I`m starting to think it might be the coolant thermostat.
Funny you mention that...
I'm lucky enough to have 3 pain in the *** trial and error problems:
-The dreaded Oil/Siren noise
-Thermo fans coming on full blast while car is off randomly
-This rev hunting up and down while slowing down to stop.
I can probably confirm it isn't the coolant/temp sensors, as I've replaced them both due to the zombie thermo fans problem, just so you know...(which by the way did not fix that either, and after forking out for both temp sensors plus a brand new thermo fan assembly coupled with the fact that VCDS reports nothing; and never will - due to the fact that when the car is ignition on and/or running the ******* works perfectly just as it should, so had to rig fans up to a dedicated switch wired into the dash.
But what I'm getting at is I'm starting to think the other 2 problems (Oil/Siren noise and this rev surging) could and probably are related.
Car is mapped stage 2 generic
Returning to stage 1: both problems still exist
DV is standard but seems fine...if the diaphragm is ********, is it obvious? Cause seems nice and tight but it's the first time I've looked at one...
PCV is forge
I'm very very suspicious on the hard corrugated pipe (pic attached) but how to replace this one? Never trusted anything about this pipe seems likely to crack and the way it connects is not ideal