[dr.electron]

I will add that at idle, the pump duty cycle is just under 50% - like 47% or so. If the 0.23bar is real, wouldn't the low pressure pump be pushing harder to try to bring that up? Or is it driven by the high pressure measurement - meaning that when the high pressure pump needs to pull more the low pressure pump pushes harder? I'm just puzzled as to how the car can drive "normally" with what looks like practically now fuel pressure on the low pressure side. If it weren't an expensive part, I'd just swap out the pump to see if the problem went away.

 

[dr.electron]

Whoops - yes, the low pressure sensor. #2 in the diagram. I replaced that with an actual Audi OE part. The fact that the engine doesn't seem to struggle does seem to point to a sensor problem. So since I've replaced the sensor, maybe it's a wiring issue. I'm going to see if I can rig up a way to check the sensor voltage while the engine is running.

 

[dr.electron]

It may be worth noting that I replaced the fuel filter and sensor at the same time. So if the filter was actually the problem, and the replacement sensor was "bad", then that could explain this. But the likelihood of getting a bad sensor strikes me as extremely low. I haven't tried swapping the original sensor back in yet.

 

[dr.electron]

SOOOOO.... I disconnected the new sensor and plugged in the old one out in the open air. Turned on the ignition but didn't start the car: block 103-0 reads 1bar. As it should in atmosphere. I plugged the harness back into the sensor on the engine - still without starting it - and it read 4.5bar, which is where it should be. Started the engine and it dropped to about 2.6bar with duty cycle running at 70%. Listening carefully, I could hear the fuel pump screaming quietly beneath the back seat.

The pump is clearly bad - so I'll be ordering a new one.I'll report back when that's been replaced.

 

[dr.electron]

The pressure regulator for the low pressure side is built in to the fuel filter, as I understand it. Is there another low pressure regulator at the engine? Or are you talking about one on the high pressure side? I haven't messed with anything over there since the pressures on the fuel rail seem to make sense and track the command from the ECU.

 

[dr.electron]

There could be a regulator there - and I definitely welcome the brainstorming! There's surprisingly little general knowledge about the 2.5L TFSI engine out there in the TT community, and the diagrams on the Audi parts website, the repair manuals, and LLLparts are not especially useful. This diagram from the self study guide for the engine doesn't show a regulator, but that doesn't mean there isn't one:

Since I actually heard the pump straining this time, it's now my #1 suspect. I know that noise because I installed the wrong fuel filter in my TT 3.2 several years ago and forced the fuel pump to work hard. If the pump doesn't fix the problem, I'll be looking for another low pressure regulator next.