2.5tdi intermittent powerloss

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karl210

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My audi has been fine for a while until today. Everything was fine when i pulled up at some lights, i noticed when i set off the exhaust was louder, i decided to give it some boot but there was no power, felt and sounded as though there was no turbo atall. I did this a few times to try and see if i could figure the problem out, but after fully thrashing it up the revvs the power came back and it felt better than ever. All was fine again but after another 10 or so miles it happened again and went away.
It seems strange how the exhaust goes louder and deeper, any ideas? I will scan it tomorrow and hopefully that will bring something up.
Thanks
 
Sound like sticky waste gate/egr.
When u scan, monitor live data on egr position & air mass. Confirm egr only during idle and part load. If ur scanner allows, actuate egr at idle and monitor air mass -> should drop proportionally as exhaust gas introduced.
 
egr has been fully removed, last time i checked the vnt it was free not too long ago, but i will give it another wiggle and maybe spray some mr muscle or whatever they say to spray in there. That did cross my mind, would that cause the louder exhaust though?
 
I bought the car with the egr already blanked. Im not too sure, i have vcds, will that monitor it? I have also fully gutted the exhaust system though, was around 2-3000k ago
 
No. No exhaust pressure sensor/measuring block for this. I've had honeycomb sliding sideways inside exhaust and choking the car...but if uv seen to the exhaust, not likely...
 
I recognize your symptoms (2.5TDI as well). Lack of actual boost (it'll still do 120mph, but it'll take significantly longer to get there, so no true limp mode and no CEL activated), and a much more audible drone in the exhaust.

I've had this problem for well over two years now, seems to be getting more and more frequent (up to the point I'm used to the lower power, I guess), and no garage was able to find anything wrong with it. It should have around 200bhp now (190-205-ish, depending on weather etc.), even when running badly it'll likely make 170-180bhp, so it'll still feel a tad faster than a stock car, so garages will say it's good). Initially, thrashing it ("loosening stuck valves or actuators by hammering them" method) seemed to work; burned off some excess carbon, forced the actuators to move.. etc.. however the issue state seemed to return more and more often.

My exhaust is a 3" (turbo downpipe to tip) decatted and fully open, no meshes/combs or baffles in there. Only have one resonator, which is a free-flow absorption type (wadding behind a perforated mesh wall), so exhaust back pressure is not an issue in the slightest.

I would describe it as VNT being stuck, but it isn't (checked). What I also noticed is that the brakes feel OK, but not as sharp as they can be. This leads me to conclude that there is a problem in the actual vacuum system (used for brakes and VNT actuator). This has been mentioned on several forums as a problem, in which they replaced the vacuum pump and/or some hoses, which fixed both problems (separate cases). Odd thing is - according to the local Audi dealer, there is not a single connection between the vacuum of the VNT and that of the brakes! Which, in itself is odd, considering that both use vacuum and there is only one vacuum source in the car: the pump..
 
The vacuum pump supply vacuum to both brake servo and n75, egr solenoid but are on separate circuits. Ther's a check valve on the vacuum pipe to servo supply which has a nipple that tee's into the turbo system. I would say that your n75 is dying or the vnt mechanism is cloged but there is also a connection between a dirty fuel filter and the fuel pump supply- that will also put the car in some sort of limp mode however you should get the Abs/esp light on the dash. I will start by replacing the fuel filter along with a new, genuine Tee valve, then N75 and as a last instance i would check the exhaust manifolds for breakage- had a few turbos with mangled hotside due to manifolds breaking internally ! As with everything, start with the cheaper bit first then work your way towards the expensive bits... One more thing, is your intake manifold anti shudder flap still working when you turn off the engine ? I also had a few that bleed vacuum from that valve, it's next to the abs pump on the same bracket...
 
cheers guys, i've been ill so not had much chance to have a look.
I scanned the car and the only fault that came up engine related was low level fuel sensor, this has been showing ever since i got the car a year ago, fuel gauge works fine though.
It did it again yesterday, i pulled over a few times and switched engine off and on to see if power would come back but it didnt, Hopefully the issue is still there when i go out to the car shortly and i will listen for the anti-shudder valve. I will plug vcds in to check the actuator isnt sticking, is it worth doing any logs to help pinpoint the issue?
I really gave it a good thrashing yesterday and it wasnt getting any better after 10 miles of driving, the car feels like it has about 50bhp (no overexageration) i decided to rev the engine as far as it would go and it got to 3500rpm'ish and it just stopped, seemed like it cut power completely but i panicked and pressed the clutch and power came back.
 
Ok so the shudder valve isnt operating when i shut the engine off, i do remember it working before though. Also i ran the n75 in basic settings and the wastegate rod would move up and slowly come down, should it not stay fully up for 5 seconds untill vacuum is released?
I logged n75 duty while running, i think it went from between 85-90% at idle and down to the lowest of 75% at full power, isnt it meant to go lower than this?
The car seems to permanently have these symptoms now. In my previous post where i said it wouldnt allow me to go over 3500rpm, it did today.
Thanks
 
If the shutter valve isn't working then you have low vacuum, leak or faulty vac pump.
 
Ok thanks, are there any ways to test this? Ive just had a quick look up and the engine is quite loud to be fair. But the brakes work really well
 
Pull the vac pipe off the anti shudder valve and suck it , see if you have a leak.
 
Will be attempting this soon so before i do whice pipe? Is it the one that goes from the vac pump to the anti shudder or another one? Thanks
 
Disconnect the anti shudder valve all together, plug the hoses with 2 small bolts and see if you get more vacuum to the turbo...
 
audi upgrades
Ok i disconnected the anti shudder valve on the inlet and pluged the hose, still no better. Ive tested wastegate diagphram and thats ok.
 
Still baffled by this one. Ive just disconnected the wastegate hose, sucked on it and clamped the hose shut to keep a vacuum and she runs well, really well!!
The vacuum pump seems to sucking quite strong and brakes are good. We still have no anti shudder valve noise though. Any further ideas? Could it be the n75?
 
Just tested the resistance in the n75, it just drops to 0, and seems to have continuity, its beeping on the continuity test, is this normal?

Edit - its reading 15 ohms

Edit again - ive bypassed the n75 from the vacuum pump to the anti shudder valve and that is working now, all seems to point to the n75, do we agree?

Thanks
 
Last edited:
This sounds just like mine.
2005 a4 cab, 2.5tdi 163
Feels quite flat, doesn't seem to quite Rev straight up
Feels like the turbo doesn't boost like it should but it does boost, as you can feel somthing.
 
Mine was completely flat, felt as though it had 50bhp and sometimes wouldnt rev over 2000rpm
 
b16 will said:
This sounds just like mine.
2005 a4 cab, 2.5tdi 163
Feels quite flat, doesn't seem to quite Rev straight up
Feels like the turbo doesn't boost like it should but it does boost, as you can feel somthing.
Click to expand...

Exactly. Took a VCDS log yesterday of groups 3, 8 and 11 (MAF, injection and boost pressure). MAF values (actual) fluctuated, and were higher on high RPM (470 @ idle, somewhat normal for a tuned car with shut EGR, up to 1000+ at 3800rpm), however MAF specified was ALWAYS 879.2, regardless of RPM (742 idle, 4200 in the log). Used 3rd gear. Car felt slow, lack of torque.

Boost was up to 2500mbar, with 2400 specified at high RPM, so turbo did its thing. N75 duty cycle changed, between 50 to 90%. Together with the boost pressures being made I do think that my N75 is still functional.

Was quite sad to see that boost pressure was actually at or slightly above requested, was hoping to see a lack of boost as that's exactly what it feels like.. :(
 
I'm not so computer orientated (unless it's the ****** playstation) so always give up looking at figures and actual values.

I'm going through mine after work and whenever I get chance (as I'm a garage mechanic) and will keep you posted.

Because I drive so many cars, one minute you think it's not bad, then you think 'God this is crap.

It just feels so lethargic!! It really could do with that extra kick up the **** !!
 
Ps I've now got a 2002 a6 2.5TDi Quattro, it's a car we used for collecting parts etc and a customer T boned a dopey bird in it last week = parts raider. I'll try the usuals and swap the vac pump etc & report.
 
bastet said:
Exactly. Took a VCDS log yesterday of groups 3, 8 and 11 (MAF, injection and boost pressure). MAF values (actual) fluctuated, and were higher on high RPM (470 @ idle, somewhat normal for a tuned car with shut EGR, up to 1000+ at 3800rpm), however MAF specified was ALWAYS 879.2, regardless of RPM (742 idle, 4200 in the log). Used 3rd gear. Car felt slow, lack of torque.

Boost was up to 2500mbar, with 2400 specified at high RPM, so turbo did its thing. N75 duty cycle changed, between 50 to 90%. Together with the boost pressures being made I do think that my N75 is still functional.

Was quite sad to see that boost pressure was actually at or slightly above requested, was hoping to see a lack of boost as that's exactly what it feels like.. :(
Click to expand...
Then replace the Maf, it should go over 1000 pp on full throttle, it looks like it might be slowly dying...
 
MAF readout did exceed 1000, it's the ecu specified value that didn't change.
 
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