A4 very rough idling , fuel pressure all over the shop

[a4_co_durham]

coped as i accidently posted on B6 form instead of b8

59 plate B8 2.0 TDI

to cut a long story short, car has 148k on it & has been mostly fine
had major service at independant last week all good
next day en route to work filled up @3/4 tank at Sainsburys (yes i know....) and after that when at some roundabout lights noticed car was rough/shaking when idle in neutral and revs bouncing about 1000prm...
seems to run fine when driving normal but definatley getting shaky when idle, no lights on dash at all
so next day popped back to garage where service had been and they agreed def something amiss, but no codes on ECU at all ?? we could see the fuel pressure bouncing about all over though? and when he disconnected the fuel pressure sensor the engine went back to normal (is in not bouncing ), so suspected the sensor was dodgy, but no codes ?
anyway in the office they asked a few more questions and seems they think may be filling up has disturbed some crap in the fuel so may not be the sensor?
so not sure what to do as dont want to spend money on something they dont even know will work. said they can take the car in for a day and have a proper look but a bit vague on how much this will cost (understandibly?..) or if they can narrow down the issue

been using the Mrs car this week whilst i seek a second opinion & thought i would ask here?

could it be as simple as the sensor? as it seems ok when disconnected

cheers

[Dr Mcr]

I had a similar problem with my b8 Avant on 115k miles.

When the revs dropped to about 900, the needle bounced back up to 1100 and dropped again. The engine shook a fair bit just before it jumped.

My cousin (VAG mechanic) said it's likely to be one of the two fuel sensors or possibly fuel pump. He did say if I wanted to know for sure, I could take it to a diesel fuel specialist near Halifax. This would mean a cost to diagnose, then additional cost to replace the part.

We tried to replace the cheapest sensor first, with one he had in. This didn't fix the problem. I took a risk on ordering the second most expensive sensor, as it was Cheaper than the cost of diesel diagnostics. Sorted the problem!

It was a fuel rail pressure relief valve. Took the part number from the one on my car.

[a4_co_durham]

I had a similar problem with my b8 Avant on 115k miles.

When the revs dropped to about 900, the needle bounced back up to 1100 and dropped again. The engine shook a fair bit just before it jumped.

My cousin (VAG mechanic) said it's likely to be one of the two fuel sensors or possibly fuel pump. He did say if I wanted to know for sure, I could take it to a diesel fuel specialist near Halifax. This would mean a cost to diagnose, then additional cost to replace the part.

We tried to replace the cheapest sensor first, with one he had in. This didn't fix the problem. I took a risk on ordering the second most expensive sensor, as it was Cheaper than the cost of diesel diagnostics. Sorted the problem!

It was a fuel rail pressure relief valve. Took the part number from the one on my car.

cheers for reply exact same issue and resolution here

replaced the pressure sensor (front of the common rail) with no success

then after finding a few vids on youtube went for the Fuel Rail Pressure Regulator valve and between me and a mate with more tools, managed to replace it, took a run round the block to run the air out but all good now

picked the part up on ebay for £60, local indepenadant wanted £380...

fwiw part no was 057130764H

Cheers

[Dr Mcr]

You did better than me on price. I think I paid £115 for a genuine brand new part.