ALH with too much vacuum resulting in over boost (P0234)

[Aaron T]

I’ve got a stock 03 Jetta wagon with 159k miles that is producing far too much boost and vacuum out of the n75 solenoid. Turbo veins move freely, actuator is functioning properly, n75 is good, and there are no vacuum leaks. When I disconnected the MAF and went for a test drive, the car was all around sluggish.

I connected a mighty vac to the “out” port on the n75 and at idle it reads 20 inHg. I checked another good n75 solenoid and the reading was the same. I checked a healthy ALH and it read around 5 inHg at idle and maxed out at 10 inHg with revving. It seems to me the ecu is calling for way too much vacuum. Could this be possible and I just need a tune or am I missing something?

This is my first post on here and I’m not sure how to post pictures. I’ve got VCDS and the actual boost is way higher than it should be.

Thank you so much in advance!

 

[P2B]

I’ve got VCDS and the actual boost is way higher than it should be.

That's a bit vague, best to take some logs and post them. Here's an excellent guide:

The Ultimate Enthusiasts Guide to Fully become One with the TDI, as told by Burpod....

economy, longevity, performance. fuel, timing, boost. fasten your seatbelts, get on your bi-focals, walmart cheaters, or beer goggles, and whatever else you might need :) this will be a very lengthy set of documents to explain the TDI and every aspect therein, as i understand it.... Sections...

forums.tdiclub.com

 

[fatmobile]

He used the graph in VCDS to see how the boost actual compared to boost requested.
It was obvious the boost was going way higher than requested.

He stopped by my place and we swapped my good N75 valve from the 2000 Jettas into his 2003 wagon.
It did the same thing his N75 is doing. At idle the N75 output line to the turbo went to 18.
On my car it maxed at 10 when I ran the RPMs up. When we graphed actual vs requested on mine it stayed real close.

He has good vacuum from the pump and no leaks.
Reached around to feel the vanes controller and it moves freely and stops moving at 18inhg.

See if I have this right:
High vacuum pulls the vanes into the closed position causing higher boost.
What could be causing the N75 valve to have such high vacuum from the output to the turbo controller, even at idle?
Even when swapped with a known good N75.

 

[Aaron T]

That's a bit vague, best to take some logs and post them. Here's an excellent guide:

The Ultimate Enthusiasts Guide to Fully become One with the TDI, as told by Burpod....

economy, longevity, performance. fuel, timing, boost. fasten your seatbelts, get on your bi-focals, walmart cheaters, or beer goggles, and whatever else you might need :) this will be a very lengthy set of documents to explain the TDI and every aspect therein, as i understand it.... Sections...

forums.tdiclub.com

Thanks for sharing some instructions on how to make a log. I’ve got a screenshot of the graph produced in group 11 but I don’t know how to post it here. I’ll definitely make a log too but not sure how to share it.

 

[P2B]

Thanks for sharing some instructions on how to make a log. I’ve got a screenshot of the graph produced in group 11 but I don’t know how to post it here. I’ll definitely make a log too but not sure how to share it.

How to upload photos in Xenforo and use them in your posts

This is a temporary place holder for a proper FAQ on how to upload photos to the new system: To upload images to the image server: Go to the "Media" section. The link is found in the top bar. click on "Add Media" found near the top right corner. You will get a popup on where you want to...

forums.tdiclub.com

For logs, upload to any file sharing service and post the link.

 

[P2B]

What could be causing the N75 valve to have such high vacuum from the output to the turbo controller, even at idle?
Even when swapped with a known good N75.

This is normal/expected - N75 duty cycle should be 75% at idle on a manual car.

 

[fatmobile]

18inhg at idle is more than 75% isn't it?
I thought 18 was supposed to move the vanes to the maximum closed position.

Are you sure it's normal to have the vanes closed that far at idle, when boost isn't needed?
On the graph, the boost requested is very low at idle.
Even reving it at idle the boost requested is very low.
Because my 2000 Jetta runs normal, doesn't get an overboost, actual is much closer to requested and has much lower
(edit: vacuum output from the N75 and)
boost at idle.

 

[P2B]

The vacuum pump should be making about 28" hg, 75% of that is 21. N75 duty cycle refers to the percentage of input vacuum passed to the output, not vane position.

Turbo does not make boost at idle or when the engine is revved with no load, but vanes are kept closed to minimize lag when boost is requested. When the engine is started, the actuator should pull the turbo vane lever down against the stop.

Here's a post by Jeff (Rocketchip) saying the same:

How much vacuum should N75 output on max boost?

TITLE EDIT: "HOW MUCH VACUUM SHOULD N75 OUTPUT ON MAX VANE DEMAND" Having underboost issues. Tied mightyvac into vac system and I get full 27" or so all throughout vac system and into N75. But out of N75 (to turbo) I get 10" on idle, and 13-14" on pedal floored. Since turbo requires ~18" on...

forums.tdiclub.com

 

[Geobmx4life]

How’s the length on the actuator rod? Should be between 3and 18.

 

[P2B]

Start with running the N75 output test in VCDS. Actuator should cycle smoothly between full boost (lever against the stop) and no boost. Total actuator movement should be 3/4".

 

[burpod]

also this:

understanding the n75, actuator rod length, problems, tunes - VNT and wastegate

BASICS: the VNT turbo boost is controlled by opening or closing the vanes on the turbo - vanes fully open means least possible boost, fully closed maximum possible boost. the actual amount of boost it will make depends on the turbo characteristics (size, flow etc), rpm and amount of fuel being...

forums.tdiclub.com

the HG you see at idle can vary a decent amount depending on a number of conditions.. a stock car is trying to make ~1000mbar absolute psi with 25% duty cycle as it's starting point, at idle. i believe the "%" refers to % of the max output of the n75 - which i believe is 25hg... 75% of 25 is ~18hg. which would be vanes 100% closed if rod length is 18hg.

to see if the n75 is good, at least least able to put out it's full amount, do the VCDS charge pressure control test with mityvac teed in (or plugged direct into n75 outlet). see the diagnostics section here for more info i've written if you do all the vac/turbo/n75 tests there (only takes 10 min) you will know if you have any real problems. even if those check out, i believe it's still possible to have some vanes that will sometimes stick a little, if turbo is sooted up and the pressure conditions in the turbo are just so... there is also possibility of worn vnt control ring. both of those the car should still run decent, but boost control could be a little sloppy or erratic to a small amount. the factory boost control also isn't really even all that good.

more diagnostics:

The Ultimate Enthusiasts Guide to Fully become One with the TDI, as told by Burpod....

economy, longevity, performance. fuel, timing, boost. fasten your seatbelts, get on your bi-focals, walmart cheaters, or beer goggles, and whatever else you might need :) this will be a very lengthy set of documents to explain the TDI and every aspect therein, as i understand it.... Sections...

forums.tdiclub.com

 

[Aaron T]

How’s the length on the actuator rod? Should be between 3and 18.

That's right where it is, between 3 and 18.

 

[burpod]

this post here will shed some light on HG vs duty cycle:

so being the genius i am i wrote a little test tune to map out the duty cycle -> HG. using the go pedal, i made it so it evenly steps the duty cycle from 100%, 90%, 80%, 70%, etc.. with only very tiny fuel changes. appears to be 10% corrresponds to about 3hg, this is with a new pierburg n75. not that it has any bearing on this test, my rod length is 20hg. this test is simply measuring out the duty cycle % to actual vacuum. based on this, it appears i would reach my set screw around 20hg @ 20%. i'll probably redo my test "tune" so i can get down to 0% and try it with a couple other n75s, including a known "bad" one that maxes out at 22hg with the vcds n75 test. needless to say, it's not a drivable tune

100% - 0"
90% - 0"
80% - 3"
70% - 5"
60% - 8"
50% - 11"
40% - 14"
30% - 16.5"

the goal of this test would be to test how evenly duty% coresponds to actual vane movement. i would think it should be perfectly linear, and if it's not, the n75 perhaps isn't working correctly in all ranges

i still want to do this test on multiple n75 valves, including ones that are "bad"

you should do a basic boost log - 011-001

double-check that timing adaptation (04 channel) has not been messed with

 

[Aaron T]

How to upload photos in Xenforo and use them in your posts

This is a temporary place holder for a proper FAQ on how to upload photos to the new system: To upload images to the image server: Go to the "Media" section. The link is found in the top bar. click on "Add Media" found near the top right corner. You will get a popup on where you want to...

forums.tdiclub.com

For logs, upload to any file sharing service and post the link.

Idling-IMG_2758.jpeg

• Aaron T
• Dec 18, 2023

This photo was taken when the care was just idling. The vacuum was much higher on my car compared to fatmobile’s healthy car.

IMG_2756.jpeg

• Aaron T
• Dec 18, 2023

Here is a picture of the graph taken when the car was parked and revved.

 

[Aaron T]

this post here will shed some light on HG vs duty cycle:




i still want to do this test on multiple n75 valves, including ones that are "bad"

you should do a basic boost log - 011-001

double-check that timing adaptation (04 channel) has not been messed with

Here’s a log I took a little bit ago:
Driving on a two lane highway

 

[burpod]

sanity check block 010 with key on/car off make sure baro + map are reading the same

 

[burpod]

the log file on your drive is requesting permission - share it for link that can be viewed by anyone

 

[burpod]

take a log with 011-001, and/or 011-001-004
not enough info really to determine with only boost data, need to know fueling at least

the 2nd log taken was all in limp mode

 

[burpod]

how does the car drive? is it quick? smoking? it's creating a ton of boost for the fueling, like either the vanes are sticky, it has bigger nozzles or rod length is way off. snap a pic of the set screw - is it in a factory correct spot - should be ~3 full threads showing. if that's out of whack, rod length will be out of whack

 

[burpod]

looking at the limp mode log, i doubt its anything strange with timing or bigger injectors. it really looks like the vanes are just sticking down at the near closed area. i'm not 100% sure, but i believe it's possible for the vanes to get hung up when under boost/running, even if they "seem fine" while at rest. there is (naturally has to be) some play in the vanes against the machined surfaces, and there can be pitting that develops as well as soot. when under load, i think its very possible that vanes can get pushed in/out a bit and get hung up enough the spring/vacuum of the actuator can't overcome it

 

[Aaron T]

how does the car drive? is it quick? smoking? it's creating a ton of boost for the fueling, like either the vanes are sticky, it has bigger nozzles or rod length is way off. snap a pic of the set screw - is it in a factory correct spot - should be ~3 full threads showing. if that's out of whack, rod length will be out of whack

I got to looking closer at the actuator movement and it was stopping at 15. I lengthen it a little and it is doing much better. Actual boost is still a little higher than requested but doesn’t go into limp mode now. It’s not too quick. My AAZ powered mk2 Jetta would smoke this thing. I’ll take some new logs with the specs you requested tomorrow and share them.

IMG_2768.jpeg

• Aaron T
• Dec 18, 2023

IMG_2769.jpeg

• Aaron T
• Dec 18, 2023

 

[burpod]

reach your hand back there and snap a pic of the set screw

and you can use the thinnest feeler gauge (or some folded over clear packing tape) to get a more accurate idea of when its hitting the set screw.

 

[Aaron T]

reach your hand back there and snap a pic of the set screw

IMG_2771.jpeg

• Aaron T
• Dec 18, 2023

Do the numbers written on the turbo mean anything to you?

 

[burpod]

no, numbers dont' ring a bell. set screw looks like it's in correct position, also appears you have an oil leak there, either from boost @ the compressor housing or oil drain line

 

[P2B]

Do the numbers written on the turbo mean anything to you?

They don't mean anything to me but are evidence the turbo was replaced and possibly not adjusted correctly.

Does the rod move a full 3/4" when you run the output test?

 

[PakProtector]

My first TDI would overboost and limp. With the engine off the vanes were free and the vacuum motor appeared to behave properly and smoothly. A new turbo sure as hell fixed things... Those vanes get shoved around a bit by the exhaust.

Douglas

 

[burpod]

my bet is on sticking while under boost/load, despite appearing fine while testing at rest. have seen this quite a few times and pretty sure that's what's happening. you could try to spray it cram full of foaming degreaser (like from walmart) and let it soak and actuate the vanes, spray in some more and repeat again, then blow it out the exhaust with relay 109 disconnected while starting. never actually done that but it might work well enough, else need to take turbo apart for a real clean, possibly dremel/sanding off any pitting on the machine surfaces - which is totally fine to do, have done this on wifes car.

 

[fatmobile]

I didn't try disconnecting the rod to see how the vanes moved through the whole way.
It was easy to move and smooth at the top, with the actuator connected.

That little clip holding the rod to the vanes is easy to lose.
Or maybe just loosen the 2, 10mm nuts holding the actuator on.
Those aren't quit as easy to lose.
Glad to hear you got it to run better by lengthening the rod.

Still has me wondering why my good runner has way lower N75 output.

I was thinking pulling the turbo from the top would be the easiest.
Though disconnecting the oil return and block mount might go better from underneath.
Seeing those numbers on the turbo make me wonder how recent the turbo was replaced,..
maybe the bolt/nuts will come off easy.

The oil leak burpod noticed is on the list too.

 

[burpod]

at the "top" is vanes open.

you can use a "standard" 1/4" circlip from the hardware store ($.50) and it will work, although there might be some slightly different styles which perhaps might not always work. ones i've picked up from aubuchon work fine. a little looser than the oem, but have never fallen off, also much easier to remove without flying off into oblivion.

to remove the actuator i use a little stubby 10mm wrench to take the two nuts off. its a little tricky, but doable from the top. also, pulling turbo from the top is easiest. only reason why you need to get to the bottom is to disconnect the drain line.

if the n75 doesn't put out 24-25hg during the vcds test, it might bad. but it's possible for it to put out 20hg and still be good however. if rod length is 18hg, it doesn't need >18hg, but i think it depends on why it's not putting out 24+hg. if it's simply gummed up such that its not opening past 18hg, thats fine, since that's not needed, but if for some reason it's an electrical issue with it, then the n75->hg is going to be all out of whack.

the turbo looks pretty new to me

 

[LeisureDave]

This is normal/expected - N75 duty cycle should be 75% at idle on a manual car.

Huh. Mines at 25%… I’m wondering if that’s backwards and means the same thing?