N75 Valve Flow ??

Hi guys


I was doing a few tests on my n75 valve as im suspecting it to me faulty.


If I blow down the two shorter pipes I can blow through them and nothing come out of the longer pipe.


If I add 12v to the electrical plug then you can hear it click open and the air can flow through all the pipes.


Here an example:



With no power:




With 12v power:





Is this normal as I would have thought that with power added to it it would open the longer pipe and shut off one of the shorter ones?






.

<tuffty/> said:

ECU applies a duty cycle to the valves so opens and shuts at varying speed to control boost...

When off all the boost will go to the actuator/wastegate to keep it open... duty cycle is applied to shut off boost supply to allow the wastegate to shut and produce more boost

<tuffty/>

Click to expand...

Thanks tuffty mate (as always)

Does that mean from my tests that it's working as it should?

I would have thought that when power was sent to it that the short pipe going the actuator and the long pipe would be connected together and the other short pipe would be closed off, but instead all of the pipes open up.

Oh ok.

It's the pipe that goes off to the charge pipe that I thought should get shut off.


As a test I blocked the one that goes off to the charge pipe and the car was boosting as it should.

That's what led me on to test the n75 in the first place.

<tuffty/> said:

Plausible that the N75 is goosed... they do fail from time to time

I assume you have gone down this line of diagnosis due to lack of boost?

Fault codes?

By blocking off the charge pipe one you mean the boost feed from the charge pipe to the N75 or the long outlet to the TIP?

If you mean the former then you have uncontrolled boost and potentially in a position to blow up your engine... what boost did you make or did you do this without a boost gauge connected? (that would be a pretty bad idea)

You need to check for fault codes in the first instance... then assuming thats ok you can check baseline actuator operation by joining the pipe from the charge pipe (goes into the bottom of the N75) to the actuator pipe (the one connected to the short outlet opposite the long outlet)...

This will run the turbo at the operating pressure of the actuator which is around 5psi IIRC..

Running uncontrolled boost is a valid test but only under controlled conditions... all you are trying to establish is if the turbo can actually make more boost than actuator pressure... you would need to have a boost gauge connected and essentially try a single acceleration run in second from 2000 ish rpm (not third as you can load it up too much for this test)... if the boost looks to be heading past 20psi then back off immediately and reconnect everything as normal... if it doesn't make any where near that then chances are the turbo is dead..

The safest way to check is if you have a full version of VCDS you can log blocks 115 for boost request vs actual and 118 for N75 duty... you can see then if the actual boost meets requested boost and how hard the N75 has got to work to get it there...

<tuffty/>

Click to expand...

Thanks again tuffty mate (I should just add that that to my sig to save me keep saying it…lol)



The only fault code I had was the one in my other thread about the n249 but that hasn’t come back.


Yes, I meant the hose from the n75 to charge pipe.

I’ve got a boost gauge fitted and would NEVER boost it up to max while testing it, come to think of it I wouldn’t boost it up like that when its running good…..lol



Maybe I should give you a bit of background so you can get an idea of the chain of events (make yourself a cuppa …lol)


For a long time the car has had a boost leak that I couldn’t find.


It got so bad that the best I could get my boost gauge to read was 0 and that was if I really put my foot down.


I recently tracked the boost leak down to the hose coming from the pancake pipe to the charge pipe (the stupid C-clip design).


I would push the hose on and it would last a day or two and then pop off again. I was thinking about getting on old pipe off the AGU but decided to use a silicone coupler to eliminate the problem all together.


This worked brilliantly for a few days but then went back to a similar problem to the one I had before only now it seems to just boost when it feels like it.


I triple checked the coupler and every other boost vac line on the car but everything is ok.


This led me on to looking at the N75 valve.


First thing I did was measure the resistance on the pins (they measured 33 ohms I think).

Then I wired it up to a 12v battery and I could hear it click when current was put through it.


Then I started blowing through the pipes on the valve (as explained in the first post).


I then took off the hose going from the n75 to the actuator and took it for a careful test drive.

The car felt the same as it did before with it boosting when it felt like it so I put the hose back on.


Now because when I was testing the n75 valve with the battery it resulted in all the pipes being open when current was applied, I decided to clamp off the hose going from the n75 to charge pipe and give it another carful test drive.


Immediately I could feel the difference with it boosting up. I then removed the clamp ant its gone back to boosting when it feels like it.



And this is how it stands now

badger5 said:

blow into centre pipe air should go out of other short pipe. aka n75 off aka actuator pressure.

apply power, blow into centre pipe air should exit long leg pipe

(presumes std n75 valve)

Click to expand...

Thanks Bill


Ill double check tomorrow but if i remember correctly they are as you say in the first part with only one of the pipes being closed off with no power added.

But with power added all the pipes become open (linked) and you can blow through all of them.

<tuffty/> said:

Did this come up before or after you did the test by blocking off the N75 nose?


Getting it to MAX is not really the goal... Where have you taken the feed for this from? what boost did you see when you did the test?


Smoke test needed even now...



Validate where you have taken this feed from... boost pressure needs to be taken from the inlet manifold after the throttle body... also check there is no obstruction in the pipework to the gauge...

Have you actually checked the turbo at all? sounds like its dead to me

<tuffty/>

Click to expand...

Thanks again for the reply.

I think the code was from when I was doing other tests looking for the boost leak.


I just meant I’m not in the habit of putting the car through its paces when its running good so there’s no chance of me doing it while testing things…lol


The boos gauge feed is taken from the manifold.


I would love a smoke test but I’m yet to find anywhere that has one near me. Strangely enough a lot of garages don’t even know what it is.


I had the turbo checked when I was first looking for boost leaks and not finding them.


When the car does feel like boosting it boosts really smooth (I could tell it was boosting good even without a boost gauge) so I think the turbo is ok.

badger5 said:

blow into centre pipe air should go out of other short pipe. aka n75 off aka actuator pressure.

apply power, blow into centre pipe air should exit long leg pipe

(presumes std n75 valve)

Click to expand...

Today i took the N75 Valve off and did a couple more tests as you suggested.

With no power added i blew through the middle pipe and the air exited only through the pipe that goes to the wastegate. but not the long pipe that goes to the TIP.

With 12v power added all the pipe become open and i can blow through all of them and they all become linked.



Another test i did was to block both of the short pipes with my fingers and then suck and blow through the long pipe with no 12v power added.

when releasing my finger of either of the shorter pipes i could hear a very slight pop.

Although the pop sound is slight and i was blowing and sucking hard (there's a joke there somewhere....lol) it does show a small amount of air is passing through.



If this is enough to cause problems i don't know?

I've added a picture to this post for you.

you wont get any pressure while sitting in one spot revving the engine as the car needs to be under load for the turbo to kick in properly.

The n249 valve is at the front of the engine under the plate that the dipstick is attached to.