Boost Problems - Vibrating N75 - Low boost with engine cold

Joined
Nov 8, 2020
Location
Portugal
TDI
1.9 VP 110 bhp
Hello! My car is having some troubles in the last two weeks. (I am sorry for my English)

It all started with a limp mode, I was driving in 5'th gear, I floored the gas pedal, and I lost the boost completely. Restarted the engine, and I got a charge control pressure - negative deviation error. I cleared the code, and since that I am having almost no boost before 2krpms , and overboosting around the 2,5k rpms. But, when the engine reaches the operating temperature , the problem goes away most of the times, and I am not getting any engine error codes.

I heared that the n75 valve could be causing the problems, I tested it with vagcom, and I was hearing a subtile electrical noise coming from it. So, I decided to remove the tubes connected to the valve, I removed the power cable, i cleaned it with wd-40, I cleaned the dirt and the excess liquid by blowing air with a small tube, placed everything back, I drove the car and it was a little bit better.
In the next day, I realized that the valve was doing a vibration sound, so probably I end out by destroying it. In the vagcom I got a:
17957 - Boost Pressure Regulation Valve (N75): Open or Short to Ground P1549 - 35-10 - - - Intermittent
I did a log with the software while driving, so if you don't mind please give it a look and help me diagnosing and fixing this problem. Thank you!

LOG VAGCOM FILE - GOOGLE DRIVE
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
With your N75 valve requesting more vacuum for the turbo I will bet you have a fault in the vacuum system. The N75 valve on ALH motor works opposite the regular line of thought. If the N75 has a number below 50%, the ECU is requesting more boost, and if the number is above 80% the ECU is telling the N75 valve to lower boost. I would isolate the N75 to see how much vacuum is being applied to it and start there. If you don't have 22" of vacuum at the nipple that could be the start of finding the issue. With the engine at idle pump the brake pedal a couple of times and if the pedal gets hard, there is a vacuum issue.
 
Joined
Nov 8, 2020
Location
Portugal
TDI
1.9 VP 110 bhp
When i press the brake pedal a couple of times it gets hard! I dont have any kind of gauge to measure the pressure directly in the vacuum hoses.

My mechanic is insisting in the turbo's geometry, but I don't get it, because with the engine in operating temperature I almost don't get any boost problems. If the turbo vanes were sticky, or the turbo geometry mechanism, I believe that the problem would be worse, because of the metal expansion, but I am only guessing here.

Thank you for your help, I really appreciate it!
 

eddieleephd

Top Post Dawg
Joined
May 27, 2012
Location
Asheville, NC
TDI
2002 jetta Wagon
A vacuum gauge is going to be necessary to diagnose this issue. I recommend going to the auto parts store and renting a vacuum pump, there should be a gauge attached.
Then you can isolate the issue.
I also recommend getting vacuum line and replacing the small vacuum lines presently attached.
The pump could need repaired also. Use the pumps gauge to measure the vacuum pump directly after the pump, then measure further away.
The brake booster, its seal, and vacuum pump are notorious for leaks.
 
Last edited:

eddieleephd

Top Post Dawg
Joined
May 27, 2012
Location
Asheville, NC
TDI
2002 jetta Wagon
So, now, take the file that vagcom created when you did the log.
Go to Malonetuning.com and drop the file in the browser. It will graph the file.
You will then copy the address in the address bar and post that here. Anyone can follow it and see a graph that's understandable.
 

Nevada_TDI

Top Post Dawg
Joined
Aug 17, 2008
Location
Reno, sort of...
TDI
2001 Jetta TDI
The problems you are having have nothing to do with your turbo! If you can being very gentle remove the small hose that is coming out of the vacuum pump if you have any excess tubing that will fit onto the nipple put a piece with some sort of block: screw, bolt, golf tee etc. This way you are isolating the vacuum pump and the brake booster from the rest of the system. Start the engine, wait 10 seconds or so,
turn the engine off, wait another 30 seconds or so,and then press the brake pedal once. If the vacuum booster or the hard plastic line that attaches it to the vacuum pump is faulty there will be no vacuum to the brake booster left to assist the brake pedal. I hope this is clear.
 
Joined
Nov 8, 2020
Location
Portugal
TDI
1.9 VP 110 bhp
I think that thr n75 valve also controls the brake pressure, i am note sure. So, if mine went bad, probably it will interfeir with the brake vacuum mechanis.
 

Tdijarhead

Top Post Dawg
Joined
Nov 10, 2013
Location
Lawrenceville PA
TDI
2003 TDI Jetta Daughters Car, 2001 TDI Beetle, Daughters car, 2005 Golf TDI Mine, all 5 spds
The n75 doesn’t control the brake vacuum but it gets its vacuum supply from the same source so when your brakes get hard from low/no vacuum it’s a good sign your turbo won’t function properly.
 
Joined
Nov 8, 2020
Location
Portugal
TDI
1.9 VP 110 bhp
So, my brakes get hard when I press them a couple of times, but it goes away some seconds later, is that normal? Or is it a indicator of low vacuum?
 

MOGolf

Top Post Dawg
Joined
Jun 27, 2001
Location
underneath something
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2001 Golf GLS TDI Reflex silver, rough road suspension and steel skid plate, 2004 Passat Variant, Candy White, rough road suspension and geared balanced shaft module, and much, much more. 2016 LR RR HSE TD6, 2019 Jaguar I-PACE
You have a vacuum leak. Probably the large vacuum line to the brake booster. They have a history splitting as they age.

This will cause the power brake issue and the loss of vacuum to the N75. You probably did not have a problem with the N75 until your attempted cleaning procedure. Now you need a new one of those too.
 
Joined
Nov 8, 2020
Location
Portugal
TDI
1.9 VP 110 bhp
The low vaccum reaching the n75 can cause it to vibrate I think. I saw a video recently where a guy was covering one of the valve's connections and it started to vibrate as mine does. Also, there is a kind of little filter near the valve that I think that can cause the vibration.

I only used a little bit of wd40 to clean it, but yes, probably i broke it. Or maybe it was clogged, so it wasn't reacting to the lack of vaccum, and now that I lubricated the mechanism, possibly got it free or loosen inside.

I have a large tube possibly related with this problem, it is oily and have's a weird yellow kind of tape, that looks very ugly, that should be the tube you're talking about. I am going to try to post the pictures here.

I am very thankful for your help, really, you don't even imagine how much a simple comment helps.

I hope you have a good day, and I wish you the best, thankyou!!
 
Last edited:

wonneber

Top Post Dawg
Joined
Oct 12, 2011
Location
Monroe, NY, USA
TDI
2014 Jetta Sportwagen, 2021 Atlas,2003 Jetta 261K Sold but not forgotten
The:
17957 - Boost Pressure Regulation Valve (N75): Open or Short to Ground P1549 - 35-10 - - - Intermittent
is electrical.
Check the wires around the N75 for fraying or breaks.

I've used WD-40 to clean the N75 in my 03 JSW.
It did seem help a bit as you found but did not last long.
I replaced the N75 and it was better.

I also tee'd a vacuum gauge into the vacuum system with a long hose so I could watch the gauge while I drove the car.
I think I put 2 part epoxy around the vacuum nipple to hold it tight and seal any leak as it was loose.
Not sure if your 1.9 VP 110 bhp has the same vacuum pump fitting being loose problem.
Could be your " brakes get hard " problem.
 

MOGolf

Top Post Dawg
Joined
Jun 27, 2001
Location
underneath something
TDI
2001 Golf GLS TDI Reflex silver, rough road suspension and steel skid plate, 2004 Passat Variant, Candy White, rough road suspension and geared balanced shaft module, and much, much more. 2016 LR RR HSE TD6, 2019 Jaguar I-PACE
The large tube connection with that tape over it is the main vacuum line. It is supposed to be loose as a relief for the engine vibrations. If it does leak from that location, the the vacuum pump should be replaced. Someone taped over it and over the course of time it has become oil soaked. The seal between the cylinder head and vacuum pump may not have been replaced during past timing belt replacements. Have you measured the vacuum being supplied to the N75 (top nipple on it)?

Follow that large line towards the N75. You'll find a check valve that also has a smaller diameter nipple. From there the large line goes to the brake booster. Ensure it does not have any splits in it. The little nipple is where the vacuum line attaches to supply vacuum to the N75 and EGR regulator valves. You've probably gone through all of those lines to ensure no leaks.

If it is having electrical issues (and the code wasn't left over from an accidental drive with the N75 electrical connection disconnected), then this could be another reason to replace the valve.
 
Joined
Nov 8, 2020
Location
Portugal
TDI
1.9 VP 110 bhp
My mechanic replaced that tube you're talking about, the car is much, muuuuch better, except for the fact that the n75 now vibrates when i turn off the car and sometimes ticks under light acceleration. Next step is changing the n75 right?
I still feel like the turbo sometimes takes too long to hit, but only sometimes, but with a worm engine everything feel and looks perfect. Check the URLs down:

N75 noise like mine does
Replaced tube: Replaced Vaccum Pump do Brake Booster Line
 
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