VW Passat 2.5 TDi looses power - Hamsafar
I recently purchased a VW Passat automatic, with 2.5 v6 TDi (AKN) engine. It is 4 years old and 111,000 'motorway miles'.

I keep suffering from loss of ability to accelerate.
For example, it is a bit slow from a standstill in 1st gear, but worse is when turning right at traffic lights, you can roll up and wait for a gap in oncoming traffic and press the accelerator right down but nothing much happens for 1-5 seconds, then there is a violent surge in power and a puff of black smoke.
It also does this at roundabouts, when you roll up to big roundabouts on A roads, it is a bit dangerous as it just crawls onto the roundabout and when you have gone halfway round there is a sudden boost of power (If it was like that all the time I'd be pleased)

So while in the deserted countryside I replicated this, and with CC off and widows open, I could hear a quiet hiss from the engine, and at the exact point the power boost occured the hiss stopped and there was a noise like a pneumatic valve closing, (split second rise in pitch like when you let go of an airline trigger)

It is almost like a wastegate stuck open which suddenly snaps shut, but I have read it doesn't even have one as it is a VNT turbocharger.

All obvious service items replaced, and visual inspection under bonnet doesn't reveal anything untoward.

I can see the VNT actuation lever, all pipe clips look original, but I can't hear any air leaks at this stage.

I have read lots of info about the N75 valve on 1.9tdis, do I have one of these?

Thanks for any help before I go to VW, and roll my eyes!

VW Passat 2.5 TDi looses power - Hamsafar
Sorry for spelling loses incorrectly!
VW Passat 2.5 TDi looses power - Screwloose

Yes it's got an "N75" solenoid. [On the R/H bulkhead.] It's also got an EGR solenoid and a variable intake manifold control solenoid. Both of those systems [particularly the EGR valve] might also be worth considering before getting into the clogged-up vane ring issue in the mega-bucks turbo. Getting the fault-codes and live-data examined would probably be the best start.
VW Passat 2.5 TDi looses power - Hamsafar
Thanks, I didn't know about the intake solenoid!
I have looked and think I can identify the parts you speak of.
I'll book it in to VW for starters, hopefully they'll be able to sort it out or give me clues to do it myself.
VW Passat 2.5 TDi looses power - Hamsafar
Well, the dealer says it's the airflow meter and there is a two week backorder due to a worldwide shortage!
It will be £136 fitted. I could do it myself for <£80, but they aren't in stock anywhere I have tried, and if it turns out to be a misdiagnosis, I'll be in a better position letting them do it than if I'd done in myself.
VW Passat 2.5 TDi looses power - adverse camber
MAF's seem to be a bugbear on vag tdis. Have you tried the motor factors? Not much cheaper now that vag have brought the prices down to a sensible level. Bosch (assumong its the bosch thin wire one - the failed ones usually are) also do what they call a reconditioned unit, the one I bought for my volvo (with the straight 5 2.5 vag tdi engine) was identical to a new unit - it was on the stock list at andrew page as a recon, but the box, package and unit inside was identical (I asked to look at both new and recon).

Some people get temp fix by cleaning (very carefully) the maf element - have you looked on the tdi sites ?
VW Passat 2.5 TDi looses power - Hamsafar
Hello, thanks for the info.
The price of the part is £86 and my usual factors (ECP) quote £70 and they are out of stock too. I have seen on ebay Pierburg ones to replace what the advert says are unreliable Bosch ones, but they are all 1.9 not 2.5.
Mine may be hot film, it doesn't look like any hot wire I have seen so I assume it's film, I have tried eashing it in gentle solvents with a soft paintbrush, which worked for the next journey, but maybe that was a fluke, so I disconnected it again, the trouble with this it raises the idle speed and makes shifting into D or R a bit unsympathetic! It's also noteworthy that none of the Bosch stuff I have seen on the car is made in Germany anymore, and the drill I bought made by them is also useless especially as it was 3x more than a Performance power one I got for work which is a much better peice of kit.
VW Passat 2.5 TDi looses power - Hamsafar
Well, I had my new MAF fitted today for just £117, I set off from the dealer and the car was no different at all. I stopped and opened the bonnet, a brand new MAF including housing sparkled before me. I turned around and went back, and they had another look and said the computer says the MAF is faulty again, they are having it back for all of Wednesday, hopefully 4th time lucky!?

The dealer was pleasant enough and negotiated an excellent goodwill price with VW so I don't really mind paying for the new one just to have a new one with a 2 year warranty, hopefully, it won't be so expensive to cure. I don't think it's the wiring as it's better when disconnected, but maybe there are several pins and only one has no signal?

I can still hear a hiss during low-power/smoke situations which I can hear get shutoff like a closing valve at the moment normal power resumes.
VW Passat 2.5 TDi looses power - jc2
The Bosch drill I've got was made by Scintilla in Switzerland(remember their Vertex magnetos).
VW Passat 2.5 TDi looses power - Hamsafar
Apparently the car is "all sorted" I am told the MAF was faulty, but they had to "interrogate the computer" and convince it to accept the new part. Almost like some sort of organ rejection - they'd have me believe (if they could)
VW Passat 2.5 TDi looses power - Hamsafar
Well, just got home, they have changed something, no black smoke and quicker 0-10mph, then it's like a milk float, hardly any power where there was before. What nonsense! This is the worst ownership experience of anything I've bought.
VW Passat 2.5 TDi looses power - nick62
I don't know how they have the brass neck to charge you £117 for a new part that has no effect on the problem. Surely they road tested the car afterwards and knew the problem was still there? I'd ask for my money back. But if your experience with VW (customer service at VW UK head office....not the dealer) is anything like mine, you're wasting your time.

My local Bosch specialist charged me £75/80 exchange for my 1.9 TDI PD MAF and fitted free, (plus £30 for the diagnosis test.........which was suspect of the MAF, but in no way certain to be the problem) and he agreed beforehand that if it didn't cure the fault he would take it off again at no charge.
VW Passat 2.5 TDi looses power - Hamsafar
Thanks, I will, I'm going around first thing with merry hell to play.
I have been on a test run now the roads are quiet, and twice I had my foot to the floor on 60mph limit hills and it wouldn't get above 28mph and was slowing down to 24mph near the top where it gets steeper. I have been up there at 60mph wasily before.
They have obviously trimmed the maximum fuel quantity per stroke down by changing control blocks in the ECU to mask the problem (esp. smoke). This is very naughty and I'm furious as this would make proper curing of the problem impossible without setting them back.
VW Passat 2.5 TDi looses power - Screwloose
Ashok Leyland

Seems, on the face of it, as though your dealer is digging themselves ever-deeper into a hole of their own making. Fixating on a MAF when it now clearly isn't that, has apparently led them into desperate measures. Have they considered that a restricted "cat" or exhaust, or a stuck-open EGR valve, can also give low air-flow readings/black smoke? Turning down the fuelling to match the lower air intake quantity is not exactly a useful ploy!

VW Passat 2.5 TDi looses power - Hamsafar
Thanks screwloose, I went back to VW this morning and spoke to the man who worked on it.
He said they couldn't capture data on the road because their equipment was rubbish and old!
He told me everything he had checked, and all the wiring, VNT actuator etc...

They are happy to look at it again, but it's driving me around the bend.
He also said there were signs that the engine and gearbox had been removed at some stage as some clips and things were't in place. I have also phoned the VW dealer that used to service it for the original keeper and they said it had a premature timing belt tensioner failure 6000 miles ago in October 05, and that was done under warranty and that was the last thing they did.

Does anyone know of any national VAG experts?
VW Passat 2.5 TDi looses power - George Porge

This is where I'd take it. Ask for Steve
2 Dirty VW diesels and a Honda with an 18 inch blade
VW Passat 2.5 TDi looses power - Hamsafar
Golly! Not more than 4 miles away! Thanks!
VW Passat 2.5 TDi looses power - Screwloose

Mmmm - I do like their non-PC webpage: she can come and polish-up my tools any time she likes!
VW Passat 2.5 TDi looses power - ianden
Well, having had trouble with my 1.9TDi following yours and screwloose's hints finally tracked the trouble to a MAF. However your symptoms seem to be exactly the same as when I accidentally left the vacuum supply pipe disconnected, not at the turbo (which certainly stops the turbo working but you still have some power) but near the EGR control valve. Car barely accelerated at all and only to 30mph tops. Your vacuum plumbing is almost certainly different but it's worth a check, some connections are well hidden.
VW Passat 2.5 TDi looses power - Hamsafar
Thanks for the info.
It is booked in to see Steve at C&R as recommended above in the morning.
I did a visual check a couple of times, but you know when you haven't had a car for long, you can't see the wood for the trees under the bonnet sometimes!
There was certainly a lot of vacuum pipes and valves under there, more than any other car I've had.
VW Passat 2.5 TDi looses power - Hamsafar
Thanks to Dox and all, the matter is now resolved *Touch Wood*
The problem turned out to be the EGR valve which wasn't faulty itsself, but was being opened at the wrong times by a faulty vacuum control solenoid.

In the meantime, it was found that the VW dealer (owned by VW and with all the latest VAG VAS 1550 diagnostics) had put vacuum pipes to the VNT on the turbocharger on back to front, thus disabling the boost pretty much altogether, they also replaced my perfectly OK MAF and the new replacement and wasted a lot of time and money and fuel.

Do you think I should get my £200 back from the dealer?

VW Passat 2.5 TDi looses power - Hamsafar
The MAF issue, is because when the signal is sent to open EGR valve, the ECU looks for a corresponding reduced air reading from the MAF and because it already had one but didn't know, it substitutes heuristic values and registers a trouble code.
VW Passat 2.5 TDi looses power - George Porge
Steve's the real deal is'nt he. Glad you're sorted. Looks like Steve has another customer :-)
2 Dirty VW diesels and a Honda with an 18 inch blade
VW Passat 2.5 TDi looses power - Hamsafar
Yes, and he is only 7 mins away!
I have to say, C&R Enterprises of Nottingham is the best garage I have been to and will happily talk 'techie' with the customer.

VW Passat 2.5 TDi looses power - Hamsafar
Oh, and it seems like the girl on the website has left since the pictures were taken. :o(
VW Passat 2.5 TDi looses power - Screwloose

Haven't got her phone number - have you? I need a new apprentice... [Steve says his missus is even better looking.... but then he would be wise to, wouldn't he....]

Someone should have told you that an exhaust or EGR problem could cause low air-flow readings/black smoke......
VW Passat 2.5 TDi looses power - Hamsafar
Thanks screwloose, just like you suggested. It's a shame I haven't had the car very long, or I would have checked such things but I'm still reading up, and have failed to get my VW manuals working due to not working with Win XP, so hopefully will be more clued up in future.
VW Passat 2.5 TDi looses power - Screwloose

What have you got - ELSA? That works on XP - but needs a new "rolling code" for every new install.
VW Passat 2.5 TDi looses power - Hamsafar
I can't remember, I ordered two sets of CDs and DVDs off ebay from different sellers, with different numbers, both said some weird error.
It accepted the number (provisional 30 day one) and I created a new user in the admin utility, and then went to install the actual DATA VW/SEAT/AUDI and it won't accept the disc, I can't remember the message.

I also had EKTA parts CD, that just said "Incorrect Parameter" - very helpful!
I also tried a new install of Windows XP - same result.
Then I noticed the instructions said Windows 2000/NT, well XP is NT related, but maybe they meant Windows NT 5
Anyway, I don't really use Windows much I use a Mac, so it means I have to keep booting between XP and Mac OSX and I got fed up!
VW Passat 2.5 TDi looses power - Screwloose

This isn't an anti-spyware issue is it? Some of mine hate ElsaWin with a vengeance and block it's registry hooks to stop it installing. Tried disabling your anti-virus/spyware and seeing if that made a difference? IIRC those 4 data DVDs contain about 9 Gig!

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