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Discussion Starter #1
Thanks for your help so far - great website!

I have a 2001 Passat TDi 130bhp, 145,000 miles. The car lacks power at lower revs but picks up at circa >2500rpm before, when giving it more boot, going into limp mode. I am told by the previous owner that the reason is over-boost. I have VCDS but due to laptop lead problems I am yet to confirm the fault or run the necessary tests. However the previous owner had replacement N75 and MAS fitted which, he says, gave temporary improvement but the problem returns.

In the light of the the component exchange so far, I suspected sticky vanes or a leaking vane actuator. However, on engine start the actuator pulls the rod sharply up against the stop. So I then decided to assess whether the vane operation was sticky. I detached the actuator from its bracket (two nuts on studs) and, with the engine off, and with the actuator still connected to the rod, I was able to lift the actuator and manually operate the rod to the vanes. To my surprise the force required was minimal. In fact the principal force was the occasional and unavoidable catching of the actuator studs against the holes in the bracket. There was no apparent variation in force along the rod's travel, or any hint of 'slop', varying friction or hysteresis.

This led me to suspect that the vane rod was operating the crank and nothing else: perhaps it was disconnected from the annular ring inside the turbo? So I tried to detect actual vane operation. I failed to remove the flexi to the cat so, instead, I ran the engine at fast idle whilst disconnecting the vacuum line from N75 with and without kinking the line to retain the vac. Occasionally, a slight variation in exhaust note was detectable and, at other times, disconnection or reconnection caused the engine to stutter momentarily. On the basis of this I suspect some or all of the vanes are moving.

My questions please:

For a VNT with perfect vane operation, is the operation force on the rod significant or minimal as I have observed?

Are there any failure modes whereby some or all of the vanes become disconnected from the actuation lever/crank on the turbo and actuation force reduces?

Are there better ways of checking for correct vane operation without the hassle of having to remove the turbo altogether?

Thanks in anticipation for your assistance.

Bartfarst, Redditch, UK
 

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From the videos I've seen the vane lever should move with a fingertip effort if the rod is disconnected.

Have you checked the vacuum lines? I would bet that there is a moment when it's going off spec. How about vacuum testing the actuator? Maybe the spring inside is off.
 

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Post moved to mk4 forum since it has technical info


1. As stated, it's fingertip effort.
2. Yes. If the VNT lever wears down inside the turbo it can move without moving the ring. See this: http://www.myturbodiesel.com/1000q/a4/VNT-turbo-removal-vanes.htm



If it's not catching or binding, my best guess is that it's OK internally.

3. Do datalogging of boost with VCDS. It will tell you right away if boost requested is roughly the same as boost actual. In other words, fix your cable before chasing other time consuming problems. Also look through the limp mode FAQ article.
 

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Discussion Starter #4
Success! When monitoring group 011 with VCDS-Lite, the measured manifold pressure was static at approx 2900 mBar. Disconnected it and it went to circa 800 mBar so I reckoned that the sensor was bust. I swapped the sensor and, hey presto, the value now tracks desired pressure. I logged the parameters whilst I took it for a blast. The error signal (specified minus actual pressure) statistics were as follows:
Max 940.2
Min -1298.1
Mean -50.8

In other words over the whole run actual pressure was on average 50mBar higher than specified, which I hope is OK?

I have another problem now: a very strong oil vapour smell coming from the engine compartment. Darkness prevents me from investigating further, but my nostrils are still stinging...

Thanks again for your help!
 

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Success! When monitoring group 011 with VCDS-Lite, the measured manifold pressure was static at approx 2900 mBar. Disconnected it and it went to circa 800 mBar so I reckoned that the sensor was bust. I swapped the sensor and, hey presto, the value now tracks desired pressure. I logged the parameters whilst I took it for a blast. The error signal (specified minus actual pressure) statistics were as follows:
Max 940.2
Min -1298.1
Mean -50.8

In other words over the whole run actual pressure was on average 50mBar higher than specified, which I hope is OK?

I have another problem now: a very strong oil vapour smell coming from the engine compartment. Darkness prevents me from investigating further, but my nostrils are still stinging...

Thanks again for your help!
Did you wash down the engine bay? There could be some oil residue burning on the exhaust. TDI EGR valves tend to weep oil too.

The most likely thing is that the restored power and higher engine and exhaust temperature is burning out all the crud that was built up.
 

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Discussion Starter #6
No I didn't wash down the engine bay, and I was getting the oil smell whilst the engine was idling on the drive before and after the fix.

Would the EGR benefit from a clean do you reckon?

Much of the intake air system was pretty oily. I suspect that there could be a fair bit of blowby from the engine. The pressure sensor I took out was wet with oil, and there was a fair bit around the intake of the turbo. Is this normal or the sign of problems?

Thanks again!
 

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:thumbsup

I will add a note in the limp mod/low power article to check for that problem.

2nd on the burning residue. After I fixed a car that was driving around with a strong boost leak there were massive smoke clouds behind the car from all the build up. Like so thick that the car behind me turned on their headlights and I could still barely see them. After driving around for a while and then a few hard runs, it cleared up but it still left an odor in my nose. Give it a day and see if it's cleared up.
 

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Discussion Starter #8
:thumbsup

After driving around for a while and then a few hard runs, it cleared up but it still left an odor in my nose. Give it a day and see if it's cleared up.
Drove down to London last night - exhaust devoid of oily odour on arrival. Looks like a good long run cleared things up as you predicted.

Overall the car went like a train. Just a couple of moments of hesitency on going from over-run to light acceleration. Other than that, brilliant! I had forgotten what it is like to drive a classy car, after five years putting up with my old Citroen Xsara HDi! Thanks again.
 
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