VNT turbo - actuator arm movement

[Izalman]

Hi Guys,

Anyone any idea how much travel there should be on the actuator arm between the vacuum chamber and VNT linkage at the back of the turbo.
I can move mine a little bit but how far should it move up & down?

[Frank.A]

The total movement of the rod should be about 20mm.Get someone to start the engine while you watch the rod.The rod should move up and return when the engine is switched off.

Best,
Frank

[Mike McKinstry]

Is the above right/Does the actuator move when engine is started.

[Frank.A]

Hi Mike,

This is correct according to what I have read.This is the easiect check for VNT operation.You can also disconnect the vacuum pipe from the diaphragm capsule and apply either a vacuum pump or just suck on a piece tubing.Unless you have plenty of suck you will only get about 10mm of movement.If you now nip the tubing the vacuum should hold proving that the diaphragm is OK.The movement should be smooth and when the suction is removed the rod should return smoothly to its limit stop.
As I posted in the caravan thread,my VNT appears to be OK using this method and I suspect the MAF or the MAP.

Best,
Frank

[Mike McKinstry]

Frank,
Thanks for this.The vacuum pipe you are talking about is presumably the
one pushed onto the diaphram (something resembling tin can)from above?
This would presumably only confirm the VNT was not stuck solid because with an intermittent fault factors such as load,revs etc will all play and VNT may still jam at certain positions.
Maybe a good way of checking for a vacuum leak although mitivac would be even easier.

Mike

[Col]

A couple of things;

1, Sat here at 5am with tooth ache !

2, I don't think it will move at idle as the ecu is not and should not be calling for boost at idle. Note the vanes on the vnt rest with no vacuum in the low / no boost position.

3, The other weak link in the system is the N75 solenoid actuator valve which controls the vacuum to the turbo. Check this as well before you condem the map & maf.

4, A good session with vagcom should identify which sensor / actuator (if any) is playing up.

5, The vnt mechanism can still move but be sticky..

[Frank.A]

Hi Col,

Thanks for your input.

A point to remember is that this PD130 AWX which I have had from new, is running perfectly with plently of grunt and 50+mpg.The turbo dropping out has only occurred a few times with a 1200kg van in tow in fifth at around 60mph (2000rpm) when accelerating past an HGV uphill on M way.No fault codes shown.I have attempted to replicate the fault solo but it's perfect.

The info I have posted concerning the actuator position is correct according to threads on US forums,and is definitely the way mine behaves.

I will be towing up to Scotland in July,if/when the fault occurs again,the next time I come upon an HGV I will try dropping down a cog to see what happens.

Best,
Frank

[Col]

If you can, hook it up to vagcom and do a few runs (top gear, up hill, from 40mph and floor it).

Log the boost requested vs actual. It will probably show a major descrepancy and / or drop you into limp mode.

My money would be on a sticky vnt mechanism.


A cheap fix to this could be as simply as giving the car a damned good thashing over say an hours run. Thereafter giving it a good thrash for once week to keep things free.

I have exactly the same AWX engine with 136k on it, mine gets a regular thrashing which I believe does a TDI the power of good.

[dlyan]

If the movement is too short this will definately cause limpness under pressure. This was the problem with my 98 1.9 tdi.

[Izalman]

Hi guys,

Checked the movement of the linkage as suggested, indeed it does move when the engine is started - approx 50% of the total travel.

Returns when engine switched off.

I would agree that the ECu should not call for boost but does it work in reverse?. IE drops the applied vacuum to move the vanes to give greater boost when needed.

Handy link to Pieburgs (N75 valve) home page on another thread today.