N75 valves - loss of full boost

[Toady1]

Update on this fault. I bought a full new top breather pipe assembly costing £128 to replace the broken part mentioned above, as if one part is brittle enough to break I assumed the rest isn't any better. Whilst replacing this pipe I noticed the pipe that comes up from the V of the block under the inlet plenum was making creaking noises as I moved it to disconnect it, and sure enough, a little pressure on it to try to disconnect it from the horizontal breather pipes and it also snapped clean off!

This I'm now thinking was my main culprit to the hissing noise on idle as it is right underneath the RS6 engine cover. I've managed to repair the broken pipe with some tube and heat shrink so hoping this will fix the leak. I've finally got around to putting it all back together tonight in the dark with some lamps, and have now left the car on charge over night to top up the battery. I didn't want to start it up this late at night and annoy the neighbours.

I'll start it up again in the morning and report back on the hissing noise - hopefully it has gone and my car will drive how it did on part throttle like it did for the first week I got it! The symptoms were poor performance on part throttle, little to no boost until over 3k revs or WOT. On WOT it still felt like it had full boost, but drivability on part throttle or when short shifting was flat, little boost and not nice at all. The gearbox was quite jerky when changing gear, almost delaying changes a bit like the engine was stalling during the gear change, and appeared to slip more before it would lock into gear too, possibly because the ecu wasn't receiving the correct boost pressure reading for the revs?

I thought my diverter valves may have been part of the culprit too, and so bought some Forge FMDV008 valves to fit, but when I checked my oem valves they appear to be working fine. The diaphragm is holding pressure anyway, but they don't take a lot of force to open up, about 1/4 the amount of force required to open the forge valves. I have learnt that the oem DVs don't last long above standard boost so I may still fit these, but for now I wanted to put it back together knowing what I've fixed.

[c3nturi0n]

have you checked IC (incl. hoses) on both sides?

[Toady1]

Yes I checked and tightened up all boost hoses last time I was under the car. I don't think it is that though as when it gets to about 3-4k it boosts hard. If it had a boost leak externally it wouldn't boost properly. Its the part throttle lag of boost I'm getting. Almost like on part throttle the turbos aren't spooling. I don't think it is the oem bovs as they'd be more prone to losing full boost. It could actually be mechanically failing N75's.

Car is currently at Vass-tech having the rear springs replaced, 4 wheel alignment, and gearbox o&f done. I've told them about my issues so hopefully they may be able to diagnose something.

[Toady1]

So picked the car up from Vass-tech today and listened to another RS6 engine on idle. I couldn't hear any hissing noise from the area I have a noise, so I obviously still have some form of vac leak. It looks like I'll be removing the inlet plenum this weekend to find the culprit! The car isn't losing full boost. On WOT it picks up and goes! If the oem dump valves were dead it would leak boost and not return full power. I have full power, and its simply the lack of responsiveness/boost on part throttle which to me points towards a vacuum leak, and with the noise I can still hear it must be this.

On the plus side, my gearbox feels smoother since having its oil and filter changed, and the car handles nicer with un-snapped springs too!

[douglas96]

So picked the car up from Vass-tech today and listened to another RS6 engine on idle. I couldn't hear any hissing noise from the area I have a noise, so I obviously still have some form of vac leak. It looks like I'll be removing the inlet plenum this weekend to find the culprit! The car isn't losing full boost. On WOT it picks up and goes! If the oem dump valves were dead it would leak boost and not return full power. I have full power, and its simply the lack of responsiveness/boost on part throttle which to me points towards a vacuum leak, and with the noise I can still hear it must be this.

On the plus side, my gearbox feels smoother since having its oil and filter changed, and the car handles nicer with un-snapped springs too!

Just in case you are interested, and at the risk of me sounding like a salesman (I can assure you, I am not!); Snap On is currently running an "Automotive Test Kit" on promotion. It looks like quite a nice kit and would be ideal for tracing a vacuum leak, etc.

Anyway, the part № is - YA4000B.

Hope this helps.

[Toady1]

To update. Replaced my oem bovs with forge fmdv008 and sorted my issues. Must have been leaking after all

[RS6-T28RPH]

To update. Replaced my oem bovs with forge fmdv008 and sorted my issues. Must have been leaking after all

How is it feeling now under part & full throttle?

[Toady1]

Full throttle was still ok, but part throttle was down on power and it was also affecting the way the gearbox behaved. made it hang before locking into gear fully, throttle output didn't replicate throttle input if you know what I mean, boost was pretty much nothing until 3500k revs then whack. But all I've done is replace the oem bovs with Forge items, and reconnect it all back together and its back to normal again.

I did notice the larger diameter pipe that goes from a swirl pot to the oil tank wasn't fully seated into the swirl pot, so potentially leaking some air there. But I don't think that would have been the cause.

The car is now back to being super smooth on part throttle, smooth gear changes and locking into gear properly as expected, and effortless to drive quickly again with only minimal throttle. Too easy to be brekaing the speed limit with only feathering the throttle again!