Tdi limp mode/N75 problem?

[scotty33]

Hi All

I have had the dreaded limp mode kick in today. Car is a 98 passat tdi 90hp with the AHU engine with Garrett wastegated turbo.
The last few times, this has been down to a bad maf sensor. I only fitted a new VW exchange one, last month, and I don't think it's the problem this time as unplugging it does not reduce the symptoms.
I had a look around, found there is a hose going no-where off the bottom of the boost control valve. There is one hose going to the turbo coldside, another going to the wastegate actuator, this third hose, (which points straight down out of the very bottom of the N75), has a barbed connector still fitted to the loose end of the hose, I guess it branches off another rubber hose, but which one?
I'm not sure if this loose hose IS the problem, it looks like it may have been loose for some time?
I know the engine relies on good information from various sensors, any one of which can trigger this limp mode. Does anyone know if any of the other sensors can be unplugged (like the MAF) to get me a temporary fix?
I know I should get the codes read, seems like a good excuse to get kitted out with vag com!:(

[kenney]

Hi That is a non return valve which is connected to a vacuum reservoir (AFN engine) i dont think the AHU has this reservoir.I have seen it, not in use, many times, even on the T4.Send your e-mail address and i will send a copy

[onzarob]

Get the codes read, if the vacuum pipes for the N75 are not connected then you will get a overboost error and limp mode.

A good indicator is if you turn the engine off the start again do you get full power back, then when you increase the load the car seems to slow down,until you restart the engine again.

This is what happen when my pipe became detached ;)

[ini]

This early N75 info should be useful:

http://i118.photobucket.com/albums/o99/inisj/N75Plumbing.jpg

http://www.ms-motor-service.com/ximages/pg_si_0051_en_web.pdf

[scotty33]

Thanks for the help, my N75 is the same as the one above, the line that is not connected is the one that goes into the Air pipe from air filter (2). I can look for the port it should be connected to now I know where it is!
I get the feeling this is not going to cure my problem but we'll see. I plan to get myself a Vag com set up, but in the meantime, if any body can offer any knowledge on how to test or eliminate the various temperature and pressure sensors, I'd be very grateful.
Onzarob, as it's not the boost or wastegate pipe that is disconnected, (I guess it is just a breather line?) I'm not getting overboost, it is just very flat even when first started.

[ini]

Yep, line to atmospheric pressure.

You have unmetered air entering the system (after the MAF) this could be reducing your fueling causing 'limp mode'/underboost?

[scotty33]

Yep, line to atmospheric pressure.

You have unmetered air entering the system (after the MAF) this could be reducing your fueling causing 'limp mode'/underboost?

Ahh, didn't think of that - hope you're right, will post findings tomorrow...

[scotty33]

I just had enough time to reconnect the N75 vent line, it has not really made any difference. I ordered a vag com cable today, hopefully it will work with the laptop I need to scrounge!
In the meantime does anyone know any resistance values for components I could test with a multimeter? eg, the N75 is a solenoid valve right? The coil I guess would be no more than a few hundred ohms. and which switches are normally open/normally closed?

[schmal]

Scotty - I made a fortunate discovery in resolving this problem on my A3 (same engine). My friend calls it an Italian tune up...
I drove up to Scotland from South Wales over night, made sure that I didn't over rev in any of the gears (which is usually when the problem kicks in) and maintained a constant 60 - 80 MPH in 5th gear for the entire trip minus a few coffee stops.
The performance of the car has been vastly improved since then. I'm guessing the long drive gave the engine a chance to clean itself out. The problems only started after I let my old man use my car while I was working away and he never really gives it any welly so all kinds of deposits build up.
Hope this helps....

[scotty33]

Schmal thanks for the tip, but my car is in the more severe form of limp mode. 60 might be possible given enough time, but 80 is out of the question at the moment! must be a different problem?

[kenney]

charge pressure control solenoid valve. Measure resistance between valve contacts.
Engine code AHU
Specification: 25...45 ω

[kenney]

(Only engine code AHU)
Check conditions
Engine oil temperature min. 80 °C
Test sequence
Pull off charge pressure control solenoid valve (N75) connector.
Start engine and raise to maximum speed briefly by operating throttle. The operating rod for charge pressure regulating valve must move
If the operating rod does not move:
Check charge pressure regulating valve lever for ease of movement. When corroded together, renew turbocharger.
If the operating rod does not move, even though the lever is free to move:
Renew pressure unit for charge pressure regulating valve .

[scotty33]

Thanks Kenney, useful info, My N75 is 28 ohms but the wastegate does not operate as per the second test.
No more time tonight, I will look into the turbo and wastegate actuation tomorrow. Beginning to worry I'm one of the unlucky ones whose turbo IS dead?!

[ini]

Wastegate actuator may be full of oil, stopping it working.

If you suspect the turbo N75, swap it for the similar EGR control valve to test.

[scotty33]

Wastegate actuator may be full of oil, stopping it working.

If you suspect the turbo N75, swap it for the similar EGR control valve to test.

Thanks for that info:beerchug:, it could be full of oil, the N75 itself, had oil in last time I removed it. I presume the EGR control valve is piped up to the EGR actuator? I have not noticed another valve similar to the N75 - but I have not looked for one before:confused:

[scotty33]

Well, at this point it looks like my earlier fears are confirmed. Turbo coldside impellor can be moved but it's not what you'd call 'free spinning' guess I'm looking for another turbo ...
I'll do battle with it in a bit, when it cools down. Maybe I can coax it back to life for a short while, as I ponder my options.
It's a Garrett T015 028145702, Does anybody know if any other car uses this turbo, eg mk3 golf/Audi 80? I'm guessing the 90hp A4 does. Also can these turbo's be rebuilt? I guess I could manage this, but where might I get the parts? £500+ exchange from the dealer won't be an option..
+

[scotty33]

I found the problem, a small disc of thin brass sheet was folded up into a ball and jamming the exhaust turbine. I removed this and it's now got boost again. I'm hoping the blades have not been damaged/strained, but I'll still be looking out for a cheap spare turbo just in case.
I guess I'll have to make a new EGR blank-off plate,it seems brass sheet is not up to the job!:(
A big thank you to all who helped me with this, :beerchug:.