2004 Passat BHW 2.0 TDI - rare random turboboost loss

Issue:
At random times, driving down the road or highway, my Passat wagon will suddenly lose all turboboost and gradually slow to a 20mph crawl.
What I do know:
-Doesn't happen every day. Completely random.
-Never happened more than twice in one continuous trip, but when it happens it is extremely frustrating especially if I am on the interstate.
-The only temporary remedy I have found is that I have to pull over, shut it off, crank it back up again, and it is fine.
-I have only had an engine light twice from this but it disappears before I finish my trip. Was able to catch it once and the trouble code description said: "Turboboost control range not reached". Can't remember the code.


I have 247k miles on it.
I have read around on here searching for some insight but am striking out.
All I can surmise is that it could be one of a few things.
1. EGR
2. VNT Actuator mounted on top of the turbocharger
3. N 75 valve? (Enlightenment on the purpose of this valve, anyone?)
4. Any of the little vacuum hoses that run between the N75 valve, VNT Actuator, and air filter box above the turbo.

Since January of this year when I bought the car, I have already shelled out over $5000 in major catch-up maintenance the previous owner neglected and didn't know better than. This little issue is an ongoing thorn in my side and I would like to correct it as soon as possible. Can anyone help me?

Sounds like limp mode, but I never had mine drop down to 20mph when it occurred - certainly had a sizable reduction in power, but not 20mph reduction.

That said, the N75 is a valve which, when controlled by the engine computer, regulates exhaust flow into the turbo. By doing this, it increases or decreases the speed of the turbo compressor wheel / turbine, which increases or decreases the amount of boost. If the engine computer senses to much or too little boost, it goes to "limp" mode where the engine will still run, but the boost is pretty well shut off.

The key is to find what condition is causing the limp mode. The best way is to find someone who can scan the car, and preferably capture a data log of requested boost vs actual.

The N75 can cause this, the VNT can cause this, a boost leak, and a number of other possible causes. The key is proper diagnosis, otherwise you are going to be making some potentially expensive guesses.

Logging boost requested vs actual, as well as a good visual inspection of all components, and a vacuum test of the VNT actuator is where I would start.

Thanks for the insight, Windex. Do you have a specific suggestion for what kind of tool I can purchase to test the vacuums on said parts of my engine?

Boost issues are pretty frustrating. And it feels pretty dangerous when it happens. But you don't wanna shell out if you don't have to. Read this. Not sure if there is one for our platform, but the ideas are the same:
http://forums.tdiclub.com/showthread.php?t=179589

I can think of one cheap thing to try that's worth doing regardless of whether it's the cause of your issue and that's buy new vacuum hoses and replace every inch of hose in your engine compartment. It's cheap and easy and you won't believe how fried the one going into the VNT actuator probably is by now! There are a million posts here on what's the best replacement. I just ordered OEM from idparts and used that. There are two sizes and I think they reference how much you'll need. To remove old ones, I suggest cutting parallel to hose with a razor to avoid stress on the plastic tees they attach to.

I believe over boost doesn't usually set the CEL but there will be a code if you attach a reader. Over boost probably isn't vacuum related unfortunately. (unless the vent hose from N75 to airbox is plugged) Vacuum applied to the VNT actuator yields more boost so a vacuum leak is going to cause under boost.

This was the issue that finally drove me to buy a VCDS cable. I'm glad that I did. They hold their value well so I figure I can just sell it if needed.

Start with replacing your vacuum lines. Start with the one from the VNT to the N75, then the ones below that. Next REMOVE the turbo pressure pipes and inspect them. If you don't find anything with those items, Replace the N75

Thanks vwztips and mgoff5000,

My initial plan is already to replace each of the vacuum lines pertaining to the turbo system.

vwztips, when you say remove the turbo pressure pipes, where are you referring? I'm not quite 100% on which pipe is which on the turbocharger. Schematic, screenshot or picture please?

This might help. Copied and pasted VAGCOM scan using a friend's VCDS cable.


Chassis Type: 3B (3B - VW Passat B5 (1997 > 2005))
Scan: 01 02 03 08 15 16 17 19 35 36 37 46 47 55 56 57 58 75 76 77

VIN: WVWVE63B14E308181 Mileage: 394140km-244907miles
-------------------------------------------------------------------------------
Address 01: Engine Labels: 038-906-016-BHW.lbl
Part No SW: 038 997 016 T HW: 028 101 143 3
Component: R4 2,0L EDC G000AG 0314
Revision: 12345678 Serial number: VWZ7Z0D9465243
Coding: 0150034
Shop #: WSC 06103 444 84163
VCID: 2F6611ABBD0B7046D7-5120

1 Fault Found:
16683 - Boost Pressure Regulation
P0299 - 000 - Control Range Not Reached
Freeze Frame:
RPM: 2268 /min
Torque: 282.0 Nm
Speed: 88.0 km/h
Load: 62.7 %
Voltage: 13.91 V
Bin. Bits: 00001000
Absolute Pres.: 2335.8 mbar
Absolute Pres.: 1897.2 mbar

Readiness: 0 0 0 0 0

There are 4 rubber hoses in the pressurized side from the turbo to the intake manifold. The 2 main culprits that go bad are the one from the turbo outlet to the crossover pipe and the short elbow directly in front of the intake manifold.

The turbo outlet usually gets a hole from the ground wire rubbing it or some random slit. The short elbow usually develops a leak on the inside of the elbow and is very hard to see without taking it off.

I have never experienced a problem with intake and outlet hoses at the intercooler. But there is always a first time for anything.

Ah ok the large hoses with the snap fittings? I shall inspect those this weekend when I don't have to drive anywhere and let you know what I find.

I checked the intercooler connections below the radiator a little over a month ago when I had the radiator replaced and the passenger side connection was slightly out and not clipped securely. Corrected that.

I am pulling out the intercooler hoses today. Will post photos of the results.