Limp Mode. N75? Turbo?

[SevilleSharan]

I have a sharan minivan with about 110KKm (66K).
Since it was 20K it has sporadically lost about half its power output when accelerating uphill.
The dealer first changed the fuel cut-off valve (no change).
-ECU (no change)
-Turbo (OK for about 20K)
-ECU (OK for 5K. Probably just luck)
The dealer always shows a printout from the 1551 with no codes.
Now the problem is back. As I am tired of the dealer not fixing the problem (and not changing the N75 and the lines that go into/out of it) I decided to study the problem myself and bought a VAG-COM and the injection system repair (includes the turbo) from VW Erwin.
When I read the engine log I got:

1 DTCs Found:
00575 - Intake Manifold Pressure
17-00 - Control Difference

According to the manual:
-N75 or lines or Turbo

The output test looks OK but this does not realy mean that there is not an sporadic N75 or turbo lock.

I decided to monitor the MAP preassure and try to produce the failure while logging data. Yesterday I was lucky and got the data while the car went into limp mode and in http://icaro.eii.us.es/~civit/noturbo.htm you can find the data for 2 minutes before the failure and about 1 minute after. When the car goes into limp mode the N75 control output is almost 100% and the turbo produces no boost (SDI like mode).
It can be seen that either the N75 or the turbo gets locked producing a boost pressure of about 2500Mbar and, thus the ECU decides to cut off the turbo to safe the turbo and the engine.
What do you think?

I have been trying to convince the dealer to change the N75 for a long time but they will always tell me that is OK. This van is much more difficult to self service than passats or golfs as the engine area is much more compact.

 

[GoFaster]

I have a hard time understanding the graph, there are many things labelled MAP and it's unclear which one means what.

Are you saying that the actual measured MAP goes to 2500 for a while and then the engine goes into safe mode? That means the problem is NOT the infamous hose between intake manifold and ECU, because the ECU is seeing the boost pressure.

If this is the case, I would just change the N75 yourself and be done with it.

Sometimes the N75 valves clog up with gunk from the crankcase vent, so make sure the hoses to it are clear (and not full of oil or crud).

 

[GoFaster]

One other thing, is this a 90hp (wastegate-type turbo, wastegate activated by pressure) or a 110hp (VNT turbo, wastegate activated by vacuum)?

 

[SevilleSharan]

I have added a simplified version of the graphic which is clear enough (only RPM and meassured MAP) in http://icaro.eii.us.es/~civit/notusim.htm . When the N75 (or turbo) locks and the ECU sees 2500mbars the ECU turns off the turbo. When the engine is restarted turbo returns.
I will change N75 and report back

 

[VelvetFoot]

One member, Brodie, who is in England, has a Sharon. I think he has said that N75 is part of the EGR valve. In any event, things could be quite different on your engine when compared with others.

 

[VelvetFoot]

Judging by your first graph, it seems that specified maximum turbo pressure is 2000 mbar. The ECU sees, or thinks it sees, 2500 mbar and therefore reduces fueling.

I was going to say that maybe the MAP sensor is wrong because it seems to start at a value of 1400 mbar vs specified of 1000. Maybe that would explain why the actual follows specified, only higher. Then I noticed that it did decay to 1000 mbar at the end so I guess that is not the problem.

 

[GoFaster]

Either the N75 valve is sticking, or the VNT mechanism is sticking. Get a vacuum gauge and install it in the hose that connects to the VNT mechanism, and have an assistant watch the measured boost pressure while driving. What the vacuum gauge is doing at the time that you have high boost pressure determines which component is at fault.

High boost pressure, high vacuum? This means the N75 is bad. The ECU is asking for the N75 to produce low vacuum (which acts to reduce boost pressure) but the N75 valve is not responding. Replace the N75 valve.

High boost pressure, no vacuum? This means the VNT mechanism in the turbo is seized up. The N75 is properly giving a no-vacuum signal, which is the signal to the turbo to reduce the boost pressure, but the turbo is not responding, and the only possible cause is that the VNT mechanism is seized. This one is bad news, it requires disassembling and cleaning the VNT mechanism, which requires disassembling the turbocharger.

 

[SevilleSharan]

This is a euro B3 110Hp with VNT turbo.
You are right the ECU is seeing over 2500mbar and then it cuts the turbo.
I don't think there is any problem with the ECU as what it does is very reasonable. When it sees 2550 it puts the N75 output at almost 100% (92.5% fixed) and turbo boost disapears.
Thanks.

 

[SevilleSharan]

Go faster what would you like in the graph. I can produce one for you.

 

[VelvetFoot]

I thought the turbo was fairly new?

 

[Eric Maurier]

Originally posted by VelvetFoot:
I thought the turbo was fairly new?

<font size="2" face="Verdana, Helvetica, sans-serif">A friend of mine with an Audi A3 TDI 110 has his VNT seized. The car has 22000km (no digit forgotten)

 

[SevilleSharan]

Originally posted by VelvetFoot:
One member, Brodie, who is in England, has a Sharon. I think he has said that N75 is part of the EGR valve. In any event, things could be quite different on your engine when compared with others.

<font size="2" face="Verdana, Helvetica, sans-serif">EGR and N75 are completelly different valves. They are just side by side.

 

[Jasper23]

I have EXACTLY the same problem with my 110 hp Golf A3 (AFN engine)..

Keep me updated..

 

[Nutsnbolts]

Guys- if the VNT is stuck in the "boost" position, then it seems to me that the N75 would shut down, and the turbo wouldn't respond, It would keep on boosting because the VNT is stuck. I only see 2 possibilities:

1) The N75 valve is bad, allowing far too much vacuum into the servo line at any given time.

2) The VNT is out of adjustment by a couple of turns- this one seems pretty likely, since the boost is so far out of spec.

It seems to me that even if the N75 was bad, it wouldn't generate the type of boost we're seeing here (it has a very linear overshoot). That being the case, I would say that the VNT is overreacting to a given amount of servo input, which would mean that the stem was out of adjustment. I'd look at that first since it would cost nothing to fix if it is out of adjustment. Turn it in a couple of turns and put some kind of thread lock on it to make sure the heat and vibration don't work it loose again. Let us know how you make out!!!

-Rich

BTW- the VNT on my 2000 Jetta siezed up at 120,000 miles...

[ February 18, 2003, 08:46: Message edited by: Nutsnbolts ]

 

[PWN]

slightly off topic

Is the N75 valve supposed to click loudly and constantly?????

 

[GoFaster]

Worry when it STOPS clicking!

 

[danix]

This sounds like a classic MAF failure.
Yes, I can ship to Spain, but I would hope they are cheaper there.

 

[T5TDI]

Originally posted by VelvetFoot:
One member, Brodie, who is in England, has a Sharon. I think he has said that N75 is part of the EGR valve.

My van is a Transporter (Eurovan) not a Sharan
Karl. Only the 5 cylinder TDI engines (like my ACV) have that combined N75/EGR solonoid. Hope mine never fails, it looks expensive!

I'd go for N75 first too.

[ February 18, 2003, 23:52: Message edited by: Brodie ]

 

[VelvetFoot]

Seville,

From another thread it looks like you've fixed it with a new N75. Congratulations!

I'll keep my fingers crossed that the fix holds!

 

[SevilleSharan]

I hope so. Thanks.
Turbo preasure follows theoretical preasure much better after changing N75. We will see.