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Old October 23rd, 2009, 06:28   #1
TurboBugger
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Cool boost deviation: negative charge pressure 02 tdi

I think my 02 tdi is going into limp mode after overboost

I get the code for boost deviation: negative charge pressure over and over and over.

I can not find any leaks. I did some test on the lines but I do not see or hear any leaks.


Is there something I can do to find the leak. I think I overboosted because my n75 valve measures only 17ohms not the 25-35ohm as specified by the haynes service manual.

Does anyone know what this MT2 valve is on the tdi?
I see 3 hoses going to the n75 valve. one from the egr, one from the throttle body and one from the actuator. I think I can bypass the n75 by connecting the throttle body and actuator; is that true?
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Old October 23rd, 2009, 06:40   #2
Curious Chris
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You need to test the actuator using MityVac or equivalent to confirm the actuator works and there is not a leak in that line.

You can use IIRC N14 as a test device but then you will get a EGR flow low DTC.

Far and away the best solution is to buy VCDS ( www.ross-tech.com) or find someone with VCDS near you.
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Old October 23rd, 2009, 07:26   #3
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Bugger when was the intake last cleaned? For more information read the writeup in my signature. Good luck and pls post the fix once you find it.
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Having trouble with limp mode... Troubleshooting Low Power

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Old October 23rd, 2009, 07:52   #4
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It's not an overboost that is causing the issue...

Read the code!
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Old October 23rd, 2009, 09:54   #5
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yes I got a friend with vcds here.
can I read boost with that program? how will vcds help me?

I can try to test the actuator with a pump

*yes the code says negative deviation which is not enough boost*
Doesnt make sense to me. There are no boost leaks
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Old October 23rd, 2009, 10:02   #6
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Quote:
Originally Posted by Drivbiwire
It's not an overboost that is causing the issue...

Read the code!
x2.

Sounds like you have an issue with the VNT, but you need to drive the car and see where the requested and actual boost is. I would use a vacuum pump to check the VNT actuator first.
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Old October 23rd, 2009, 10:22   #7
Drivbiwire
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It's not the MAF: ECU sees enough flow of air volume

It's not the Intake, This will not trigger low boost pressure in fact the opposite! Clogged intakes result in sufficent boost being generated however the restriction throttles the motor lowering Mass Air Flow hence reducing injected fuel quanity and then results in lower injected fuel. The end result is NORMAL boost pressure BUT LOWER TOTAL POWER OUTPUT! Typical of this condition is that No codes are triggered! Why? Simple, you are achieving specified boost pressure, You are getting the proper ratio of air volume for the amount of fuel injected and the engine is not at risk of producing an increase in emissions! As long as that pressure to volume ratio stays within normal ranges you will simply suffer from low power output and no associated engine codes!

BUT you do have a code so we know now that it is not a plugged intake!

The MAF sees the air volume, so the MAF is working properly. The MAP sensor is reading pressure and temperature correctly so its not the sensor. The engine is injecting the fuel but for whatever reason it is still not getting the bang for the buck so to speak from the air and fuel it is telling the engine to inject!

Therefore:

It is possible you have a leak in the intake system that is venting the compressed air out of the induction system. A very close inspection is warranted and this code is what will be triggered if you have a pipe or connection that is loose or not connected.

However you can't stop there!

A leaking N75 may cause the vacuum to be dumping and not driving the VNT to the full boost position. The N75 may be getting the signal to pull the VNT vanes to a full boost position however if you have abused the motor by NOT pushing this motor to normal load ranges (lugging and short shifting as if it was a 1900 rpm Redline) then you may have have a VNT that cannot move thru its required range of motion and is jammed in a low boost position hence preventing correct HIGH boost from being achieved.

If you lug your motor around and short shift at 1500 or 2000 rpm then you may have trashed your VNT system and have jammed up vanes. Carbon will form on the VNT system and prevent the guide vanes from moving accross their full range of motion. This will stop the turbo from making rated boost pressure. The VNT is commaned to full boost at idle to permit the fastest response from the turbo. As you accelerate and the ECU sees the boost pressure rise (500 samples per second) the ECU commands the N75 to decrease the duty cycle of the VNT actuator and thus reduces the guid vane position to a lower boost setting. In your case the VNT actuator is full open and the engine is still not seeing the ECU's expected/anticipated/calculated boost pressure!

If you drive the car properly (Shift at 3000 rpm, floor it on a regular basis and understand what Load is) then it may be the last item below.

The last thing it could be is a jammed or jamming fuel shut-off selenoid. This restricts the flow of fuel to the plunger and prevents full boost pressure due to low gas energy. This is a last resort if you have verified:

-Full range of motion of the VNT
-Verified good N75 condition in respect to sealing
-Good/new vacuum hoses in the system
-Verified good sealing of all the hoses and ducts on the pressure side of the turbo.
-Have no leaks in the intercooler from rocks
-No other component codes that can generate incorrect ECU base data for determining "Specified" boost pressure.
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Last edited by Drivbiwire; October 23rd, 2009 at 10:39.
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Old October 23rd, 2009, 10:36   #8
TurboBugger
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how can the n75 be checked for good operation or good seal? leaks?

can the n75 be bypassed?
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Old October 23rd, 2009, 10:41   #9
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Bypass that N75 and you WILL BLOW THAT TURBO!

Don't even go there!

Swap the N75 with the EGR control valve...If the issue is still there then you have to dig further. If the code is no longer triggered, replace the N75.

The two valves are interchangeable!
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Old October 24th, 2009, 06:03   #10
TurboBugger
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so should i move the n75 to the other harness plug or just change the hoses around from egr valve to n75 and n75 to egr.

i think i need to swap the locations of each n75 egr valve so the ecu can control them correctly
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Old October 24th, 2009, 07:12   #11
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You need to move them...
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Having trouble with limp mode... Troubleshooting Low Power

Having trouble searching? Read this thread on advance search in forum http://forums.tdiclub.com/showthread.php?t=133579
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Old January 5th, 2010, 11:58   #12
TurboBugger
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Quote:
Originally Posted by Canadian_Grizzly
You need to move them...
well I moved them twice and doubled checked. Still no luck. I get boost negative deviation cel every time I drive.


I tried to clean the MAF this last week. now after cleaning the MAF I get the P0101 maf CEL

LOL
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Old January 5th, 2010, 13:30   #13
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In order to be 100% sure the VNT vanes are not sticky, pop off the clip connecting the VNT arm and the actuator (put a piece of tape or a magnetic grabber on the clip if you can- those suckers really fly!) then flip the VNT arm back and forth- should move freely- see video clip:
http://pics.tdiclub.com/data/500/2006_0603_175432AB.AVI
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Old February 2nd, 2010, 08:56   #14
TurboBugger
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Quote:
Originally Posted by Tom W.
In order to be 100% sure the VNT vanes are not sticky, pop off the clip connecting the VNT arm and the actuator (put a piece of tape or a magnetic grabber on the clip if you can- those suckers really fly!) then flip the VNT arm back and forth- should move freely- see video clip:
http://pics.tdiclub.com/data/500/2006_0603_175432AB.AVI
sure; the VNT arm moves Freely!
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