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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old June 15th, 2018, 08:44   #16
AceTdi
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JRmonster huge help.

It's stock auto.

So sounds like I had open/closed terminology mixed up.

So at idle max closed vanes on auto we're looking for 14" out of N75? Please confirm I'm understanding that correct.

So if only 14" expected out of N75, that would say N75 is working correctly. Please confirm I'm understanding correctly Jeff. Thanks!
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Old June 15th, 2018, 13:36   #17
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For what it is worth, I believe you will see the actuator pull the arm down solidly against the set screw when the engine is fired up (idle). At least that is my observation on the ALH engine in my 84 Vanagon........... and, it works just fine!
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Old June 15th, 2018, 14:03   #18
AceTdi
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Great news guys. Did swapnostics with N75 and N18 as BobOH, Enabled, JRMonster suggested.

With N18 in place of N75, vacuum outputs (MightyVac tied with Tee fitting into N75 out to turbo while driving), I got between 13-15in/hg at idle/non-vane demand.

On vane demand got full 18in/hg of vacuum. Most importantly no underboost. Proper turbo function.

(Old/failed N75 was 10in idle, max 14-15in on vane demand. Not enough to fully move vanes, which require 18in to actuate)

This points with 99% certainty to failed N75 which I will now replace confidently.

Thanks for the suggestion there. Sorry if a few got offended by my very focused question.

Been fixing long enough to know diagnosis MUST confirm part failure with absolute certainty (multiple methods if in question), before replacing parts. Throwing parts is not pragmatic.

Hopefully this helps a few people who haven't used this diagnostic process on N75 before (I hadn't seen info). Will try to post results after new N75 arrives for extra confirmation, but as you know sometimes when it's fixed it's forgotten. Thanks again all.

Last edited by AceTdi; June 15th, 2018 at 14:07.
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Old June 16th, 2018, 13:02   #19
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We are always here to help, and sometimes we all get a bit testy when suggestions aren't followed after being asked for. I have been a member of this website for a long time, and I still run across stuff I need help with. OP, thank you for letting us know you found the problem.
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Old June 19th, 2018, 12:18   #20
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Quote:
Originally Posted by AceTdi View Post
(Old/failed N75 was 10in idle, max 14-15in on vane demand. Not enough to fully move vanes, which require 18in to actuate)
Mine is about 10 inches or so.
Sounds like your making headway.
Bottom port on both of those valves should run to a tee that goes to the bottom of the air filter box. Does yours?
Make sure the hole in the filter box is clear.
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Old June 24th, 2018, 16:00   #21
AceTdi
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Bad news. Installed new N75, getting full 18" of vacuum to turbo, but problem still occurring It's better, but intermittent underboost and limp.

Pulling turbo tomorrow to inspect vanes. Seems weird that I'm only getting underboost, no overboost. Last time I had sticky vanes I got lots of overboost. Will likely start new thread on this. Thoroughly stumped by now though.

New thread with continued diagnostics and background here:

http://forums.tdiclub.com/showthread.php?t=490103

Last edited by AceTdi; June 24th, 2018 at 19:51.
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Old June 25th, 2018, 09:13   #22
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I refuse to post to a new thread about the same situation.
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Old June 25th, 2018, 09:32   #23
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I'm a sucker, I looked.
Seems we're diagnosing Limp Mode.
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Old June 25th, 2018, 13:05   #24
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Quote:
Originally Posted by BobnOH View Post
I'm a sucker, I looked.
Seems we're diagnosing Limp Mode.
From day one.
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Old June 26th, 2018, 13:33   #25
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good news......Looks like he may have it cornered, only 6 threads. I guess sometimes it just can't be helped.
http://forums.tdiclub.com/showthread.php?t=490103
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Old June 27th, 2018, 14:12   #26
AceTdi
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Yup, fixed!

The washers holding turbo cartridge to housing squared off and lost contact, causing cartridge to wiggle free from the housing. So exhaust lost pressure through the space created between the cartridge and the housing. Too little exhaust pressure to drive compressor and create requested boost.

Lesson: Use very overlapping and thick washers to fasten cartridge to housing so it doesn't vibrate itself free. More info on the thread BobOH linked.
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