read codes. Found 17965 charge pressure control positive deviation p1557 - 35-00 intermittant

[kaneelschep]

So I had a bit of a hickup wich mostly ended up in limp mode about 2 times per 100km. Which was gone after restart. Sometimes a few days without.
Got worse in the last 2 weeks. Now it sometimes goes into this very fast hickup which feels like its fluctuating between no and yes boost very fast. And then goes into limp mode. Or it just continues to do that when pushing the pedal. Anyway I restart and its all good again,
Read the codes and this came up.

Read a lot about it.
Most people point at maf, n75 or vacuum lines.
Now I do have some shady vacuum lines. But they did not get worse and they were just as shady 20000km a go.
I replaced the ones I could reach and which looked the worst.
Didnt help.

So I suspected maf.
Did the test from:
MAF sensor FAQ, troubleshooting, and replacement for VW and Audi TDI (diesel) | VW TDI forum, Audi, Porsche, and Chevy Cruze diesel forum (http://www.myturbodiesel.com/wiki/maf-sensor-faq-troubleshooting-replacement-vw-audi-tdi-diesel/)

Now I get a bit of a strange result. They say actual and specified should be around the same.
But actual is actually higher than the specified. At around 3000rpm actual goes up till just over 1100. while specified never ever goes over 850.
And even at lower rpm the actual is always higher.
Then the limp mode kicks in and like the page sais, actual never goes over about 540.

I recently had sort of same issue with my volvo 850 benzine. Also was a maf problem in the end.

But maybe someone has any other ideas or things to test?

Thanks!

[kaneelschep]

Read a bit more and higher actual mostly points at bad egr flow. So could be clogged egr.
I could not make a graph of the data with vcds lite. It sais I need the registered version.
But I made a log though. Dont know what to see at the egr data though. Will have to read a bit more about that.

[arman123]

Turbo clogged up with carbon you need to replace the turbo for a recon unit or get your one refurbished.

[kaneelschep]

Just replacing the whole turbo is a bit drastic to my taste ;-)
To me its seems clear it is something controlling the boost.
The turbo works perfectly fine most of the time. Today i had almost no issue.

I think something else and a lot cheaper is the problem here.
Either maf, vacuum line, egr or n75.

The higher actual maf seems to point towards egr.

Thats what i will check now. ;-)

[arman123]

I had the same problem when I got my car and tried n75/maf/vac lines and in the end it was the turbo the car is over boosting observe your turbo actuater arm try put a hand held vac on it see if it moves freely most likely it’s sticking and causing the oberbooste code you getting common fault on these engines turbo just get build up of carbon people putting crap diesels you need to give these cars a good drive using all the Rev range every couple of weeks to blow all that crap out

[kaneelschep]

I followed the egr vacuum line to where it connects to the valve on a big elbow. The n18 right?
Everything there was all oily and the vacuum elbow quite worn out. The vacuum line hanging almost out.
So i cleaned it all and reconnected.
Also replaced all the little pieces of vacuum lines there.
It immediately ran a lot better. Good boost. But after pushing it hard, it went into limp mode at 3500rpm in 3rd gear. No hickup or anything. No other symptoms. Just no boost. So I think the elbow let the vacuum go again. But its dark and i have to get up early.
Tomorrow another day..

[kaneelschep]

So it seemed i had two things going on. The fast fluctuation of power, no power was appearantly a seperate problem. As it went from intermittent to a permanent thing yesterday. A Misfire on cylinder 2 vcds would tell me later at home. 25km later..
I found a fix for that in the injectorplug, which seems to have worked. 100km later and now the innitial problem of overboost only throws it into limp mode without drama.
Given that the maf shows high enough figures for it to have degraded, i suspect the n75 to have failed.
Didnt do a test for that yet though.
But its big culprit in overboost vnt situations.

[kaneelschep]

All points at the n75 valve. So I checked for replacement. Appearantly the car is supposed to use the H serial model pierburg. The one with the square plug. The n18 valve on the car also has that square plug. Which is basicly almost the same pierburg valve as the n75.
Though this seems to be a bit of a mixup car, cause my n75 is the one with the newer oval connector. The one With the same serial but J instead of H.
Which is 1; a bitch to get off. I hate those oval connectors. And 2; more than two to three times the price! :P

The whole placing of the n75 in the audi b5 is horrible btw.
I could just get the top nut off after taking the whole intake hose of.
But it seems to held in place by another nut on the bottom. Which is quite impossible to reach without taking the nose of.
I will try from the bottom after I get the replacement valve.
And some more vacuum line. Cause while I am there, I just replace every line I see.

I am using 3.2mm now. It was the only thing they has closest to 3,5mm. Will 4mm fit too?

[kaneelschep]

So I changed a lot of vacuum tubes till I ran out of new tube. Need to get a few more meter and then will change all. Changed the n75 valve. Cleaned the EGR valve and the pipe.
Nothing made the difference. Everytime back to limp mode.
Till by chance I leaned on the pipe coming from the brake booster going through the firewall through a rubber.
And it did tsssssss.. Anytime it moved a tiny bit it leaked vacuum. On both pipes/sides of the rubber.
I greased up the rubber and the pipes going in it. And my problem was gone..
No more overboost. Think I still need a new rubber though. Dont want it to pop loose or out again.

Immediately I had another problem. totally unrelated though. Broken regulator on the alternator.
Local prof checked diode, bearings etc. Cleaned alternator. And I bought and swapped in a new regulator.

So now i could finaly properly test the car.
it does not go into limp mode anymore. no matter how much I push it.
When boost comes on it feels strong and responsive for a 410000km engine. Just this morning it passed that ;)
car feels nimble and fast again. More clear clean boost than I ever felt.

Only difference is that it seems to need a bit more time to spool up than I was used to.
When you dont let go of the gas, it is never an issue. But when you change gear slowly it falls back to idle too long and needs a few seconds to spool up again.
Dont know about that. Back to searching the fora again ;)
Anyone know?

[kaneelschep]

So my hack with the grease was fun for some time. Until it started drying out and on longer boost situations the same problem of overboost and limp mode occurred.
I bought a new part at Audi. Not even 10 euro. Now it runs like normal again.
Always power. And no more delay.
Cheap fix in the end!
Just comes to show; Vacuum vacuum vacuum :)