ocles_inc

I have two techs. One of them diagnosed and fixed some issues with the fuel supply on my silver 2000 Audi A6 4.2 V8. That included correcting the fact that the fuel lines had been switched around. Even so, the fuel pump in the car still had a bad check valve, so a few weeks later (as in today) I asked my second tech to swap out the fuel pump. He did. Afterwards, the car would not start.

I suspected that he had switched the fuel lines around but he seemed pretty sure he hadn't.

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 tells 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? Well, kinda. 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.

I have a similar 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. My tech corrected this, and then the silver car started and ran well.

Key point: fuel flows through the fuel pressure regulator (the brass-colored cylinder atop the fuel rail at the rear passenger side) on its way to the tank ... not from from the tank.

Tip #1: As to the cans we used: these are the kind in which Campbells soup is sold at places like Walgreens's for around $1 each. They work well as temporary gas containers for testing. I like my test containers to be metal so that static electricity is not a risk.

Tip #2: I found it impossibly hard to get the cans titled enough to retain fuel, unless the passenger side engine cover was first removed.

I hope this helps someone who's puzzled about this same issue. :-)

~Tanya