VNT Operation ? Turbo boost problem :(

[A4 Lad]

Hi there folks. I was looking to run a problem past you for advice. I have a VW Passat 2.0 engine (CBDC) that is going into limp home mode when under load. The fault code 16683/P0299/000665 - Boost Pressure Regulation: Control Range Not Reached is in the ECU after each time this happens. I have checked the N75 for operation, checked for vacuum leaks etc and found no faults.
The question that I want to ask is, when operating a vacuum gauge directly onto the VNT on top of the turbo, should this operate the waste gate ? The reason that I ask is that when I try and put any vacuum into it, it does nothing. No vacuum is held at all and the waste gate does not open as a result. I suspect that the diaphragm is blown inside the VNT but just wanted to see if anyone could confirm that indeed, the unit should hold a vacuum ?

[zollaf]

yes, the actuator should move when vacuum is applied. it needs vacuum to actually get the turbo to work, so yours must have very little power. as you apply vacuum the rod moves and this turns the vnt inside the turbo. its not a wastegate though, it directs the gas onto the impellor. no vac means no boost. new actuator needed.

[A4 Lad]

Thank you very much for the reply zollaf. The other thing that really confuses me is the fact that the car actually drives well (with noticeable turbo lift) until under load and then the limp home mode comes in.i disconnected the vacuum pipe that goes into the the actuator and Tom it a test drive up the road and still the turbo appeared to kick in which totally threw me ! So basically if I put a vacuum pump direct onto the actuator and prime it, there should be some sort of vacuum?

[zollaf]

yes, it sounds like your vnt or actuator is somehow jammed in position. most unusual. i would remove the link in the rod connected them and see if the vnt moves. thats the lever that enters the turbo. it should move about 19mm iirc up and down. then try moving the actuator . something is broken or stuck. basically, when you first start, the actuator gets full vacuum and moves the vnt. as you drive off and reach higher revs, the vacuum drops and moves the vnt to produce less boost, rather like a wastegate but its not. normally they carbon up ad won't move far enough so you get an overboost.

[A4 Lad]

Great. lol remove the actuator in the morning and check that the arm going into the turbo is free and go from there. I was 99% sure that it was the actuator as it would not hold any vacuum what so ever but as I said, the fact that it appears to boost ok under normal driving threw me off the idea as I tho that possibly the electronic side of the actuator done some part in the operation. So basically when I start the car at idle, the arm should fully extend out and when the turbo is being boosted, the vacuum is dropped and closes the arm, is that correct ?

[zollaf]

yes, thats right. the electronic part is there to tell the ecu where the acuator is at. later they moved to a fully electronic actuator.

[A4 Lad]

Massive thanks for the advice :)

[Teflon]

I really must write up the diagnostics for this, complete with internet links to photos,videos, VCDS measurement blocks, etc. There’s some really good info out there, but it takes some finding.

Firstly, although the actuator can be duff, the more likely case is that the inside of the turbo is gummed up so the mechanism isn’t going full stroke or indeed, sometimes no stroke. If this is the case then no amount of vacuum will move it.

As Zollaf says, disconnect the actuator at the turbo end. (DON’T meddle with the adjuster!). No need to measure movement – you should feel free movement in both directions with a noticeable ”clonk” at each end. Cold it will feel as if it is almost too loose and there is too much play in the mechanism. If it is in any way stiff or gritty along its stroke then your turbo is gummed up and needs to come off for a clean out. Forget the snake oil solutions on offer to put in or even oven cleaner – you might get a week or two of psychological benefit but only a strip and clean will permanently solve it.

Of course, it could be the actuator, or the MAP sensor (boost sensor) or several other things. But start with the easy to test stuff first.

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