There are a lot of threads indicating problems with underboost, but not many with resolutions.
Mine is a 03 Jetta. The symptoms were the classic underboost symptoms. I couldn't get the engine to spool up to 4K rpms in 3rd gear. It would go into limp mode and at times throw a code. The symptoms got worse just after Matt-98AHU did my 2nd timing belt at about 189K, which threw another element into the troubleshooting process. I assumed that this was just coincidental from the start. I still believe that.
So to track this down I did the following:
Checked the vacuum system-
25" Hg at the vacuum pump. This is perfect.
about 20" Hg at the actuator. This is plenty.
All original hoses were replaced 50 K miles earlier.
Checked the VNT actuator-
Pull 25" of vacuum on the vnt actuator and it holds that vacuum. VNT operational. Good
Connected the vacuum tester to the VNT actuator, it moves smoothly through it's whole range. Starts moving at 4" and hits the stop at 18" Hg. That's right at spec. Good
Check operation of the N75.
Watch the actuator move when someone starts the car. Good.
Matt used his VCDS to exercise the actuator. Good.
Replaced Fuel Filter.
Replaced Air filter.
Swapped the MAF with a known good one.
Check for boost leaks-
Build a boost leak tester. Photos here
https://plus.google.com/u/0/photos/1...93054411129757
Basically this is isolating the boost path, which is between the output of the turbo and the EGR. So disconnect the rubber hose at the turbo and plug it, remove the end of the rubber hose at the EGR, push air into this end using a compressor. You have to make plugs from PVC and the air
injector of PVC and a compressor fitting. The boost reaches 30 - 35 psi so it has to be able to hold this much pressure from the compressor.
I had no leaks in the boost path. It held until the upper fitting blew off at 40 psi. Good.
Finally bought a VCDS, which I was hesitant to do. This didn't tell me
anything I didn't know already, but it does give a common language with
which to communicate with on line resources.
The group 11 log told me that I was underboosting until limp mode kicked
in.
Removed the EGR to check to see if it was getting stuck open by gunk in
the valve seat. Cleaned the minute amount of gunk out of the valve.
EGR functional. Good.
So at this point there should not be any underboost, but there is. The only things left are turbo and exhaust system. I talked to 3 TDI mechanics who all said that they had never replaced a catalytic converter and never seen one clogged up.
On sage advise from a seasoned TDI mechanic, I tested the exhaust system by loosening the 3 flange bolts holding the downpipe to the exhaust manifold.
Bingo. No more limp mode, no codes, and the car runs faster than I've ever seen it run. I can accelerate on hills in 5th gear, where before it would go into limp mode on a level roadway. I expect that performance will be reduced a little when I put a new cat on it, but am 99% sure that I have found the problem.
So in summary, check the vacuum lines and vnt and vane movement first, look for oil splatters indicating a cracked hose. Don't rule out a clogged cat. It's easier to check than removing the turbo or even the EGR.
Thanks go to Matt-98AHU (Matt), oldpoopie (Justin), runonbeer (Robby), and fixumhausrich (Rich) for answering my pleas for guidance.
Good Luck,
John