[SOLVED] Lack of power with 2,5tdi AFB

Hi!

I'm desperate here and at my wits end, so you are my last resort. I'm sorry for my rather poor english, and especially the technical terms. But I'll try :)

I have a 1999 Audi A6 2,5tdi Quatro Avant with AFB engine. I haven't had that much problems with this car, but I have had one problem for ever, that noone seems to be able to fix. The problem is that _sometimes_ (or maybe 75% of the time) the car doesn't have the power it should have. But the rest of the time, it's going very, very good. The car is chipped, but I have also tried without chip, and the problem was the same. In addition to this, I get sometimes error with vag-com about exceeded manifold pressure. Sometimes (maybe once a month-ish) it goes into limp mode because of this. There are no other error codes - and never have been, really.

Sometimes, when I'm going very slow (10 km/h) and then push the throttle a bit, for half a second it feels like it's dying, before it's starts getting revs again. Like it's not getting diesel for a brief second. But I don't know if it's related.

The last two years, I have changed/done the following (not in order):

N75
Removed EGR totally
Replaced all vacuum hoses and one way valve
Replaced hoses to/from intercooler
Pressure tested intercooler system
Replaced turbo
Replaced diesel injector pump
Replaced air filter and diesel filter
Replaced MAP-sensor
Replaced MAF
Replaced crankhouse breather
Removed catalytic converter
Changed cam-shafts and timing belt
Temporarily changed ECU

I am ofcourse not 100% sure that my overboost-problem is related to the problem with periodically poor performance. But it seems likely. Anyway, I'm really at my wits end about what could possibly be causing an overboost here - every part that is related to charge pressure is replaced...

I have been thinking about the build up of soot in the inlet, but I have been taking a look and it doesn't seem to bad. Anyway I guess that should be the same all the time, not sometimes working perfectly, other times having loss of power.

What makes this so dificult is that it's sometimes it's going very good, other times not. It's not really going bad - but it's extremely big difference. When it's going properly, it's really satisfactory :)

If anyone has an idea here, I would be very grateful. But I'm afraid this is an unsolvable problem...

Regards,
Stian

What about the vacuum pump ? If all the parts were genuine i would say timing is not right...

Hi!

Thank you very much for your answer. I was hoping you would reply - have read some of your answers on this forum and you seem to know everything :)

All parts are genuine, and even fitted by a VAG shop. I _think_ I have had this particular problem for the five years I have owned the car, but I'm not sure. There have been so many things wrong, so it's hard to tell. But I have always felt that it is sometimes having more power than other times. Even when it has always been bad. But regarding what you wrote about timing, it _might_ have started after I got the injector pump replaced...

I have been thinking about the vacuum pump, but I have been driving with a gauge on the vacuum-connection, and it has never been below 18 in/Hg, usually it's around 25. I guess that's ok? And if I disconnect a vacuum hose even hours after the car was shut down, it's still a considerable vacuum.

What you say about timing is interesting. I know to little about this, but can the symptoms be that it's sometimes good and sometimes not so good? The car is always very easy to start. Can I check this with vag-com, or do I need to look at marks on the belt and cams and whatnot?

Thanks again for your reply! :)

After some googling I see that I can check timing with VCDS. Though I can't seem to find the button to start the TDIgraph. Whatever. When it's warm I get 15,6 degrees on early and 1,8-2,0 degrees on late. As I understand, this is good numbers...

And I guess it shouldn't be the timing. I have for sure had this problem since I changed diesel injector pump (and maybe longer...), and seven months later I changed timing belt (where the timing would have to be redone?) and eight months after that I changed camshafts. So the last two years there should have been three times where the timing would have been controlled/adjusted :)

As I initially said, I don't really know if the overboost is related to this problem, but it probably is. Can an issue with fuel cause an overboost situation..? I hope so, because I really can't see what else is causing this, other than pump/injector/needle sensor, etc. Everything about the charge air system is replaced... I will post an updated graph with boost tonight, if that would help anything...

Hi!

Sorry for the delay. Attached is graph with logging on block 11. I don't have much experience in reading this, so I don't really know how close specified and actual intake pressure are supposed to be. But for me this doesn't look too bad.

I think the big question for me now is wether anything related to fuel supply can possibly cause exceeded manifold pressure, and secondly if it's at all likely that something with the injection pump or even injectors can cause this erratical performance issues? It's mostly bad, sometimes worse, sometimes quite good. I have not been able to find a pattern in when it's good or when it's bad.

I have spent truckloads of money on this old car, and it would have been very nice to finally be able to sort it out...

Your turbo is faulty, it is a very common problem and shows itself as overboost.

Well, maybe it is, but it was replaced a year ago (as I said in my first post). The problem was there before, and it was still there after. So I really don't think the turbo is to blame (at least not by itself)...

Well you would hope not so in view of it having been replaced I would now fit a new boost pressure sensor (what VAG call a Thrust sensor) and if that did not cure it, a new N75 valve but the latter tends to be responsible for under boost codes rather than over boost.

If you by "boost pressure sender" is talking about the MAP sensor (G71) it is replaced, as well as the N75.

I stated in my first post what parts I have exchanged, and everything is done by a VAG shop with only genuine parts...

Well from your graph it is definitely boosting more than the ECU is requesting. If the N75 valve, MAP sensor and turbo are new plus the pipework and electrics as they should be then the only straw I can pluck at is the Air Mass meter, after that you have to go back to the beginning and start again. Recently I have had a couple of genuine VAG N75 valves go faulty within a very short time.