# Has your N75 FAILED?



## FRAX (Jul 21, 2013)

I ask as I have just gone through a long process of trying to find a boost fault on my QS.
The fault turned out to be the N75.
It just turned 45k today and I know that the N75 has been replaced at least once before.
Is this a common fault with all TT's


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## YELLOW_TT (Feb 25, 2004)

Mine failed on the qS at 6k and just under 3years old


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## EnthusiastOwned (Mar 14, 2014)

YELLOW_TT said:


> Mine failed on the qS at 6k and just under 3years old


So that's a yes then? :lol:


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## FRAX (Jul 21, 2013)

Is it more of a problem with the QS then rather than all TT's


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## brackett (Feb 1, 2004)

Idiot question - what's the symptom of a failed N75, and what is it (the N75)?


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## YELLOW_TT (Feb 25, 2004)

brackett said:


> Idiot question - what's the symptom of a failed N75, and what is it (the N75)?


No boost from the turbo. The N74 is the little black box attached the the TIP


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## FRAX (Jul 21, 2013)

Here ye go not mine but 
Here goes....
As i see many of you are totally confused about the N75 valve (what it does, how is it working), just exactly as i was before digging myself into the topic quite deeply. My car was seriously down on power - i even posted some desperate topics here too, but noone was able to give me any usable suggestions/answers. At the end, i found that there was some problem with my N75 valve (though it was working OK) which prevented the car to produce more boost than stock. 
So, if any of you has problems with the N75 valve - or just got problems but don't really know what is it related to, then read more further. 
The turbo, the boost it produces, the N75 and the ECU. 
These are the most important things which you have to focus on. As we know, we got a turbo on our engine which produces boost. The turbo is "driven" by the exhaust gasses our engine produces. Exhaust spools up the turbine in the turbo which is mounted on the same driveshaft as the compressor which then generates boost. I persume it is all clear for everybody. 
There is a wastegate on our turbos (as usually on every turbo) which looks like a simple valve, just exactly like in the cylinderhead. There's a difference though, the valves in the head are operated by the cams, the valve (wastegate) in the turbo is operated by pressurized air. When pressurized air is introduced to the wastegate, it will open, enabling the exhaust gasses to escape and skip the turbo. This will result in a dramatical loss of boost, because there will be no gas which would spin the turbo. I bet this is (was) also clear for everybody. 
But how is this all hooked up??? 
Quite simply. The wastegate is operated by the turbo itself. The boost the turbo produces is directly fed to its own wastegate. What does this mean? When the turbo spools up, it will create boost and this boost will open its own wastegate, shutting itself automatically down. 
But where's boost control? And wheres ECU? 
Here comes the N75 valve. The N75 valve is inserted between the turbo's high pressure output and the wastegate. If the turbo would directly linked to the wastegate, it would not be able to produce lets say more than 0.3-0.4 bar of boost, because it would shut itself down immediately. When the ECU wants more boost (because you are stepping on it for example), it WENTS some air from this loop (turbo--->wastegate) so the wastegate will not open at all, or will open later. 
The N75 has got 1 boost input, and 2 outputs. At the input, boost is entering into the N75 and exits on one of the outputs going to the wastegate. When N75 is closed (default state), all the boost goes to the wastegate. When ECU opens the N75 valve (because it has got an electrical plug too coming from ECU), so when ECU opens it, some of this boost (which would operate the wastegate) is leaving/leaking back to the inlet (and not opening wastegate). Since the N75 valve has NO state in between, is is opened or closed. The ECU switches it ON/OFF all the time (several times a second) to generate the desired amount of boost. Doesn't it reminds you to something???? ... Yeah, it's surging. But it is absolutely normal, this is how ECU controls the wastegate. When a car is chipped, the boost is a LOT higher than it is designed to operate on, so this kind of boost control will be noticable. 
By adjusting the N75 valve, you can adjust the amount of air which should leave when the N75 is opened. If you adjust the N75 to the right, you will "close" the N75 valve, so just a few amounts of air is wented from the wastegate which will result in a lot smoother drive (eliminating surging, because the boost will be more constant), BUT at the same time it will introduce more boost to the wastegate, causing LESS overall boost. Adjusting the N75 valve to the left will "open" it, enabling more air to escape from the wastegate when N75 is opened, which will result in more boost (coz wastegate wont open), BUT at the same time it MAY cause surging as the N75 opening/closing will be more obvious.


Hope the owner of this wright up does not mind me posting it :wink:


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## Wak (May 6, 2002)

YELLOW_TT said:


> brackett said:
> 
> 
> > Idiot question - what's the symptom of a failed N75, and what is it (the N75)?
> ...


Sort of!.. actually when they mostly fail by typically causing an overboost that gradually gets worse.

Theres a period of time when you may feel the cars more powerful because of this until the overboost triggers an overboost fault and then no-boost limp mode.

There seemed to be a bad batch with TT QS's that failed sooner than most and in generaly I find they are only slightly behind maf failures in terms of frequency.


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## FRAX (Jul 21, 2013)

Thanks for the info WAK I thought a man like you would know as you have probable fixed a lot of them in your day.


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## EnthusiastOwned (Mar 14, 2014)

Wak said:


> YELLOW_TT said:
> 
> 
> > brackett said:
> ...


Hmmm,

This makes sense as my car boosted seriously hard and now it's flat as a pancake.

The overboost fault/limp mode, is this recognisable through VAGCOM?


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## Wak (May 6, 2002)

EnthusiastOwned said:


> Hmmm,
> 
> This makes sense as my car boosted seriously hard and now it's flat as a pancake.
> 
> The overboost fault/limp mode, is this recognisable through VAGCOM?


yes should be unless the flat as a pancake is something else! :?


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## Mil-Keith (Aug 23, 2018)

Wak said:


> EnthusiastOwned said:
> 
> 
> > Hmmm,
> ...


I hate to hijack this from years ago but could anyone help with my problem in this thread? I'm replying to this because I know Wak is pretty clued you on this stuff! https://www.ttforum.co.uk/forum/viewtop ... 9#p9261939


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