ouwdee

alright first off this is my first thread im posting on the audi forums, and ill try to keep posting about every problem i encounter with my TT as long as there isnt posts about the same problem already. the n75 valve is a common topic.. yet i have not found a thread about it which completly explains EVERYTHING. so im gonna give it a go. if anyone as anything to add please do so. cheers.

EXPLANATION
The N75 valve controls the turbo wastegate actuator via directing air pressure to it by actuating a tiny solenoid when the ECU decides its neccesary. this solenoid can turn on and off VERY fast to keep the boost at the proper constant. basically it prevents overboost electronically instead of relying on a set tension spring on your wastegate. Its a plastic "T" with a small solenoid and the 2 prong plug on it. 3 hoses: 1 from the intake hose before the turbo, 1 from the intake pipe right after the turbo, 1 from the wastegate actuator.

SPECS: the long part of the "T" which is connected to the intake tube before the turbo SHOULD NOT leak if you blow air into the other 2 nipples. If it does AT ALL you need a new one. if you take a resistance reading between the 2 prongs in the plug it should be between 25-35 ohms.
*****VERY IMPORTANT!******* this is where i was stuck. I had the proper ohm reading, yet mine was still acting up. SO! run extremely hot water on the solenoid, even put it in a boiling pot of water, because when this little sucker heats up from engine heat and moving back and forth the resistance reading goes UP! i just ran hot tap water on mine and i got 40 ohms, 5 over the tolarence. completly made sense why my car would slowly stop boosting once it warmed up. ALSO. it in NOT true that if your n75 is malfunctioning it will throw a code. it the resistance can get out of whack, but something it doenst go past the ECUs set parameters to throw the code, so electronically the ecu finds it acceptable, when in all reality its not mechanically functioning properly.

IN MY OPINION: (dont bite my head off for this one people.)
don't buy the RACE n75. get an N75J and stick with BOSCH, unless youre going to a manual boost controller of course.
mine which failed was a race. not only is it lower quality and shorter lifespan then bosch. it also spikes your boost pretty hard, boosts quite late, and gives a stiff boost curve. the N75J has a nice smooth boost curve, it doesnt just come outa nowhere and kick you in the ***. but the last parts just my opinion.

BEFORE YOU EVEN START THINKING ITS YOUR N75
basics. make sure there are no leaks in your system. do you have a rough idle? change your little braided vacuum hoses ASAP. theyre garbage. rev your engine in neutral, even take a hose off after the intake and listen for your turbo spooling. your turbo will always spool, unless its shot of course. CLEAN YOUR MAF! the ecu will not close the wastegate unless it senses a massive amount of air being sucked into the engine by the open throttle plate. if the maf is not sending the correct number you might not boost. check your wastegate actuator diaphram by applying pressurized air in small bursts to the hose that goes to it from the n75. you should hear a "pop!" sound when you let the pressure off, since pressure opens the valve, when you let the pressure out you hear the valve seat itself/ the diaphram get pushed back by the spring inside it. if you havent already, GET A BOOST GAUGE! it will serve you better with your investigation then a VAG com.

I am a certified mechanic (a young one) , so if you have any questions whatsoever just shoot me a PM and ill do my best to help you out. I dont see a whole lot of german cars at my shop, though id love to. but you learn something every day..

anyways feel free to add to this guys! the more the merrier!

ouwdee

EDIT- end of SPECS

ALSO. it in NOT true that if your n75 is malfunctioning it will throw a code. if the resistance is out of whack, but it doenst go past the ECUs set parameters to throw the code, you wont get a MIL. electronically the ecu finds the n75 acceptable, when in all reality its not mechanically functioning properly.

Peter_Tremonti

It was my understanding that the N75 valve bleeds pressure away from the wastegate to purposely raise the boost level. And by raise the boost level I mean more than what the wastegate spring will allow mechanically if intake manifold pressure alone were acting upon it.

ouwdee

to an extent. yes. especially if you are running an apr flash.

codeman_11901

alright guys, i got a symptom for you to help try to diagnose. i have a 2001 audi tt quattro roadster. these cars are supposed to run 14-15lbs of boost at max. and hold it there. heres the issue. if you press the pedal down slowly, the boost will climb to 10-11 pounds and hold there, never going up. if you push the throttle down very quickly to the floor(only in second and third gear) it will go to fourteen. it cant be a leak, because it wouldnt change with throttle response. it has to be electronic. i havent done your little trick with running air to the wastegate from the n75 line to actuate it. does this sound like something the n75 could be doing? causing these inconsistent, yet consistent, boost pressures? its weird, but i have checked pretty much everything else i can think of. im not a certified mechanic, but i can usually fix issues with cars like this. what do you guys think?

ouwdee

well good news is- theres nothing wrong with your car.
being that the turbo system is completly electronically controlled- your tt will only boost to 11 psi at partial throttle. partial throttle- partial psi.
you dont hit full 15 psi boost in 1st gear because its simply too short of a gear. (dumb gear ratio, really ruins the car.)
and like you said- 2nd and 3rd- full throttle you get your boost. the turbo has enough time to spool and your throttle position sensor is letting the ecu know that youre requesting full potential.
i actually noticed with my car that the ecu picks up on my driving habbits. i have the 93+ octane program on my 225hp.
with the program it pushes 20 psi, peaks out at 22 for a split second.
i drove the car sorta like a grandma for about a month, and i tried to open her up and i couldnt get the boost past 17 psi.
so i kept opening her up for the next couple days, and its like the car realized that it had to boost harder, it started, and kept boosting to 20 consistently once again.

codeman_11901

hmm. because now it will hold 12 psi. even in sixth gear at 2500-3000rpm's. i dont have a revotune or anything like that done. do you think the boost will keep creeping up as i keep spooling up the turbo over and over? if thats all it takes, consider it done. haha i also noticed that as i slowly push the throttle down, it will hit the twelve psi at a certain position. but it isnt at the floor, and if i push it the rest of the way down, its like it didnt even register that i moved the pedal. sound normal? what tune do you have on your car?

ouwdee

i have the apr 93+ octane program.
give this a try- whenever you want to hit full boost, floor it rght from 1st gear, or downshift so your revs over 3000 and floor it. that always works for me.

codeman_11901

it will do it in second and third. but fourth through sixth wont budge. seems like it should hold fifteen regardless of how fast you mash the throttle. idk maybe im being too picky lol

hotgrass

i just found out that i am chipped by APR i guess. i had the car for 2 years. when i press and hold the set (cruise), my engine light blinks once 4x then after that it blinks 2x consecutively. so i tried the first 1 time blink and release it and turn of the ignition and turned it back on. i drove it and floored it in 2nd and 3rd gear getting only 10psi boost. i tried the second program which blinks 2x consecutively and floored it in 2nd and 3rd gear boosting only 15 psi max. from what i heard is im suppposed to boost 18-20 psi. i believed the car was chipped in CA because thats where i got it and im now in TN and im using 93 octane gas. would that make any difference since in cali we only have 91 octane? could that be the reason im not hitting 20psi?