Limp mode experiment

turtle1026

Veteran Member
Joined
Feb 9, 2010
Location
Florida
TDI
2004 Passat
Traveling down the highway in limp mode I can max speed about 72 mph. I have a 16683 under boost code. If I clear the code while driving, I get about 10-25 seconds of boost until it lays flat again. I shot a video of this but I don't know how to post it on here. I did this repeatedly on a 150 mile trip. Any time I needed to pass someone I just cleared the code and took off. It was like having my own little turbo boost button. Because of this, I am convinced there are no boost or vacuum leaks, and the turbo vanes are not sticking. I had also applied vacuum to the turbo, and I get full range of motion on the actuator. Am I down to the only possible culprit being the N75 Pressure Converter? 2004 Passat BHW. Thoughts please.
 

auntulna

Veteran Member
Joined
Mar 13, 2005
Location
Springfield, MO
TDI
05 GLS Passat wagon, mit panzer plate
I have had boost leaks from both of the rubber pressure hoses. The lower one by the turbo was obvious from rubbing. The one on the 90 degree hose was very subtle, and I think was present for many months. I suggest taking that hose off and looking inside and out very closely. You might find a fine crack there.
 

rydogg

Veteran Member
Joined
Jan 29, 2008
Location
Indiana
TDI
Passat
You have a vacuum leak somewhere. You are probably overdriving your turbo and could ruin it quickly...and they're friggin expensive!
 

turtle1026

Veteran Member
Joined
Feb 9, 2010
Location
Florida
TDI
2004 Passat
I posted the video on YouTube at the following link:

http://m.youtube.com/watch?v=r2TEM4YoJO4

Maybe this will explain my point a little better. If a vacuum leak existed then how is it that I can simply turn it on and off? I know the turbo works because I get boost. Since the N75 controls the vacuum pressure, I was drawing the conclusion that must be the problem. I am aware that boost leaks can cause underboost conditions as well, but again how can it be cured simply by clearing fault code? This leaves either the engine computer or the N75. That is all I am seeking expert opinion on here. Am I missing something else?

I too have experienced a split elbow hose in the past. That is not the current case. Also, since exhaust gasses turn the turbo, I don't understand what you mean by "overdriving" the turbo.
 

rydogg

Veteran Member
Joined
Jan 29, 2008
Location
Indiana
TDI
Passat
"Overdriving" was a poor choice of words. Maybe I should have said over-boosting or over-speeding. If you've got a crack or a leak in your lines, your turbo has to work harder to give the same amount of boost that it did before the leak. This is extra wear and tear on your turbo. You are basically resetting the computer in it's baseline configuration and it ASSUMES no leaks. With this assumption your ecu tries to make the correct amount of boost for your engine speed for a short time. Once the ECU figures out there's an issue (10-25 seconds in your case), it puts your car into limp mode. Your ECU is basically saying "Hey dummy, I've got a problem here. Fix it!"

Over-speeding is a cause of turbocharger failure that is fairly difficult to diagnose. The compressor speed, or boost pressure, is determined in most cases by a waste gate actuator that simply regulates the speed of the turbine by allowing exhaust to bypass the turbine wheel once a desired manifold pressure is achieved. If there is a leak between the compressor and the engine, proper pressure will be harder for the turbocharger to obtain; the waste gate will remain in the closed position until the turbocharger overcomes the leak and provides pressure within the intake manifold. Even a small leak can cause the turbo to work 20% harder than normal to reach its desired pressure, so great care should be given to the plumbing system on the pressure side of the turbocharger. Inspect all hoses and fittings for fitment and tightness, looking closely for piping that could rub and wear holes that could cause leaks. Also inspect the intercooler core and compressor, bypass valve, if equipped for proper function.
http://www.workturbochargers.com/knowledge-base/turbo-failure-101.html
 

turtle1026

Veteran Member
Joined
Feb 9, 2010
Location
Florida
TDI
2004 Passat
Okay, fair points thanks. A few other points. I have another thread on here that identified my recent head gasket change. With that of course there was a ton of disassembly and reassembly. I also thoroughly cleaned the intake manifold and changed oil and coolant. I then drove a 300 mile trip flawlessly until the very end when I went to limp mode. I have inspected and touched every hose but I will go forward with a true boost leak test.

That said does anyone know what kind of start up test the N75 goes through? Why does it seem to "allow" vacuum to flow for those moments right after code clearance? I really want to understand this system better. Thanks for the help.
 

turtle1026

Veteran Member
Joined
Feb 9, 2010
Location
Florida
TDI
2004 Passat
The intermittent nature of my boost coming and going was annoying me for a while. I tested for boost leaks using compressed air and found none. I tested various vacuum lines with the gauge taped to the windshield to get various readings. Sometimes, it would work fine for a while after testing, and then it would go away. Finally, the problem showed itself. I started the car after work the other day and started to drive. No boost and no power brakes. Sure enough the 2 inch piece of vacuum hose at the tandem pump (upper passenger side) had broken apart. The dry rot must have caused intermittent leaks. 2 inches of hose and a cheap fix, and all is well now. I am glad I never bought a new N75.
 
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