Still limp mode/sinking brake pedal 2001 Audi B5 TDI AJM

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
Hi all.

I have been rummaging with a limp mode and sinking brake pedal issue for some time now.
Had a sweaty buttcrack situation twice of sudden no brakes.
In normal situation also cannot make the wheels block or abs out with pedal on normal surface.

I basicly swapped and checked all vacuum lines. changed n75 valve and the rubber which connects the main vacuum pipe to the brakecilinder through the firewall.

That made a big difference allready.
Went from always limp mode quickly when power was asked, to just sometimes.
When it works, it always has plenty of boost and feels powerfull enough like any tdi I driven without issues.

Now after about 3000/3500 rpm the boost sort of ends. When I continue it will go into limp mode.
That is quite a given.
But sometimes it will just go into limp mode cruising in 5th gear asking no power at all.
Maybe related to going over bumps/holes. Well or it was coincidence.
We have a lot of holes in the roads here ;)

So I know a few things on these cars suck.
The pipe coming from the brake cilinder to the engine.
I checked that pipe. Nothing wrong with it.
Where it goes in the rubber through the firewall sucks. I changed the rubber.
Brake cilinder itself sucks.
it looks good. And dont want to change a 250 euro part for nothing.
Is the connection flange on the vacuum pump supposed to be able to turn?
it does.
So while typing this I found this:
http://forums.tdiclub.com/showthread.php?t=457763

I guess I found an issue :D
Gonna read more about that.
but maybe someone can comment and confirm?

Thanks!
 
Last edited:

Tdijarhead

Top Post Dawg
Joined
Nov 10, 2013
Location
Lawrenceville PA
TDI
2003 TDI Jetta Daughters Car, 2001 TDI Beetle, Wife’s car, 2005 Golf TDI Mine, all 5 spds
Sounds like a leaking vacuum pump, which I think you’ve just discovered.
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
I will take it apart this weekend if I have the time. And glue it together somehow ;)
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
With the sudden -10c cold here since last week, the issue became worse again.
At any longer boost it will go to limp mode.

So yesterday I had some time to spend on it.


Around the top hose of the intercooler its very oily.
So the intecooler it out and tried to pressure test it. seemed ok. Cleaned the hoses, the intercooler and around it. Now I can keep an eye on it and check where the oil comes from.


At the top of the plastic pipe going down to the intercooler its also very oily.
Could this plastic pipe maybe have a tear or crack?


Glued the vacuum pump nipple to the pump with metal epoxy. That cant leak anymore too.
Tried it today, but the issue hasnt changed.


Think I have to follow the oil here to find the leak..
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
Its always the same. Overboost. Which basicly means vacuumleak :p
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
Naaah. Turbo feels strong as ever when it keeps boost. Still good push from 1500rpm up.
And the irregularity of when it occures to me clearly points away from turbo.


Yesterday, still rather cold engine, I drive slowly uphill and plop. Limp mode.

I restart while still driving, keep it slow a bit. 10km further I decide to push it till 4000rpm again up a hill. No problem. Flies up. Leave everyone behind.

And no problem for the next hour too.

On the way back still all is well. Road gets a bit bumpier, not pushing at all. Limp mode again..
No pattern besides: More limb mode when cold and bumpier roads.
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw

imo000

Top Post Dawg
Joined
Dec 13, 2005
Location
Cambridge
TDI
2009 M-B ML320 Diesel & '05 Passat TDI Manual 5-Speed
You are starting to throw parts at it without proper diagnosis. This can be a slippery slope.
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
I did not buy a maf. I just tested it. ;)
And it didnt do anything. If all it just ran worse and it still went into limp mode.
With the maf connected it clearly ran much better.
Also maf values show quite normal.


So I keep coming back to the vacuum. I put it back into the garage and checked everything again.



Now I have one thing which is not on any vacuumdiagram I can find.
Its a vacuum controled valve in the coolant line going to the heated diesel filter.
This is directly connected to the line coming from the brake booster.
From the brakebooster a little line goes to a split under the coolant reservoir.

To connect to the n18, and then way further to the n75 and the vacuum reservoir.
Thats what all the diagrams show.
But mine has a 4 way split instead of 3. And also directly connects to this coolant line valve.


And if I blow or suck on this line, Air leaks out at that valve quite easy.

I seperated it and it only leaks there. Without the valve, the rest does not leak.

Could this be the basterd that makes my precious vacuum disapear?
 

imo000

Top Post Dawg
Joined
Dec 13, 2005
Location
Cambridge
TDI
2009 M-B ML320 Diesel & '05 Passat TDI Manual 5-Speed
One thing I've learned over the years. Follow the diagnosis as opposed to what you think the problem is or else you will keep disregarding the results and going to run around in circles. For example, if you already check and double checked the vacuum system then that's not where he problem is. Time to move on to the next on the diagnosis list.
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
In nornal cases yes.
But as i said, i missed that line before. And this has been going on for half a year. Did at least 15000km in the meantime. Things change. Gotta doublecheck. And it paid of!
The car came back alive. Not perfect yet though.
Allthough it didnt immediately go into limp mode at the first hill, and turbo was strong and over the whole band again. When over 4000rpm and power on low rpm in 5th gear makes it go into limp mode.
We soldier on..

Big improvement!
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
Did about 500km since then. It runs great again. Issue was definetely vacuum related.

I am happy I doublechecked ;)
WIll try next weekend to do some test runs with vcds and the laptop. To see what happens at higher rpm.
 
Joined
Apr 7, 2014
Location
Finland
TDI
VW Passat 2000 1.9L TDI 85kW AJM
Or sticking turbo vanes
I would guess the cause is sticking turbo vanes. Our old TDI Golf with AFN engine had sticking turbo vanes. I never put the turbo to pieces put it felt like the turbo vane ring had rust pieces or big sooth particles that caused it to move erratically. I had adjustable one way valve from boost pipe to turbo control vacuum pipe (T-fitting directly to the pipe that went to turbo). The idea is to configure to valve no not put any air unless the difference between the control pipe and the actual manifold pressure is too high (around 2.0-2.5 bar) and if that is exceeded the manifold pressure is leaked to control pipe to practically push the VNT mechanism closed (the push will obviously stop once the boost pressure drops to sensible values).

Before going with the valve I tried to manually move the turbo vanes back and forth every time the system seemed to have issues. The sticking does not always mean frozen in one position but just having problems with smooth operation. If you move the VNT mechanism manually over the full movement range for say 10 times and the movement is not smooth all the time you have sticking turbo vanes. The problem with the Audi/VW implementation in these cars is that the ECU will try to adjust the vacuum in the control pipe assuming that everything will work. The ECU does not know if the VNT mechanism moves at all, it does not know the rpm of the turbo. And still it tries to use vacuum solenoid to control the turbo rpm! The only feedback is has is the manifold pressure and there's pretty strict limits when it considers that the manifold pressure does not follow the assumed behavior (=everything works). The VNT mechanism does not need to be stuck for more than 0.5 seconds around 3500-4000 (engine) RPM and the ECU will enter limp home mode.

In our car, the colder it was outside, the more sticky the turbo got. I guess it was just thermal expansion in cast iron turbo housing causing already too small tolerances to get sticky.
 
Joined
Apr 7, 2014
Location
Finland
TDI
VW Passat 2000 1.9L TDI 85kW AJM
I had adjustable one way valve from boost pipe to turbo control vacuum pipe (T-fitting directly to the pipe that went to turbo). The idea is to configure to valve no not put any air unless the difference between the control pipe and the actual manifold pressure is too high (around 2.0-2.5 bar) and if that is exceeded the manifold pressure is leaked to control pipe to practically push the VNT mechanism closed (the push will obviously stop once the boost pressure drops to sensible values).
The thing was called Dawes TDI valve, similar to https://dawes-devices.com/products/dawes-devices-mk-ii-hybrid-boost-controller

Note that the valve is pretty hard to configure correctly. If you allow it to leak too much, you never get high boost and the acceleration will suffer (I never got limp mode so it's a start). If you set the limit too high, the device will do nothing. After I had used it for around half a year, I removed the device and the car seemed to continue to work nicely. I guess the device caused the VNT mechanism to move more than previously and it luckily cleared the particles that caused sticking. The car will be better without this kind of hack but it will help with the limp mode as long as the VNT mechanism is not totally frozen.

Be warned that this may cause turbo RPM to drop pretty rapidly once the valve opens which will cause drop in performance and some claim that it might be bad for the turbo bearing. I don't believe this to be bad for the turbo but performance drop is never a bonus...
 

kaneelschep

Well-known member
Joined
Nov 4, 2017
Location
Holland
TDI
ex-66kw passat, ex-132kw a6. Audi a4 b5.5 tdi 85kw
I really wanted to mr muscle this weekend, but then I had to find the hole in/and fix the fence cause the dog kept escaping to the street.
And sort all the cables in the living room, cause our little girl started walking.
;)

I am definetely getting a can today. :D
Its so easily accessible in the b5 audi! Cant go wrong.
 
Top