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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 October 13th, 2018, 23:16   #1
Join Date: May 2018
Location: Langley, Canada
Default ALH Boost issues

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?

Last edited by Dhawk12; October 13th, 2018 at 23:22.
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Old October 14th, 2018, 10:30   #2
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Join Date: Aug 2018
Location: Silver Spring, MD
TDI(s): 2000 Golf

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.
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Old October 14th, 2018, 16:02   #3
Join Date: May 2018
Location: Langley, Canada

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.
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Old October 14th, 2018, 17:39   #4
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Join Date: May 2016
Location: Hampton Roads, VA

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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