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Discussion Starter #1
I've searched around and am still having trouble with limp mode on my 1999 Beetle TDI. I've been having this issue for a few months now. It fist started ONLY when in 5th gear, cruise control @ about 70 MPH on a steady hill. I'd just turn the car off and restart it on the downhill and problem solved. The 2nd time it would happen during my commute it would throw a CEL with a code of P1550 - Charge Pressure Control Deviation - Intermittant.

Here is a list of the things I've done to date:

1) Replaced all the vacuum lines.

2) Swapped the N75 Valve with the EGR. No change. (I never swapped them back, could this be an issue?)

3) Unplugged the MAF, the car ran horribly, so I plugged it back in

4) Sprayed the VNT rod and actuator with PB Blaster and went for a hard drive.

5) Replaced the VNT Actuator with a new one. The old one was bad because when hooked up to a vacuum pump the rod didnt move until about 10 and made a popping sound while the new one moves at 3.

Today with the new actuator installed the problem (limp mode) happend in third gear under load, the problem got worse with the new actuator! I adjusted the new actuator rod length to that of the old one and when I had it off of the turbo, I was able to move the turbo VNT rod with my fingertip. It moved very easily.

6) I ran VAG-COM and went to measuring blocks group 11. I was told that the graph lines should stay realitively close together but when I gave throttle they didn't.

Does this mean that the turbo vanes are'nt sticking? Is it a bad N75? Any help or suggestions would be appreciated!
 

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First swap the EGR-N75 back. I don't think this is causing your problem though.

The MAF is probably fine from your rough test.

If the VNT LEVER moves easily by hand when the rod is off then the vanes are fine.

If the rod is the same length and starts to move around 3, the VNT actuator is fine. You have now ruled out the turbo and actuator.

If you stomp on the gas pedal some small spikes in manifold pressure actual vs. requested are normal. If you drive smoothly they should stay relatively close. If they are not, you know the problem is most likely related to the turbo control.

Possible problem: bad manifold pressure sensor - this is unlikely.

Bad wires to the N75 - more likely. Bad N75 - more likely. Bad vacuum line somewhere - more likely. My guess is that a vacuum line was bad, by disturbing it, the hole or defect is getting worse. Even if it looks fine, replace the lines going to and from the N75.

When you tested the VNT actuator, did it hold vacuum? Did you use the same line that is on the car? If not, this is probably the problem. I say replace the lines anyways since they're cheap, probably worn out, and more likely than other causes. Inspect all the vacuum lines back to the black vacuum bulb and to the vacuum pump at the driver's side camshaft.

As a last resort, the ECU could be bad, this is unlikely.
 

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Discussion Starter #4
I've already replaced all my vac lines (unless I missed one somewhere) and yesterday I switched back the N75. Drove 300 miles today...still no change!

I tested the VNT actuator with the same line that run from the N75 valve and it held vac.

I added an EGR delete kit a few years ago, that would'nt create any sort of problem with this would it?

Here's a crappy screenshot of my VAG COM graph in group 11. Thanks for all the input so far!

 

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Discussion Starter #6
The EGR delete kit does throw a CEL but I regularly clear it with my ScanGauge. I believe group 11 is the VAG-COM group that show requested boost vs. actual, I think. I read about it here on the forum as something to check but I dont fully understand the data that I'm seeing on the graph. My graph dosen't look like the graph in the example of a properly working VNT Turbo.
 

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Small spikes are normal, what you are seeing is what the boost pressure the ECU is requesting and expects to see vs what boost pressure it's actually seeing. The spikes look normal, the big raised yellow section doesn't, it looks like a sticking turbo or other problem that is causing strange boost response.
 

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Discussion Starter #8
I guess I'll have to double check the turbo vanes, I thought I ruled that out because the VNT rod moved easily with my finger tip. I'll have to check it again.
 

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Group 11 measuring block is pictured below w/captions,


If the VNT lever moves easily then the vanes are not sticking.

In the picture below, I assume when you say VNT rod, you mean the item labeled VNT lever. The rod is part of the actuator. When the rod and lever are disconnected the lever should move easily. The rod should have spring resistance. When they are connected you should feel only the spring resistance.


Something is not right with the turbo control system, you just have to figure out what. Your screenshot of the graph suggests a problem with it. Replace the vac lines, check the vac pump thick hose, something isn't right. You already ruled out the VNT actuator and you say the vanes are free so that means n75 valve solenoid, upstream vac lines, vac pump, wiring to n75 valve, or ECU. It might be a bad boost sensor or wiring to the sensor too.
 

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Discussion Starter #10
Thanks for all of the suggestions, at this point I almost wish it was the turbo! I think it may be easier to take the turbo apart and clean it than chase down this other little gremlin. I'll keep everyone posted! Oh and I did mean to say that the VNT lever moves easily with my fingertip with the VNT rod removed.
 

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Discussion Starter #11
Update

Just to update, I took the car to my trusted TDI mechanic and he was able to adjusted the length of the VNT rod which solved the problem. Simple fix but it was driving me crazy!!!! Thanks for all the help.:D
 
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