How to replace the VNT turbo actuator on ALH TDI engine or
adjust the rod
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difficulty: 2/5
Introduction
This article shows how to replace a turbo actuator on a 1999-2003 mk4 TDI with ALH engine and a possible solution to avoid replacing it. It looks like a wastegate vacuum can and uses a lever to move the VNT vanes inside the turbo. The manual even calls the VNT actuator a wastegate and they are similar in location and function, so don't get confused if you see the term wastegate used instead of VNT actuator.
The biggest difference in the can's function is that a wastegate can uses pressure to move it's rod and is found on non-VNT turbos. All VNT turbos (all mk4 and newer TDI) use a VNT actuator under vacuum to move it's rod. A sticky turbo actuator can cause limp mode by not moving the vanes as the car's computer expects. Dealers often replace the entire turbo because they make more money from replacing the entire turbo and it increases the chance of fixing the symptoms in 1 visit. Other mk4 TDI engines and 2004-2006 mk4 TDI are similar. It is possible to adjust the VNT rod as well to adjust the movement but before you try this, you must 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 it can cause dangerous boost spikes or lack of response.
See 1000q: limp mode fix for other things that could cause limp mode.
Parts (click links to compare prices)
1 VNT actuator - VW# 038 198 716, available from kermatdi VNT actuator, or tdiparts VNT actuator
Procedure
First, you should diagnose a possible sticking VNT actuator. Have a helper turn on the car and the wastegate lever should move. If you have a vag-com, with the engine running, go to group 011. Go to "basic settings". Click this on and you should see the n75 cycle the VNT arm. These cars also have a problem with soot and carbon clogging the VNT vanes, which causes the same symptom as a sticking wastegate. You can also use a vag-com to log requested boost vs actual boost. If it's not close then this could indicate a defective or clogged VNT actuator.
The first possible fix you should try is to use a few drops of PB Blaster on the joints and then going for a test drive. This may loosen rust enough to help loosen any seized parts and might temporarily solve the issue.
A seized actuator could also be caused by soot buildup in the turbo. As good practice, you should rev the engine high and hard to raise exhaust gas temperatures (EGT). Raising EGT will burn up and blow out carbon and soot build up out of the turbo 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 minute or two of normal driving afterwards. You don't have to cool down by idling a minute or two after every time you drive, just drive sensibly before shutting down the car and this counts as a cool down period. A good place to do this is on highway on ramps (always observe posted speed limits), or on the highway. 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. This only applies to mk4 and newer cars (1998+ New beetle, 1999+ Jetta, Golf, Passat) because earlier TDI use a conventional turbo. See 1000q: turbocharging for a more general FAQ on turbos.
This procedure, aka the Italian tune up, is also good practice for any car since it can help burn up carbon deposits in the engine (such as on the valves) as well. Before you order a VNT actuator, try it and see if it helps. You might go into limp mode a few times but it's free and it might solve or help if the problem is soot buildup instead of a bad actuator. One common myth is that the actuator can won't move throughout it's range without the Italian tune up. The actuator can's lever moves every time you start the car and it moves at low rpm. Raising EGT is what helps burn up the carbon deposits since the vanes inside the turbo are where carbon collects. Rust on the lever arm is a different mode of failure.
Here is the TSB for VNT replacement with pictures and procedure: VNT
actuator technical service bulletin. Some further tips on removal:
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. It is possible to do it from the top but it
should be easier to see it from the bottom.
2 bolts hold the actuator to the turbo. Spray them with some PB Blaster and let soak.
There is a small circlip holding the rod to the lever
arm, use a screwdriver or a pick to remove the clip and the other end of the rod
should slide off. Also remove the vacuum lines. Pictured below with
arrows are the bolts. Pic from tdiparts.
To adjust the rod, 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.
Loosen the locknut and turn the rod to adjust the length. Try 1/2-1 turn at a time. Tighten the locknut when satisfied. The best thing to do is to match the length of the old rod so that there are no change in how the turbo works. I do not suggest adjusting it if you are on the stock turbo and the car is working properly.
You can also apply suction to the can and check it's movement. Specs on a stock turbo are at about 18 in hg full travel. The official method is to use a .05mm feeler gauge between the stop and vane mechanism. Pics and further details are in the TSB linked above and here. Unlike a conventional wastegate, it uses suction from the N75 (boost solenoid that controls air to the wastegate/VNT actuator can) instead of boost.
Torque specs:
lock nut on shaft rod: 53 inch pounds, or 6 Nm
2 bolts holding the actuator: 53 inch pounds, or 6 Nm
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