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VW B5 Passat TDIs This is a general discussion about B5 Passat(>98 (2004-2005 in North America)). Non TDI related postings will be moved or removed.

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Old November 7th, 2018, 03:01   #1
kaneelschep
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Default Still limp mode/sinking brake pedal 2001 Audi B5 TDI AJM

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
Gonna read more about that.
but maybe someone can comment and confirm?

Thanks!

Last edited by kaneelschep; November 7th, 2018 at 03:14.
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Old November 7th, 2018, 04:15   #2
Tdijarhead
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Sounds like a leaking vacuum pump, which I think you’ve just discovered.
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Old November 8th, 2018, 02:36   #3
kaneelschep
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I will take it apart this weekend if I have the time. And glue it together somehow
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Old November 8th, 2018, 03:34   #4
Tdijarhead
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I’m not sure what the vacuum pump for your Audi looks like but if it’s anything like this, I doubt you’ll be able to fix it. Other than fix the rotating nipple, there is a fix for that as you’ve discovered.

https://www.idparts.com/vacuum-pump-a4-alh-p-2031.html
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Old December 20th, 2018, 01:24   #5
kaneelschep
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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..
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Old December 21st, 2018, 09:34   #6
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Have you scanned the ECU?
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Old December 26th, 2018, 06:51   #7
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Its always the same. Overboost. Which basicly means vacuumleak :P
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Old January 2nd, 2019, 11:54   #8
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Quote:
Originally Posted by kaneelschep View Post
Its always the same. Overboost. Which basicly means vacuumleak :P
Or sticking turbo vanes
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Old January 3rd, 2019, 14:34   #9
kaneelschep
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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.
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Old January 14th, 2019, 06:41   #10
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Pfff.. Read all the typical threads again.
http://forums.tdiclub.com/showthread...gnosing+fixing
http://forums.tdiclub.com/showthread.php?t=215165

Going for the maf.
Just got a complete belt set, thermostat, top control arms and a dieselfilter.
a maf wont break the budget that much more

I totally missed the part where I could just unplug it to test..
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Old January 14th, 2019, 07:07   #11
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You are starting to throw parts at it without proper diagnosis. This can be a slippery slope.
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Old January 16th, 2019, 13:23   #12
kaneelschep
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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?
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Old January 16th, 2019, 14:07   #13
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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.
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Old January 17th, 2019, 00:11   #14
kaneelschep
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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!
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Old January 21st, 2019, 10:40   #15
kaneelschep
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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.
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