A6 2.5tdi - testing and cleaning the n75 valve

[bluezie]

I have an A6 2.5TDI Q (AKE) mapped to 208BHP , and for several years it has had wobbly boost. It boosts up to 2500mbar (Requested 2300) then sags down to around 1700 for a few seconds, then back up to near 2300 etc etc.

Looking at VCDS graph, its obvoius the actual boost is badly wobbling around the requested. Although NEVER throws and overboost code, or goes into limp. The obvious thing is stuck VNT. I checked that by disconnecting the actuator, and it is REALLY free and smooth. Aso cycling the VNT using VCDS output test, it snaps quickly, smoothly and fully open and shut.

Vacuum leaks have been eliminated, and the actuator diaphragm holds pressure without leaking.

So, the only thing left is the N75 valve, which is working fine when slammed open and shut by big changes in duty cycle, but might be sticking when attempting to fine tune boost control.

Before committing (wasting) nearly £100 on a new one, is there a proper way to test a N75 for stickyness, and / or clean it?
I have heard carb cleaner might work, but might ruin the diaphragm, and I have though of a quick squirt of WD 40 to see if it improves.

The EGR valve cannot be used as a swap out on this, as the connectors are keyed differently.

I have lived with this problem for so long as I my wife nearly alway drives the car, and doesn't thrash it so much so doesn't really notice anything wrong. Recently while pulling a loaded horse trailer, it became very noticeable - the boost sags sometimes causing the car to slow down when accellerating up hills!

[CharlesElliott]

I have cleaned/removed my N75 valve. Disconnected it, cut a slot in the metal band and opened it up to split the two halves of the N75. Then cleaned as per this guide. I didn't bend the metal ring though, just cut it, opened it to remove, then put it back, closed it, and used a tie wrap round it to keep it in place.

[bluezie]

Many thanks for link to cleaning guide! What engine was your N75 controlling, and what were the symptoms before cleaning, and how did it improve after cleaning?

[CharlesElliott]

It's an AKE. I was trying to fix lack of boost at certain times issue - in the end I think that issue was caused by something else.


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