Allroad 2.5 TDI intermittent power delivery... (sometimes)

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Adam C

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Hi,

Can anybody point me in the right direction?
My car is 2001 2.5 TDI Allroad. 79K miles

My car for 99% of the time performes to (I think) normal specifiation although on a few occasions it has suffered from temporary power loss. The power loss only happens under very heavy sustained load. This is most noticible when towing a (1650KG) caravan up a hill at highish speed, ~ 50mph in say 4th gear or maybe about 3500 RPM. What happens is that the car is maintaining speed, but there may be a period of time where power drops, but then it comes back again about 10 seconds later. I have managed to replicate this while not towing but you really need to be going some in order for it to happen and it doesn't always happen.
My thinking is that is could be to do with fuel delivery issues, or maybe a MAF sensor, or maybe some sort of sticking vnt mechinism? I understand that the vnt mechanism can be physically wiggled without dismatling the turbo and any stickness felt.?? Has anybody got any pictures they can post of where this valve is ?
Although the fault is not present most of the time, I need to get it fixed because of the towing issue, don't want to be stranded somewhere with the wife and kids and half a mile of traffic behind.

cheers

Adam
 
Hi, An update - my Ross-tech Vag-com cable turned up at the weekend.
I think my theory about the VNT mechanism may be correct, Vag-com tells me:
"1 Fault Found:
00575 - Intake Manifold Pressure
08-10 - Control Limit Surpassed - Intermittent"
Which I Interpret to mean on my car - that the turbo's still presurising the inlet when the throttle backed off and the protection mechanism isn;t working -m i.e VNT
If anybody can tell me if there's anything I can wiggle before I send the car to the garage I would be very grateful.

thanks

Adam
 
I had a similar problem with the Passat i used to have and it turned out to be the MAF, but symptoms returned about 5/6000 miles later. Having experienced a similar but more drastic power failure on my A6 its possible your N75 may have failed/is clogged up with crud or you have blown one of the vacumn hoses. Its also possible there is a split in one of the inlet hoses that is opening at high boost pressure.
Cheapest option first, I would firstly thoroughly clean your MAF using a brake cleaner or similar, and reassemble the connector using a dielectric grease, all available from Halfords. If that has no effect, next is a bit of a toss up. You could get an exchange MAF from your local indie/dealer, around £70, but the Vagcom code seems to point at the N75. I'm not sure of the price as mine got swopped under warranty. I would go for the N75.
 
Hi there!, Well I've had an interesting day so I thought I'd share it with you. Before swapping the MAF I wanted to do some more testing, especially to see whether my slow VNT mechism theory was backed up with data.

So we went for a drive this afternoon to get some plots done of the N75 Duty, requested and delivered boost. Here is the result of a hoof in 3rd gear to approaching red line, green is requested, yellow delivered, white is N75 and red is engine speed.
http://dev.voip.co.uk/~adam/vag/pre_pipe.png

so its maybe 14:00 in the afternoon, it's raining and I'm wondering why the delivered boost is off the scale. The boost starts to drop as soon as the N75 releases it's grip though so anybody's insight to this would be great. Maybe it's normal. Maybe the car's had a remap. I don't know. According to logging boost delivered peaks at exactly 2560.2 mbar. Does this ring any bells?

Anyway, I turned the car around to come back and repeat and there was a very loud bang as I pulled away. After a brief stop we limped back to bicester keeping the load and rpm as low as possible. The car at this stage smelt like a parrifin heater.
Here is a graph showing the trip back to Bicester. Note no Boost delivered:


Rather than go home we popped round to AMD in Bicester to ask them for assistance. I am extremely grateful to very very helpful guy who had a look for us and within minutes had picked up a shiney jubilie clip and refitted one of the big pipes that had blown off. He also freed up the VNT mechanism and advised me to clear out the faults on the ECU and retest, not forgetting to check the MAF requested and delivered. Again, I'm very impressed with the help received thanks once again.
There's a bit of a question mark over the extent of possible of accident damage to the offside front part of my car. My audi dealer told be about it when I has the cambelt changed a few weeks ago. I've only had the car a few thousand miles and it's possible the jubilie clip wasn't fastened correctly if this was disturbed during repairs. I don't know.

The graph below is the best one I've got so far since the repair, I'll try and get some more done when I have time.
On the left I can still see over delivery on boost pressure, but this is delt with quickly, Is this normal? MAF requested and delivers seams pretty much to stick together. I'm guessing that this is normal too. Colours have changes on the graph.


thanks for your attention

Adam
 
hi there was surfin about on the net and spotted your post, you seem to be having the same problem i have with my a4 2.5 tdi quattro ever since i have had the car it hasnt been right coming up with that fault you stated also the un uniform power delivery sometime brilliant sometimes rubbish and feels like its strangled. when i first got the car it felt a bit lacking so had it chipped the bloke reconed the maf was ******** so i changed that then all the problems of power delivery being all over the place going into limp home mode half the time. was wondering if you have had any joy with it?? recently with mine i have re timed the fuel pump (as it and the cams were out but separate problem) changed the maf meter 12 months ago freed up the variable vane mech (didnt seem that sticky) changed the map sensor on the ns intercooler checked all the hoses to no avail. unwilling to take it to a garage as i would imagine thay will tell me a i need a new turbo when the car drive fine 75% of the time with no smoke under boost. i have also got the ross tech cable and vag.com but as with most of it if someone else had found a solution im all ears to save my crying over the engine bay whilst getting no further with the car .

thanks, M
 
oh also i changed the n75 valve as that was a possible cause, its a right pain as i want to sell that car and its worth ****** all not working properly especially when its awesome when its working well. M
 
Interesting stuff. Did they say what was wrong with the turbo that caused the fault to be intermittent? And if its not a rude question how much did a new turbo cost?
 
Interestingly enough the Turbo isn't an exchange part, so I've got the old one in the garage waiting to be taken apart when I have some time.
The Turbo was Very expensive, at about £900 for the part alone.

This is the problem with running 6 year old cars that are still worth more that £10k!


Adam
 
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