3.0tdi boost hesitation

[Gmac]

Morning folks… I’m looking for any insight in to this issue i have with the car… I’ll try and best describe it…. But boost hesitation would be the best description for it. So when im cruising about 1800rpm, and say in forth or fifth (doesn’t actually matter what gear I’m in, but definitely the rev range around the 1800 mark) and i press the accelerator theres a definite delay whilst it feels like boost pressure is building and then sort of goes ‘there you go, there’s your boost’ and off the car goes and accelerates on boost as normal. It almost feels as if the car decelerates initially, like theres no power at all. I often box down in to a lower gear (in to higher rpm band) to drive round the problem. The car wasn’t like this when i first got it about a year ago now And I’d say the problem is getting slowly worse. I cleaned out the egr and intake about 5 months ago as they were pretty coked up.

What do we think the issue it here? I’ll scan it for codes this morning using vcds see if that throws any light.

 

[Charlie Farley]

Could be a boost pressure loss , a scan may show more, maybe worth checking for loose or damaged hoses too.

 

[northpole]

Dirty vnt mechanism on the turbo and if that is the case you might want to clean the complete intake too.

 

[Gmac]

Dirty vnt mechanism on the turbo and if that is the case you might want to clean the complete intake too.

What does vnt stand for?

 

[northpole]

What does vnt stand for?

Ask Google... basically you got a turbo that is variable because of the VNT... that mechanism gets stuck when it's dirty... clean it and power delivery will be smooth again.


Variable Nozzle Turbine aka VNT.... look it up you'll need to understand what to clean and how to do it.

 

[Gmac]

Scanned the car, no fault codes relating to this issue. Watched a couple of YouTube videos on how to remove the turbo, quite an involved job. It’ll need to be done when i can get it off the road for a few days without needing it. I had a basic look over some hoses, but still working out what’s what under the engine cover! A lot of stuff packed in to the engine bay with these v6’s!

 

[northpole]

Not that hard find the vacuumlines... and follow those 9 out of 10 times they have cracked if oem or if they have been replaced with silicone ones they've touched a heat source and melted. But if that was the case it wouldn't go hesitate... it would simply not give you that boost at all and go into limp mode.

This is why I pointed to the vnt mechanism as this is how it shows there is a problem with it's mechanism. To go worse overtime and go into limp mode.

Speaking from 15 years of experience driving vnt turbocharged diesel engines (not all from the VAG concern by the way) this problem is basically part of having the pcv gasses with their oil mist return into the intake + having an EGR system returning exhaust gasses with soot particles into the intake too. This turns into some tar like sticky goo that coats the complete intake and the VNT mechanism too.

Either delete the egr or plumb in an oil catch can and this will never happen again. (That is if you keep emptying the oil catch can )

 

[Gmac]

So just to keep this theres up to date…

Removed the turbo… BorgWagner KKK BV-50… took it apart, cleaned up the internals with Mr Muscle oven cleaner, replaced an o-ring on cold side, and put it all back together. Still got the same problem of a boost hesitation, like a slow boost build from around 1800-2000rpm. I checked the values of the output test (VCDS engine module) of the actuator arm to make sure that i had reinstalled this at the correct point. These were going from 20.9%-80.9%. I’ve read they should be 20-80%. So pretty darn close. Maybe these can be tweaked, but not sure how much movement relates to what % change. Leaving it as it is for the moment.

So i think i need to look at cleaning the boost sensor, which on my 3.0tdi is a combined IAT sensor (seems to get called a thrust sensor) and is located under the nearside headlight, so that’ll be a bumper off/headlight off job.. argh.

The car is still totally drivable but just has this slight lag. No codes and no limp mode.

 

[northpole]

@Gmac I replied with a link to something that is accessible, a direct link to an exploed view of how the turbo is assembled etc in your other topic... hope it'll help.

 

[Gmac]

Still got this, and I’m wondering if the exhaust back box delete has had anything to do with it? The timings of when i did the exhaust and the problem starting could be around the same time. Differing back pressure perhaps, i get that diesel whistle now on overrun… so may be its a pressure thing from exhaust side..if the revs are built slowly from low revs, 1300rpm, and it builds slowly then by 18-2000rpm, when the problem is most obvious, the problem doesn’t happen.
Thoughts?

 

[northpole]

Nah it is too far down the line to have an impact... unless you went big bore straight pipe from the turbo, in that case you'll need a remap.

The vnt mechanism.might have ware or still has some dirt in it... you can always load it up with some penetrating oil and see if that solves it.. but if that doesn't last you might have to replace the vnt mechanism.. and instead of doing that I'd go for a rebuild turbo or a hybrid (and have it remapped to suit that hybrid turbo)

 

[Gmac]

I was thinking it might be along the lines of wear in the vnt mechanism. As i would say the problem has been getting very slowly worse. Wonder how much a replacement turbo would be. Plus I’d need to be certain that that is what the problem actually is.

 

[northpole]

If you really want to test it I would buy a scrappy turbo but chances are that it could have the same problem... alternatively if you happen to have someone with the same engine and turbo (that works fine) swap 9ver to test and see if it's gone...

But if I was in your shoes: Hybrid turbo and a remap... I mean if you have to replace it you might as well gain some extra torque and bhp for your hard earned money and for remapping audi's the only person I would trust in the UK with a 3.0 tdi = Bobby Singh = BS Racing. And I would let him tell me what to get and do the remap.

 

[bobby singh]

If you really want to test it I would buy a scrappy turbo but chances are that it could have the same problem... alternatively if you happen to have someone with the same engine and turbo (that works fine) swap 9ver to test and see if it's gone...

But if I was in your shoes: Hybrid turbo and a remap... I mean if you have to replace it you might as well gain some extra torque and bhp for your hard earned money and for remapping audi's the only person I would trust in the UK with a 3.0 tdi = Bobby Singh = BS Racing. And I would let him tell me what to get and do the remap.

Thanks mate

 

[northpole]

Thanks mate

No thanks, you've done stuff others still don't seem to get anywhere near when using the same hardware, let alone making it reliable at those figures. And the fact that every time I've contacted you the information I got was always spot on.

 

[Gmac]

Thanks mate

If i was closer Bobby, I’d have the car in to you to get it sorted! Any chance you can have a read through this thread and share your thoughts? TIA

 

[bobby singh]

If i was closer Bobby, I’d have the car in to you to get it sorted! Any chance you can have a read through this thread and share your thoughts? TIA

Without logs its impossible to guess whats going.
Maybe disconnect the MAF and go for a quick drive - if it feels better then you know the MAF is faulty.
MAF is used by the smoke limiter in the ECU - if low air flow is seen then you get low levels of fuel.