N75 valves - loss of full boost

[Toady1]

My car is down on power as it isn't boosting fully, boost gauge on car says it is, but VCDS shows that it isn't reaching the required boost pressure, actual is way below demand on the graph. Also on gear changes it has gone from smooth to jerky.

I can also hear a hissing sound at the back of the engine near where the oil filter is (around the RS6 cover bit). Ive not stripped it down yet in that area as haven't had the chance, but it could be a broken vacuum line

Edited as I've located the N75 valve and I don't believe these to be the fault - I have NO codes on Vagcom at all, apart from a duff a/c pressure sensor!

[douglas96]

Page 29 of the study guide shows the locations of the N75 and N274 solenoid valves.

The study guide can be found on the topic below;

viewtopic.php?f=47&t=121159

At the rear of the upper intake manifold are several vacuum hoses that connect to it. Hopefully one has popped off. If it has you may need to remove the bolts for the upper manifold to be able to move it slightly to get access to crimp the Oetiker ear clips on the hose(s).

The attached picture shows the part where these hoses connect to.

Hope this helps,

Douglas

[Stiggles]

I've also got a hiss from underneath the RS6 cover, when removed you can tell it's directly below. Without having anything to compare against I have been assuming it was a characteristic of the car as opposed to a fault, although it may be possible something was nicked or not replaced when Audi did my engine drop.

[ATinOf]

At the rear of the upper intake manifold are several vacuum hoses that connect to it. Hopefully one has popped off. If it has you may need to remove the bolts for the upper manifold to be able to move it slightly to get access to crimp the Oetiker ear clips on the hose(s).

Be aware If you are referring to those 4 lower stubs for those Oetiker ear clips on the pipes at the back of the inlet manifold they are coolant pipes, not vacuum pipes
https://audi.7zap.com/en/rdw/audi+rs6+r ... 121024/#33
Only the odd one at the top on one side is vacuum; if your brakes work fine its unlikely its this as it goes to your brake servo.

I would say hissing creating suspects under that cover or in that vacinity could be; the vacuum suction line from the vac pump; the secondary air pipes(unlikely I would say); the PCV link pipe

The hissing is unlikely to be your N75 valves which are next to your turbos, but your loss of boost may be related to these or their vac lines (you could have more than one issue!)
https://audi.7zap.com/en/rdw/audi+rs6+r ... -145010/#3

Assuming you have checked your charge tube clamps all round (that was what my trouble was here; viewtopic.php?f=47&t=129234)
I would check your MAF sensors (Charge Pressure Senders) havent broken their seal at your charge tubes

[douglas96]

At the rear of the upper intake manifold are several vacuum hoses that connect to it. Hopefully one has popped off. If it has you may need to remove the bolts for the upper manifold to be able to move it slightly to get access to crimp the Oetiker ear clips on the hose(s).

Be aware If you are referring to those 4 lower stubs for those Oetiker ear clips on the pipes at the back of the inlet manifold they are coolant pipes, not vacuum pipes
https://audi.7zap.com/en/rdw/audi+rs6+r ... 121024/#33
Only the odd one at the top on one side is vacuum; if your brakes work fine its unlikely its this as it goes to your brake servo.

I would say hissing creating suspects under that cover or in that vacinity could be; the vacuum suction line from the vac pump; the secondary air pipes(unlikely I would say); the PCV link pipe

The hissing is unlikely to be your N75 valves which are next to your turbos, but your loss of boost may be related to these or their vac lines (you could have more than one issue!)
https://audi.7zap.com/en/rdw/audi+rs6+r ... -145010/#3

Assuming you have checked your charge tube clamps all round (that was what my trouble was here; viewtopic.php?f=47&t=129234)
I would check your MAF sensors (Charge Pressure Senders) havent broken their seal at your charge tubes

Ahh, thanks for the correction! I just assumed (obviously wrongly) when I was taking my inlet manifold off that they were all vacuum hoses. I had already drained the coolant as per the procedure so the hoses were empty - had I not drained it beforehand, I would have realised pretty quickly!

Another thing that has just come to mind, is to check if any of the charge hoses have split?

[Toady1]

I've also got a hiss from underneath the RS6 cover, when removed you can tell it's directly below. Without having anything to compare against I have been assuming it was a characteristic of the car as opposed to a fault, although it may be possible something was nicked or not replaced when Audi did my engine drop.

Maybe its normal then, I just hadn't noticed it before now, but then hadn't had the engine running with the bonnet up.

Thanks for the replies.

[Toady1]

Douglas96 thanks for the study guide!

If the charge pipes had split or popped partially off I'd have thought the boost leaking would be audiable and quite obvious. I can't hear any boost leak, only the hissing from the rear of the engine near the top. I found a small split hose going to the rhs air box lid last night, but it wasn't that as I've replaced it with a piece of silicone hose I had.

I plugged it into Vagcom yesterday and the actual boost was half the demand boost on the graph. It didn't come up with any dtc's either, I'd have thought if the N75 valve was gone it would pick up a fault on the VCDS.

Looks like I'll be busting the jack out this weekend for a look under the car, see if I can see anything....

[Toady1]

Does anyone have a part number for an N75 valve? Found one now.....

[ATinOf]

https://audi.7zap.com/en/rdw/audi+rs6+r ... -145010/#3

You were given the link for the N75 valve - its part ref 3 in the above


If your boost pressure fails to reach target value within a predetermined time/distance you get this fault;
000665 - Boost Pressure Regulation
P0299 - 002 - Control Range Not Reached - MIL ON

If you are not getting this fault it may be something else
Suggest you take some logs with VCDS to diagnose

[Toady1]

I've borrowed the Vag Com from a friend for the weekend so I'll see what I can find out.

I have a newer part number to the one in the link.

[douglas96]

Douglas96 thanks for the study guide!

If the charge pipes had split or popped partially off I'd have thought the boost leaking would be audiable and quite obvious. I can't hear any boost leak, only the hissing from the rear of the engine near the top. I found a small split hose going to the rhs air box lid last night, but it wasn't that as I've replaced it with a piece of silicone hose I had.

I plugged it into Vagcom yesterday and the actual boost was half the demand boost on the graph. It didn't come up with any dtc's either, I'd have thought if the N75 valve was gone it would pick up a fault on the VCDS.

Looks like I'll be busting the jack out this weekend for a look under the car, see if I can see anything....

No problem, but don't thank me - thank IanH755 who is kind enough to host the English version of the study guide for us all to see.

Yeah, that's true but don't rule it out - it was a common problem that the hoses would come off;

viewtopic.php?f=47&t=122954

But as you say, it is likely you would hear this, but check simple things first!

If the N75/N274 valve had failed electrically, then this would most likely be logged as a fault. However those valves can fail mechanically which doesn't always give a fault code.

The current revision of the N75/N274 valve is - 07L 906 283 C. It is the same valve for either turbocharger. I'd be looking to find the source of the hissing sound first though.

Let us know what you find.

[Toady1]

So I looked over the car at the weekend and again no faults found on Vagcom. Even did the on/off tests for solenoids etc and the N75 valves seemed to work fine according to the software. I checked all lower boost pipes and all were fine, tightened them anyway. Still have the issue and still have the hissing sound like air escaping from a small hole by the back top of the engine, so I'm convinced it is to do with this. I'll hopefully strip this down at the weekend to investigate. I did find a split pipe going to the rhs airbox lid, but this wasn't it.

[Toady1]

Fault found. 07L 103 542 A PCV breather pipe had a crack in it leading to the escaping air.

[Woodygo]

Great all sorted then mate ready for Saturday me too I ended up doing my coolant pipes my self as mrc and unit 20 could not fit me in to 22nd onwards so done my self so all ready for Saturday see you there mate

[Toady1]

Great all sorted then mate ready for Saturday me too I ended up doing my coolant pipes my self as mrc and unit 20 could not fit me in to 22nd onwards so done my self so all ready for Saturday see you there mate

Good effort! I checked mine out whilst I worked on my car last weekend and they looked ok, though I didn't pull back the wheel arch liner to fully inspect, this is how they looked....