Okay... so yesterday I changed the HP fuel sensor for this replacement...

Audi > A4/S4 > B7 A4 > Engine Performance > 2.0T > Fuel System

The replacement item was shorter than the original which seemed a little odd but it was the one listed for my car.

Car seemed okay for the first couple of miles but then when I really booted it, the problems returned worse than ever. Now I get the same engine code only now it puts the CEL on. In running VCDS (see log snippet below with added fuel pressure delta column), the HP fuel sensor reports no more than 90 bar (actual) on the HP fuel rail despite the specified being around 110 bar. For the range 0 - 90 bar, the actual value matches specified value pretty closely but beyond 90, the sensor stalls at 90 bar. The car has almost no power and 0-60 comes up in around 30-40 seconds. Not really much fun for my 80 miles round trip commute.

Now, either the new sensor won't read above 90 bar (even though it seemed fine for the first couple of miles), or alternatively, whatever was causing the original intermittent drop in fuel pressure has started to fail more consistently, quite possibly the fuel regulator valve, as a result of my vigorous test driving. I already have on order and hope to receive it in the next couple of days.

Tomorrow, I plan to switch the HP fuel pressure sensor back to the original and see if that gives me readings over 90 bar. As you can see from the original logs, the readings with this sensor showed readings up above 90 bar and were pretty consistent with the specified values until the intermittent power loss when the specified was over 100 bar and the actual could drop as low as 6.4 bar. I'm hoping that once reinstalled the original sensor mirrors the behavior of the new one, confirming that this is not a sensor issue and is more than likely a failing pressure regulator valve.

I've also notice that the LP side is hitting high 6's which I'm wondering if has something to do with the influx of high pressure fuel from the HP rail to the LP rail via the pressure regulating valve. Just a theory but it kind of fits...? Of course, it could just be the LP pump responding to the HP fuel situation by ensuring it is delivering enough fuel.