How much vacuum should N75 output on max boost?

[AceTdi]

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 actuator to open vanes fully, would this not point to N75 issue? (IE, not enough vacuum to fully open vanes, thus causing underboost and severe lag where some boost kicks in and higher RPM?)

Trying to confirm correct values before throwing parts (don't have known working one to swap-nostic with). Can't find much advanced info on vac issues.

PS. Tested N75 connector resistance at about 17.7ohms

It's 02 Golf TDI ALH. Anyone have these numbers? Or ideas?

 

[Enabled]

The ecu doesn't give full 100% open vanes on full boost. It's highly condition dependent, as well as rpm dependent.
Got VCDS?

 

[Vince Waldon]

It's highly condition dependent, as well as rpm dependent.

Yup, unfortunately, the answer to your question OP is: "it depends".

 

[AndyBees]

Yep! You can have "full boost" under a wide range of circumstances and the vacuum applied to the Turbo Actuator by N75 will vary with each of those circumstances.

 

[AceTdi]

Kinda getting caught up in the weeds here. Aware VNT fluctuates with RPM, that's it's function.

My question is, at low RPM (from idle rev) ECU should be demanding max vane open, should it not? Or does ECU never demand full vane open?

If it does demand open vanes, the turbo actuator requires at least 18" to open them, thus, should output from N75 not be at least 18"?

Anything less than 18" wouldn't fully open the vanes. So under full open demand (low RPM curve), would result in underboost at low RPM. But boost would "catch up" as RPM's rise (the lag I'm experiencing). Unless ECU never demands fully open vanes.

Need the max spike vacuum value though to confirm/deny N75 failure. Does anyone have this number?

May try to hunt down working ALH and hook up to it if no one knows this value. Just more work.

PS. Don't have VCDS, but had it scanned (only at idle though).

Readings:
RPM 903, Boost req off, actual boost 969mbar, charge press 99.6%.
RPM 903, Boost req 999, actual 979, charge 43.8%.

 

[oilhammer]

You need to drive the car with a proper scan tool and look at the data. The VNT control system is brutally simple, but obviously some folks get caught up in it too much and cannot see the forest for the trees, which sort of sounds like what it going on here.

When you say "underboost issues", I take that to mean the car goes into limp mode under higher power demands, and sets a P0299 DTC "boost control negative deviation"?

If that is the case, here's what you can do even without a scan tool to watch the engine data:

Verify the vacuum system is intact, starting with the source (vacuum pump). If your brake assist is normal, you can assume the pump is at least working and there may just be a small vacuum leak somewhere... but still, check the big booster tube for any cracks or splits. This is VERY common.

If that is good, then you can start checking the system at the small nipple coming off of that big tube. That supplies vacuum to the three items that require it: the VNT actuator, the EGR valve, and the anti-shudder valve. There is a one-way check valve (the black and white one, we call this the 'ying-yang' valve) that is easy to check... should hold vacuum one way, and not the other. If it is open both ways, it's bad.

You can one by one check the entire system's hoses, but I can tell you that quite often by now they are rotten or loose fitting by now anyway. I remove the entire vacuum harness from the car and rebuild it on the bench.... easy to do, the whole bracket comes off with just two nuts. Remove the line down to the turbo and fish it up, unclip the anti-shudder valve solenoid on the bracket on the intake manifold and unplug its connector, unplug the connectors for the N75 and N18 solenoids, the EGR valve itself, CAREFULLY disconnect the vacuum lines from both the anti-shudder valve actuator and the reservoir ball on the front of the engine, and the vent line to the air cleaner.... and slide the whole assembly out.

The VNT actuator is a simple vacuum setup, it should hold vacuum, it is NOT self venting. Same for the EGR valve. These two items vacuum is released through the N75 and N18, respectively, to a tee which then goes to the clean side of the air cleaner.

If the control side is all good, then you may just have a boost leak somewhere, which is a different diagnostic process but mostly just visual.

 

[AceTdi]

Please read my original post. There's no vac leaks up to N75.

I know this because I have 27" of vacuum into the N75 as stated in OP, but only max 13-14" out of N75 into turbo under full driving.

I need to know correct in/hg value coming out of N75 on max vane demand.

-New brake vac hose.
-Replaced many vac lines.
-Replaced faulty check valve.
-Actuator moves freely and holds vacuum.

I've done the low hanging fruit, been diagnosing for months.

I'm looking for the correct max vacuum exiting N75 into Actuator under max vane demand.

Does anyone have this number or is able to hook on to their working N75 output line with mighty vac while driving to share what they get?

(Yes, underboost P1556 into limp under load).

 

[oilhammer]

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 actuator to open vanes fully, would this not point to N75 issue? (IE, not enough vacuum to fully open vanes, thus causing underboost and severe lag where some boost kicks in and higher RPM?)

Trying to confirm correct values before throwing parts (don't have known working one to swap-nostic with). Can't find much advanced info on vac issues.

PS. Tested N75 connector resistance at about 17.7ohms

It's 02 Golf TDI ALH. Anyone have these numbers? Or ideas?

YOU read it. Tell me where it says you did all the stuff I mentioned.

Good luck. You'll need it. Along with a scan tool. I'm out.

 

[BobnOH]

You need to drive the car with a proper scan tool and look at the data. The VNT control system is brutally simple, but obviously some folks get caught up in it too much and cannot see the forest for the trees, which sort of sounds like what it going on here............................

OH knows these cars as well as anyone. If that number were valuable for troubleshooting, he would know it off the top of his head. I would read his post carefully, you'll be almost done.

Please read my original post. I've done all that. ........................................

Please post up the logs, there are folks here who can interpret those.
Also you can swap the N75 and the N18 for testing.

You may also want to replace the rest of the vacuum lines.
The turbo boost does not follow RPM with no load. You have checked the actuator holds vacuum, it's free to move, does it move when the engine is started?

 

[AceTdi]

OH. If there was vac leak anywhere along the lines, it's not showing pre-N75 since 27" go in as posted. So it's not a vacuum leak. Or if there is a vacuum leak, it's not affecting input vacuum enough to matter.

However, since max only 13-14" come out, there could be a fail/leak of the N75 which is what I'm trying to rule out with complete certainty. Not speculation.

I am very suspect that 14" of vacuum would move an actuator that requires 18" to fully open, but VW has done much sillier.

Bob: haven't checked if actuator moves when engine started, will check when I have partner.

The only N75 logs I have are at idle (not chart):
RPM 903, Boost req off, actual boost 969mbar, charge press 99.6%.
RPM 903, Boost req 999, actual 979, charge 43.8%.

Would sure love that number. Because that give concrete proof if N75 functioning properly. If there's another way to check it without scan let me know.

Again, focusing on vacuum system here. Will run boost side once building adaptor.

If that checks out, it's sticking vanes. But don't want to yank off until ruling the rest out concretely.

 

[stomachbuzz]

I'm out.

 

[flee]

Maybe it will help if you 'bench' test your N75. You can do this easily by hooking it
up to 12 vdc and verifying that it a. holds vacuum when off and b. opens when on.
The N75 can get debris in it that can be cleared by blowing low-pressure
compressed air or 'Dust-Off' through it while cycling it with 12 vdc.

 

[Enabled]

All this strange effort. If you think it's the N75, replace it, or switch with the N18 temporarily to see of it makes a difference.

 

[burn_your_money]

If you have a mityvac you can test the vanes opening and closing vacuum to determine if you have sticky vanes. Sorry I don't have a number for you.

 

[jsrmonster]

Is it auto or manual? Stock or tuned?
N75 duty cycle at idle can be 50% (14”hg) for auto and 75% (21”hg)for manual. Full duty cycle / vacuum occurs ad idle (max vane closed for boost) and min 20% at wot min open vane, max vent.
Jeff

 

[AceTdi]

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!

 

[AndyBees]

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!

 

[AceTdi]

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.

 

[Nevada_TDI]

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.

 

[wonneber]

(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.

 

[AceTdi]

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

 

[flee]

I refuse to post to a new thread about the same situation.

 

[BobnOH]

I'm a sucker, I looked.
Seems we're diagnosing Limp Mode.

 

[flee]

I'm a sucker, I looked.
Seems we're diagnosing Limp Mode.

From day one.

 

[BobnOH]

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

 

[AceTdi]

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.