plugs, injector grommets, etc. Basically everything rubber on the intake. The emissions are removed courtesy of PO.

While doing this, I also replaced the fuel pump, filter, and send and return lines. I'm now running a deatschwerks pump & filter and -6 hose. The injectors, rail, and FPR are still stock. It has new plugs and wires. Currently the (clogged) cat is removed.

After doing this, it ran quite well, idled properly, and started surprisingly easily given the roughly 50F temperatures. After taking it around the block, the engine fuse blew. New fuse blew after running for only a few seconds. I couldn't find anything wrong with the wiring after the fuse, and according to the multimeter nothing seemed to be shorted. I decided to cut the fuel pump power line directly after the relay and run it straight back to the fuel pump.

After doing so, the fuse no longer blows, but the motor acts as if it has a huge vacuum leak, starting and then immediately dying. It clears up around 3-4k rpm, though with a pretty rough misfire. I checked and re-checked my vacuum lines, all seem to be fine. I have a hard time thinking it's a vacuum leak given how well it ran and drove before the fuse issue. Fuel check connector makes no difference, and I can hear the fuel pump operating normally.

Any ideas?

 

I would check your grounds. It sounds alot like there's not enough fuel getting to the injectors due to the pump not spinning fast enough. Check your resistances rather than just continuity. Also worth double checking the fuel filter and see if its on backwards.

 

click if you want to see the ground locations