The vehicle will have to be cranked a few times to get the bad fuel out of the far end of the fuel delivery systemif you have a safe way of collecting a sample from the fuel rail (like a fuel rail test port with a green cap) you can take samples there by switching the ignition from off to run several times until the fuel is clean.
check the fuel regulator. check the fuel regulator. check the fuel regulator.
It could be your fuel filter needs to be changed or there may be water or trash in your fuel tank. I changed drained the gas tank on my 98 silverado and that did the trick for me.
Have you checked to see if the spark plugs are firing?
Check to see whats missing when it won't start. Needs fuel & gas to run.
If you are certain it is low fuel pressure and you've changed the pump and filter the next thing would be the fuel pressure regulator. Also check to make sure none of the fuel lines are crushed closed.
fuel pressure regulator located top of motor
is the fuel filter istalled correctly in from fuel line out to engine. PS won't be the first person to install it backwards
is your a4 diesel ?
You probably have a clogged fuel line or fuel filter. I'm not sure what you're referring to when you say fuel filter regulator but I'm guessing you mean pressure regulator for the TBI. Tear down TBI and give it a good cleaning carefull not to scratch or gouge anything.
check fuel pressure regulator
Start by having the fuel filter changed.
A damaged fuel pressure regulator can cause a car not to start.