How to diagnose, test, or replace the VNT turbo actuator on ALH TDI engine and then adjust the rod
back to 1000q:mk4 general FAQ and "how to" index
back to 1000q: pumpe duse mk4 "how to" list and FAQ
difficulty: 2/5

Introduction

This article shows how to test or replace the turbo actuator on a 1998-2003 mk4 TDI with ALH engine and a possible solution to avoid replacing it.  It looks like a wastegate can and uses a lever to move the VNT vanes inside the turbo.  They are similar in location and purpose, so don't get confused if you see the term wastegate instead of VNT actuator.  Related links: 1000q: fixing limp mode and 1000q: constant low power or can't rev.  Also see 1000q: ALH/BEW turbo removal and 1000q: turbo vane cleaning.  

The biggest difference in it's function is that a wastegate can uses pressure to move it's rod and is found only on non-VNT turbos.  All VNT turbos (all mk4 and newer TDI) use a VNT actuator under vacuum to move its rod.  A sticky turbo actuator can cause limp mode by not moving the vanes as the car computer expects.  Dealers usually replace the entire turbo because they make more money and it increases the chance of fixing the symptoms in 1 visit.  Other mk4 TDI engines are similar.  2004-2006 mk4 1.9L TDI use a slightly different actuator with a sensor to see rod position.  2004-2005 mk4 Passat TDI use the non-sensor can.  Some newer TDI now use electrical motors to move the lever arm instead of a vacuum/spring loaded can.

It's possible to adjust the VNT rod to adjust its  movement but before you try this, carefully check and replace all vacuum lines and other possible maintenance items.  If it was working fine before, do not adjust the VNT rod.  Only adjust the VNT rod if it was out of adjustment before or you are modifying the turbo or turbo setup.  Adjusting it when it doesn't need adjusting will not improve turbo response or fuel economy and could cause dangerous boost spikes or lack of response.

How it should work
The lever on the outside is welded to a lever inside the turbo housing.  This is how it moves the VNT vanes.  See the below videos to see how smoothly and free the lever should move.  It should not stick or bind at all.  Vacuum is being applied to the can, not pressure.

The lever moves a ring and the ring moves the vanes.  These vanes change the angle and speed of exhaust hitting the turbine wheel.

Again, dealers prefer to replace the entire TDI turbo and actuator as a unit because it costs more and because it also rules out any internal turbo/VNT vane sticking problem, increasing the chance of the symptoms being resolved in 1 visit.

Once the engine is fully warmed up, you should rev the engine high and hard on a regular basis, like when entering highway on ramps (always observe posted speed limits), to raise exhaust gas temperatures (EGT).  Raising EGT will burn up and blow out carbon and soot build up out of the VNT vanes and help prevent them from getting stuck.  Only do this once the engine is fully warmed up and let the car cool down with a period of normal driving afterwards.  The key is both high rpm and high load.  High load at low rpm or high rpm in neutral with no load will not raise EGT high enough and long enough to burn up the soot.  

Before you order a replacement VNT actuator, spray some penetrating lubricant around the VNT lever as described below and go for a few hard runs.  You might go into limp mode a few times but it's free and it may solve the issue if the problem is only minor soot buildup.  The problem isn't from only vane movement because if you monitor VNT movement through VCDS, it reaches full movement at a relatively low RPM.  The key is both high rpm and high load (temperatures) to burn up and blow out the buildup.  Another problem is corrosion buildup around the adjuster ring inside the turbo.

Limp mode with underboost is safer, limp mode with overboost is more dangerous.  Have the car scanned for codes and use a boost gauge to confirm which you are getting.

Also see the TSB for VNT actuator replacement with pictures and procedure: below is both a clickable thumbnail picture and a link VNT actuator technical service bulletin

Parts (click links to compare prices)  

Note - The ALH engine (1998-2003 TDI) is a little different from the BEW engine's (2004-2006 4th gen TDI) VNT actuator because the BEW's actuator has the "G500 sensor" to determine vane position.  This is also called the smart actuator.  Because of the vane sensor, it is currently not available separately from the turbo.

1 VNT actuator for ALH engine (not BEW engine) - VW# 038 198 716, available from kermatdi VNT actuator, or tdiparts VNT actuator, or vwcarpartsonline, or metalmanparts actuator
1 circlip/e-clips for the actuator lever and 1 spare - no VW# available but a 1/4" e-clip should work.  The spare is for when the first one flies off and disappears.  If you know the correct size/shape, please post your comment in the myturbodiesel forums, thank you!
I avoid any actuator sold by since it's almost certainly a cheap copycat part.  I also had a bad experience with this seller so never again.

Procedure
First confirm bad VNT or isolate another problem

First confirm the diagnosis of a sticking VNT actuator because it could also be a sticking turbo VNT vane, bad n75 solenoid, or leaking vacuum line.

Have a helper turn on the car and the VNT lever should move.  If you have a VCDS, with the engine running, click on "engine" and then "output tests".  One of the tests is N75 test cycle.  This cycles the n75 solenoid, VNT, and vacuum line.

A common problem is soot and carbon clogging the VNT vanes which causes the same symptom as a sticking VNT lever.  See 1000q: clean turbo VNT vanes to see a detailed cleaning procedure.  

Through VCDS, log requested boost vs. actual boost.  To do this, plug in your VCDS cable to the obd2 port under dashboard, near the steering wheel.  Click on "engine".  Then click on "measuring blocks" in the below screen.

Then click "up" or enter 11 to view "group 11" and graph to see the below screen.  If it's not close then this could indicate a defective or clogged VNT actuator, N75 solenoid, vac lines, or sticking vanes.  Here is an example of a good N75 solenoid-vac lines-VNT actuator-clean turbo.  Suddenly stomping on the accelerator pedal will produce spikes - this is normal.  Smooth, normal acceleration should produce minimal spikes.  Swap the N75 solenoid with the EGR solenoid as a test to isolate a possible solenoid problem and check the vacuum lines.  This will isolate a boost problem to the VNT actuator or internal vanes.  To further isolate those, do the tests below.

First put a few drops of PB Blaster on the VNT lever and then going for a test drive.  This may loosen surface rust enough to get enough movement.  You may have to remove the rubber elbow before the intake manifold and turbo intake plastic hard pipe for clearance.  Below is a picture of the passenger side with those hoses removed.  While this won't free a seized VNT vane, it won't hurt anything either.  Warm up the engine and go for some very hard runs as described above.

Here is the turbo out of the car for illustration.  Put a few drops of PB Blaster around the lever and circlip to help them move smoothly.  (there's a heatshield around the can, not pictured)  If you try to press the lever down by hand, you should feel smooth, consistent resistance from the spring inside the VNT actuator.  After you release the lever, the spring should push it back up to the top resting position.  Full travel from the stop is about 1cm.  

Follow the vac line to the N75 solenoid valve (solenoid is mounted on middle of firewall - look above the brake fluid reservoir).  Remove the vacuum line at the N75 solenoid valve and apply vacuum to the line.  The rod should move smoothly beginning at about 3-5" vac and hitting the stop at 18-22".  Full travel from the stop is about 1cm.  You can also use your hand (on a cold non running engine only!) to feel the rod move as you apply vacuum to it.  It should move smoothly with no binding or clicking.  

If the VNT actuator is not holding vacuum then the VNT actuator is bad (since you've already checked the vacuum lines).  Replace it.  

If it's binding, clicking, or not moving at all, remove the actuator rod from the VNT lever.  This will let you isolate and test the VNT lever's movement.  If it's seized then you have to remove the turbo and clean the vanes.  See 1000q: turbo vane cleaning and 1000q: turbo removal ALH / BEW.  When the vanes stick too much, the internal lever and ring wear into each other and stick/create play.

Removal and replace VNT "can" or adjust rod

It's easy to remove the actuator if you have a ratcheting wrench like this.  In the pictures above, you can see the 2x 10mm nuts that hold the actuator to the turbo.  Spray the 2x 10mm nuts with some PB Blaster if they're rusty and let soak.  There is a small clip holding the rod to the lever arm, remove it.  Also remove the vacuum line.  You can now remove the actuator.

You can remove the actuator from above or below.  From above you have to remove the 2 hoses as pictured above to get access.  From below, raise the front of the car and remove the plastic lower engine bay shield, chock the rear wheels, securely rest the car on jack stands, note all cautions in your factory service manual, etc.  Make sure the car is safe and secure before getting under the car.  From below, I've found that looking behind the front subframe at the angle pictured below lets you see the circlip and lever instead of feeling for them blindly.

You might be able to adjust the rod without removing the actuator from the car.  Loosen the 10mm locknut on the rod's shaft and turn the rod to adjust the length.  Try 1/2-1 turn at a time.  Tighten the locknut when satisfied.  

If you have a new VNT actuator, match the length of the new and old rod so that there are no adjustments to rod movement.

Apply vacuum to the can or vacuum line to double check it's movement.  Specs on a stock turbo are movement at about 3-5 in hg and 18-22 in hg full travel.  Unlike a conventional wastegate, it uses suction from the N75 (boost solenoid that controls air to the wastegate/VNT actuator can) instead of boost. 

At full vacuum, the lever should be at the stop screw.  At full release, the rod should be fully extended but not hitting anything.

Tighten the 2x 10mm nuts to 53 inch-lbs, this is about 4.4 ft-lbs.

Feel free to sign up and ask a question about this article at the forums here: myturbodiesel.com forums