ALH Boost issues

Dhawk12

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I have a 2002 Golf ALH TDI with Manual trans (VNT-15 turbo) and I recently got VCDS. I was doing some logging to check the health of various components and noticed that there were some issues with actual vs requested boost. So I did a few power runs in third gear on flat from about 1800/1900 RPM to ~4000 RPM and below are my results:




I also have the log created by VCDS if that would be more helpful than the picture.



As you can see the boost spikes up to ~2500 mbar for a couple seconds and then drops below requested and doesn't really ever get back to requested during the power run. It is above the requested boost for 5.65 seconds before it drops below. It is not currently throwing any overboost codes (which is strange because in the logged data the actual boost was over 300 mbar over requested for 4.21 seconds), although it has done it once on this turbo set up (it used to regularly throw overboost codes on my previous turbo).


The duty cycle for the N75 is at ~80% at 4000 RPM.


Where should I start with resolving this? I am guessing I will need to verify the vacuum lines and actuator functionality but I will have to find a vacuum tester to do that. Could it also be the stop screw needs to be adjusted?
 
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2000alhVW

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Yeah, that's a pretty big overboost spike.
In my experience, the ECU is pretty "dumb" and has pretty bad logic on when to throw a CEL. Don't count on it to throw codes to let you know there's a problem.

On these cars...overboost can be a myriad of possible causes. Bad MAF, compromised vacuum lines, sticky N75 solenoid, sticky turbo vanes, actuator rod out of adjustment.
 

Dhawk12

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Ya I understand that just waiting until it throws an "idiot light" is not a solution.


So at this point I am pretty much going to have to follow the numerous threads for solving overboost.


I graphed the MAF actual vs requested this morning and it did not seem to be deviating by any noticeable amount, so hopefully that rules out the MAF.


My next plan is to replace the vacuum lines as they are still all factory.
 

WildChild80

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It looks like a dirty turbo to me, a clean turbo responds very quickly to pedal input but if it can't move the vanes far enough it'll take a while to make boost

Possibly actuator rod but I'd say to check how far the vanes move and see if they move freely

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Curious Chris

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What is the time scale on the x-axis? You need about 15 seconds at full throttle to get a good picture of the boost.
 

AndyBees

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The MAP sensor can be a culprit when it comes to over-boosts. But, usually the ECU will show a DTC if the MAP is not functioning properly.

Dhawk12, how did you produce the above graph?

"I also have the log created by VCDS if that would be more helpful than the picture"
 

Dhawk12

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The MAP sensor can be a culprit when it comes to over-boosts. But, usually the ECU will show a DTC if the MAP is not functioning properly.

Dhawk12, how did you produce the above graph?

"I also have the log created by VCDS if that would be more helpful than the picture"

I suppose I could try unplugging the MAP to test this right?


I produced it through much struggling with Excel's "super intuitive" (insert sarcasm) graph function.
 

Curious Chris

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The time scale is just over 12 seconds (12.05 from the start to finish).
Oh that means something is amuck. The boost control is a PID loop where it should overshoot, undershoot less, overshoot less, undershoot less, until finally you get to the A-D ripples.
 

AndyBees

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No! I wouldn't unplug the MAP. It also contains the Intake Air Temp (IAT) sensor.

If there is not a DTC relating to the MAP, I'd keep searching for other problems. However, I was having boost spikes all the way up to 25 psi but only occasionally would a DTC show for a MAP issue. I replaced the MAP and now no more boost spikes.
 

wonneber

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Long shot.
Make sure the bottom hose on the N75 is not clogged going through a tee and continuing to the bottom of the air filter box.
It's the release for vacuum to the actuator.
 

Dhawk12

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I will check that bottom hose on the N75.


Herm TDI - I haven't really looked at the intake lately, it was last cleaned in December 2015, so almost 3 years ago. I will pop off the EGR/ASV and have a look in there when I get a chance.
 

Nevada_TDI

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The N75% is a bit high, so there is likely a bleed-off issue if you have not played with the actuator arm. The one solution would be to lengthen the actuator arm a couple of turns, but based on what you have said, I would start looking elsewhere.
 

k_pt

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Thats a high overboost, would also check the play on the turbo, could be big by now.

About the overboost, can be stuck vanes, most probable, can be n75 it self, doubt its vacum hoses, that would do underboost.

The duty cycle read is what N75 is supposed to do, what the ECU is commanding, not what n75 is actualy doing.

Cat off, straight pipe, can cause overboosts too.

Disconnecting MAF will only help with overboost, since you will increase exhaust flow.

About lengthening the actuator arm, it's a solution, you can also install a dawes device, wich directs the excess boost to the vacum lines, forcing the vanes to open.

But the best solution is to check the play on the actuator. Loose the bracket that holds the actuator and wiggle it.
 

wonneber

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About the overboost, can be stuck vanes, most probable, can be n75 it self, doubt its vacum hoses, that would do underboost.
I was thinking if the vacuum release hose was clogged the actuator might not retract as fast as it should.
 

Dhawk12

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So here is where I am at. I checked the intake, and it wasn't plugged up, at least not the initial opening till it turns down to the engine.


The vacuum line running from the N75 to the airbox is clear, so it seems to be able to dump vacuum when it needs to.


I checked the turbo for play and it didn't have anything that felt out of the ordinary (very little side to side, and no noticeable axial play).

The vanes on the turbo also feel smooth (there is an occasional slight stiffness if the lever is pushed all the way away from the stop, but once that is over come, it is smooth and easy to move all the way to the stop.)

However, the actuator did not hold vacuum. I installed a spare used actuator I had laying around that does hold vacuum and adjusted it to the specs for starting and stopping (I will have to purchase a new actuator as the one I replaced it with, although it held vacuum, had a significant amount of rust buildup inside, and from what I understand, they are pretty much done when that starts happening.)


Hopefully that resolves the overboost issues!


On an unrelated note, I also just installed bigger nozzles so I should see a power gain (I am not entirely sure what size the nozzles are...its a long story, but they are bigger than the auto nozzles that were in it.) So now I need to adjust iq to see if I can stop it from being a freight train.
 
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eddieleephd

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So here is where I am at. I checked the intake, and it wasn't plugged up, at least not the initial opening till it turns down to the engine.


The vacuum line running from the N75 to the airbox is clear, so it seems to be able to dump vacuum when it needs to.


I checked the turbo for play and it didn't have anything that felt out of the ordinary (very little side to side, and no noticeable axial play).

The vanes on the turbo also feel smooth (there is an occasional slight stiffness if the lever is pushed all the way away from the stop, but once that is over come, it is smooth and easy to move all the way to the stop.)

However, the actuator did not hold vacuum. I installed a spare used actuator I had laying around that does hold vacuum and adjusted it to the specs for starting and stopping (I will have to purchase a new actuator as the one I replaced it with, although it held vacuum, had a significant amount of rust buildup inside, and from what I understand, they are pretty much done when that starts happening.)


Hopefully that resolves the overboost issues!


On an unrelated note, I also just installed bigger nozzles so I should see a power gain (I am not entirely sure what size the nozzles are...its a long story, but they are bigger than the auto nozzles that were in it.) So now I need to adjust iq to see if I can stop it from being a freight train.
Make sure neither of the vacuum lines are clogged.
You don't have any weird sounds coming from the engine after shutdown do you?
Since you just installed larger injectors, your going to need to adjust the actuator trial and error wise. Adjust, log, until it is good.
You definitely need to adjust the IQ.
Bad actuator would definitely be a large culprit.
I would work on the IQ then actuator adjustment to match.

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turbocharged798

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Set the actuator so it starts to pull at 5in/hg and comes to full stop at 20-25in/hg.

Slight drag in the vnt could be problematic. You DO NOT want any drag there at all or you will have boost issues. The ECU expects the lever to move a certain amount with applied vacuum and if there is drag, it will completely change that.
 

Dhawk12

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By neither of the vacuum lines, do you mean the two going from the N75 to the airbox? If so, they are both clear.


I do not have any weird sounds at shutdown that I have noticed? Is there something I should be looking for?


I plan to adjust iq and such later today.


"Set the actuator so it starts to pull at 5in/hg and comes to full stop at 20-25in/hg."

Turbocharged798, is this a recommendation based on me installing larger injectors? I have already adjusted the actuator to start moving at 4 in/hg (3-5 in/hg) and stopping at 18.5 in/hg as per threads on this site and a VW technical bulletin linked here.


I understand that the VNT should be completely free. I am going to do some logging to see if the overboost is resolved before I spend the time pulling the turbo and cracking it open though.
 

eddieleephd

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By neither of the vacuum lines, do you mean the two going from the N75 to the airbox? If so, they are both clear.


I do not have any weird sounds at shutdown that I have noticed? Is there something I should be looking for?


I plan to adjust iq and such later today.


"Set the actuator so it starts to pull at 5in/hg and comes to full stop at 20-25in/hg."

Turbocharged798, is this a recommendation based on me installing larger injectors? I have already adjusted the actuator to start moving at 4 in/hg (3-5 in/hg) and stopping at 18.5 in/hg as per threads on this site and a VW technical bulletin linked here.


I understand that the VNT should be completely free. I am going to do some logging to see if the overboost is resolved before I spend the time pulling the turbo and cracking it open though.
Those adjustment values aren't really worth anything in my experience. Those are the parameters by which the actuator should function, has nothing to do with the length of the arm and the adjustment.
I think it's horrible that this is what so many think. It has never worked out for me. No matter what my actuator begins to move at 4-5 and stop's around 18. The only way I can get it figured out is through logging and adjustment through log results.

I just got mine adjusted so maximum overboost is 100 on a log in 3rd, equalling approximately 1.5 psi over then dropping.

There's a more accurate method using a feeler gauge, however, behind the engine is difficult to use it.

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eddieleephd

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By neither of the vacuum lines, do you mean the two going from the N75 to the airbox? If so, they are both clear.
Two vacuum lines to the air box? Top one should go to vacuum ball and T, line below directly to the actuator, back line directly to air box for venting.


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eddieleephd

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Slight drag in the vnt could be problematic. You DO NOT want any drag there at all or you will have boost issues. The ECU expects the lever to move a certain amount with applied vacuum and if there is drag, it will completely change that.
This should say resistance to moving, instead of "slight drag," as binding of the lever when you tighten the lock nut is one of the most common mistakes at first.


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wonneber

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The vanes on the turbo also feel smooth (there is an occasional slight stiffness if the lever is pushed all the way away from the stop, but once that is over come, it is smooth and easy to move all the way to the stop.)
I've read you might free the arm up if you take the actuator off and work the arm back & forth full range for 5 or so minutes.
Never tried it myself.
 

Dhawk12

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Here are some updated logs after swapping the actuator for one that holds vacuum and adding the new nozzles in:




Here is a 3rd gear run:



And then I adjusted the IQ and the EGR adaptation:




And a third gear run:




Sorry for the large post. Though the graphs definitely look better, the spike level in third gear is still mildly concerning. Although it is only spiking to ~2448 (works out to ~21 psi) after the IQ adjustment for just a hair over 1 second and then it is back down to requested. So maybe that is normal from what I have seen in other threads.


Considering that the N75 is functioning and I have completed the above diagnosis already of the faulty actuator, does anyone have any suggestions for lowering the boost spike at this point? Adjust the actuator arm a little longer? Shorter? Crack the turbo and clean the vanes? Or is it not a concern at all?


Possible boost issues aside, I am really enjoying the extra power from these injectors in combination with my 11mm auto pump! Thanks!
 
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k_pt

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Oh, you have increased fuel delivery? Then that overboost is normal. Thats because exhaust flow is higher due to more burn. N75 map has to be adjusted now.
 
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Dhawk12

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Those adjustment values aren't really worth anything in my experience. Those are the parameters by which the actuator should function, has nothing to do with the length of the arm and the adjustment.
I think it's horrible that this is what so many think. It has never worked out for me. No matter what my actuator begins to move at 4-5 and stop's around 18. The only way I can get it figured out is through logging and adjustment through log results.

I just got mine adjusted so maximum overboost is 100 on a log in 3rd, equalling approximately 1.5 psi over then dropping.

There's a more accurate method using a feeler gauge, however, behind the engine is difficult to use it.

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When you are adjusting, are you lengthening the rod to lower the overboost?



Or is this just a poor substitute (that would cause excess turbo lag), in my case with more fuel, for a proper N75 map?
 

eddieleephd

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Tighten to increase boost and loosening to relieve boost. Shorter is more.
You really want to be 100mbar over at the peak to decrease turbo lag.
69mbar is 1psi so 100 mbar is aproximately 1.33 psi boost.

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Dhawk12

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Tighten to increase boost and loosening to relieve boost. Shorter is more.
You really want to be 100mbar over at the peak to decrease turbo lag.
69mbar is 1psi so 100 mbar is aproximately 1.33 psi boost.

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Just to be sure, when you are saying tighten to increase boost, etc. are you talking about the VNT actuator rod, or the stop screw on the turbo?
 
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