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Audi C5 A6 Quattro 4.2 V8 Direction of Fuel Flow
This article discusses the direction of fuel flow for the C5 Audi A6 Quattro 4.2 V8 as made from 1999 to 2004, with the main focus being on whether fuel flows in or out, via the fuel pressure regulator on the fuel rail.
Last Updated: 09/24/2017
Parts Group: Fuel pressure regulator
Even so, the fuel pump in our silver 2000 Audi A6 Quattro 4.2 V8 had a bad check valve, so we swapped out the fuel pump. Afterwards, the car would not start.
I suspected that we had switched the fuel lines around, so we tested. We undid the black fuel hose that is normally attached to the pressure regulator on top of the fuel rail, and I held a small metal can under the fuel hose and another can under the pressure regulator. When my tech turned the key, fuel spurted out of the hose. That told me the fuel lines were switched around because fuel is supposed to flow from the pressure regulator down the black hose and back into the tank.
How sure was I about that? Not 100%. Even so, that's how I understand Bosch L-Jetronic to work ... the fuel exits the fuel rail via the fuel pressure regulator. Was I sure that's the case with this engine? Actually, no.
We have a similar project car that runs well: a black 2000 Audi A6 4.2 V8. We repeated the test on that car, and I watched fuel spurt from the fuel pressure regulator. So, the black car fuel lines were correct whereas the silver car fuel lines were switched around. We corrected this, and then the silver car started and ran well.
Key point: fuel flows through the fuel pressure regulator on its way to the tank ... not from from the tank.