VW ALH Turbo and Vacuum Problem Determination Info

.....the tale of getting the turbo in my manual tranny 2003 Jetta Diesel working again.....
this diesel and turbo was used in the A4 series VW built between 1999 and 2007

What Can Go Wrong that you have no/not much power...the engine won't rev above 3000 RPM and has no guts.

But first:

There are all kinds of reasons you can have little get up and go....

Anyway, What Can Go Wrong In the Vacuum Control System

The following has references to the 'Bits & Pieces' section in the appendix

Bad Actuator

See here below

Check valves

Clogged or jammed

Bad solenoid valves

Not operating or valve(s) leaking. The valves are mounted on the top of the firewall at the back of the engine compartment, turbo controlling N75 on the right (driver's) side, N18 EGR controlling. in the middle.  The intake manifold changeover/anti-shudder N239 valve is clipped to the intake manifold a bit to the left of center

Bad vacuum lines

Hoses may be disconnected, broken, kinked.  Check carefully, better yet replace them with silicone vacuum hose

Frozen VNT

I won't address this (didn't have the problem), other than to say: if you pull vacuum on the actuator and it doesn't move, likely the VNT is frozen.

Bad vacuum pump

Pump may be failing, internal seals may be broken or leaking. Test with MityVac, search on posts for this. Seals may be replaceable.  It is a round flat tin can, rather like an oversized snuff tin on the right side of the engine block.

Problems elsewhere

Clogged Intake Manifold

The intake manifold can be full of carbonized gunk, see here for how to determine if this is the case.  If it is, you have major mechanic's work to get the manifold off the engine and then cleaned out.  Some people burn them out, Google "burning out a clogged vw diesel manifold".  As should be obvious, an engine that can't breathe doesn't have any power.

Clogged Catalytic Converter

Obviously, if an engine can't exhale.... See here.


Make sure that your fragile and expensive intercooler is protected by a fully intact and secured fender liner.  The intercooler cools the air fed to the turbo.  Cooler air is denser air with more oxygen.  The turbo, in compressing the air, also heats it up...basic physics..this pre-cools it.  It's on the passenger side in front of the wheel, behind the fender liner.  If the fender liner has broken free or shredded (happens a lot), your intercooler is unprotected.
There is a special RF fender line that has a grille for the intercooler available from IDparts, as are all the securing hardware.  You get extra points for checking this out and making sure it's right...an ounce of prevention is worth a ton of cure....and it's dead easy to do.


Problem Determination/Analysis

The following has references to the 'Bits & Pieces' section in the appendix which has parts diagrams and numbers....

If you have no power, it could be that the intake manifold is gunked up.  Check it with the simple quick test here by pulling the air intake hose off the air filter box and listening for the thumping sound of an engine that's breathing normally.  Also the turbo kicks in when you're reving the engine...if you have power at the low end of revs but not at the top, it's probably the turbo or its controlling system

There are basically two ways for there to be no turbo:
  1. The vacuum control system is messed up in some way. I am going to address this.  Debugging it is relatively inexpensive: the full kit of parts should be no more than $200.  It's also doable without a lot of tools and can be done over a number of interrupted days with the car drivable in between (if with no turbo)
  2. The turbo or its actuator/VNT is dead OR the actuator is out of adjustment. 
I first went after #1, which got me going again for a few months, but the problem ended up being #2

#1: Checking out the turbo's vacuum control system

Do the simple things first which don't require fancy tools (vacuum pump/gauge & a VAG tool).  What follows is in order of increasing expense and involvement. You may get lucky.  If not, then...


The valves are mounted on the top of the firewall at the back of the engine compartment, turbo controlling N75 on the right (driver's) side, N18 EGR controlling. in the middle.  The intake manifold changeover/anti-shudder N239 valve is clipped to the intake manifold a bit to the left of center. See here for more information on the N75. If you swap/replace solenoid valves, the hardest part is getting the VW's D-shaped electrical connector to release.  You can hunt around on the TDI club for hints on this, and there are YouTube vids about it. It boils down to inserting a slim flat piece of metal to release the lock, see here.  I used a medical tool from my surgeon father's surgical kit.

Logically I would have done this next, but when I checked the actuator under Further Vacuum Tests, I found it bad. There is info about this in the Diagnosing and Fixing Limp mode for A4 1.9TDI [low power troubleshooting] post

2: Checking out the turbo actuator

Problem analysis hints and tips

My car has a nylon coupling just for tee-ing in; it's just downstream of the N75 valve on the line to the actuator (that's the lower of the two lines coming off the white top of the N75


A best starting point would be to get a scan of any fault codes and doing logs of the mass air flow. If your car still has the original MAF sensor it could be the source of your low power issues. The originals were famous for being faulty. To identify it, the VW part number would read 038906461, the latest revision is  038906461C. This requires a VAG.

You can tell if the intake is restricted by taking the large hose off the intake manifold and starting the engine. If it makes a hissing sound it is likely clogged up. It should sound more like thumping on an empty tube (blue man group-like)

Pertinent TDIClub fora threads and posts

Search (Advanced search, VW MKIV-A4 TDIs (VE and PD) only), note that you want ALH engine posts, not BEW

Thread:  Code P0299 "turbo underboost" Resolution Repository Note that this is for Passats
Some pertinent posts:

Fixing/Replacing the Darn Vacuum actuator


Background Information

Parts in the Vacuum System Controlling Turbo Actuation 


 ALH stock Vacuum System Diagram (at right) Legend

Matching part numbers:
1: N239 - 1K0906283A, 7.02256.00.0 (2000 on, D shaped connector), 1J0906283A (Before 2000, square connector)
4: N18 - 1J0906627, 7.02183.01.0
6: N75 - 1J0906627A, 7.02184.01.0*
7: 046905291A
10: Part of 
Vacuum Pump to Brake Booster Line  1J0612041GD, See 2nd image below right
11: Vacuum pump, 038145101B, 7.24808.05.0;
13: 045131501C
TDI Club Forum post
Q: "Are N75 and N18 'exact' same, they have identical part numbers apart from letter A at end off N75 number, and they look identical with exact hose connections and power plug!!!
A: They look very similar and work the same way. But the N75 controls the turbo VNT actuator and the N18 controls the EGR."


Vacuum Hose

ID Parts, Vacuum Hose Kit

McMasters Carr silicone vacuum hoses
Ft. 5041K521 Metric Silicone Rubber Tubing Firm, 3 mm Id, 8 mm O.D., 2.5 mm Wall, Black (Same as 5041K52)
Ft. 5054K531 Metric Silicone Rubber Tubing Soft, 4 mm Id, 8 mm O.D., 2 mm Wall, Black (Same as 5054K53)
10 foot of black 3 mm.
5 foot black 4mm
I'd say add a foot to each, just barely squeaked it out.  You can also get silicone tubing from China inexpensively in different colors; search on EBay for '4mm silicone vacuum tubing'.

Parts Offerings/Sourcing,

Mostly IDParts here, but you can search on the part number on Ebay, Amazon, et al.  It should be obvious that a part that is very difficult to get to/replace or that gets a lot of stress is probably not something you want to get from the absolutely cheapest source.  Further wisdom: "There are only a couple of brands of these control valves (solenoid vacuum valves, N75, N18 & N239) that are known quality parts. Pierburg is one and they usually cost more than $25."  ...and Pierburg is German.  Their parts should come in a blue box with impressive German sticker and even maybe a hologram strip.


Original source of vacuum system diagram (Thanks Dave Linger!)


Parts Sources