2008 Q7 4.2 TDI V8 Limp mode on hard acceleration after engine warms up

Big Lebowski

Active member
Joined
Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
Hi all

We are hoping someone can assist with this issue that occurs on hard acceleration / load but only after the car is driven for some time and the engine has reached operating temperature.

The limp mode is quite dramatic as car can still be driven but has very limited throttle response and sound of engine changes as if the timing is then retarded.

Car has DPF delete with stage 1 tune to ECU. No error codes being thrown.

Have cleaned the MAFs and disconnected the MAF sensors and it appears the engine then uses a set of default air flow values. The check engine light comes on and the engine performance appears restricted. The engine doesn't have the same acceleration and performance that it normally does but there is no limp mode as above.

People often talk of overboost causing limp mode. From what I can tell overboost does not throw error codes?

I understand this car has VNT turbos with actuators that are controlled by electric cable from ECU?

I also don't understand why the engine will pull great without issue until the engine reaches operating temperature. Does the ECU in some sort of safety measure limit boost or fuel until the engine reaches operating temperature and that is why we only possibly get issue when operating temperature is reached? Or is the temperature aspect mean something else is at play such as transmission? Or electrical sensor becoming problematic with heat?

Any help appreciated, thanks.
 

Rrusse11

Veteran Member
Joined
Jan 23, 2014
Location
PA Deutsch Country
TDI
2002 Golf, 5spd; 05 Jeep CRD
I believe ross tech VCDS will read your vehicle. You need to do some computer diagnostics of the boost control system. Could be a loose wire to dirty VNT turbo vanes. It will help if you can find a tech/mechanic well versed with VW TDIs.
You should also post on Clubtouareg.com, lots of aussies there.
Good luck mate!
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
Hello, of course there are a lot of maps that are controlled depending on the temperature. Maybe there is a bug programmed for operating temperature and the car is going crazy. Is that an EDC16 or already an EDC17 ECU?
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
...........................snip............................. Does the ECU in some sort of safety measure limit boost or fuel until the engine reaches operating temperature and that is why we only possibly get issue when operating temperature is reached? Or is the temperature aspect mean something else is at play such as transmission? Or electrical sensor becoming problematic with heat?

Any help appreciated, thanks.
Yes, that's what the limp mode is, much the same troubleshoot tree as low power. When the ECU sees certain parameters from a variety of inputs, it won't allow full go. May not be an actual boost component. Start with simple stuff like vacuum hoses, malfunctioning turbo actuator, sticky vanes, loose piping, the big ones. At some point you may need some logs (and someone to interpret). Might also be good to look at the various temperature sensors, read them cold, then read them hot, look for wacky readings.
 

Big Lebowski

Active member
Joined
Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
And looks like I need a VAG diagnostic tool.

If the trans fluid was not correctly filled / have the correct amount of fluid, would that cause symptoms the car is experiencing?
 

Big Lebowski

Active member
Joined
Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
Does this Audi Q7 V8 4.2 TDI have vacuum operated actuators and n75 controlling valve or are the actuators controlled by direct electronic signal from the ECU?
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
BTR have EDC16 ECU. I have made a good tuningfile for. If you want, i can check your File, why the limp mode came.
 

Big Lebowski

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Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
BTR have EDC16 ECU. I have made a good tuningfile for. If you want, i can check your File, why the limp mode came.
Thanks. Excuse my ignorance but how would you do that? I have learnt the car has a DPF delete. Nox/cat canisters still there. EGR valves not operating but not blanked off. Apparently has a stage 2 tune (I said above stage 1 but I have spoken since with the tuner and it is stage 2). No error codes being thrown even apparently when it was flashed back to OEM to try and identify the problem - ongoing problem with this car over a number of owners and the car has been tuned now by 3 different tuners and limp mode persists each time.
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
I come from Germany and had a tuning company. I did it for 15 years, writing files myself because I was in contact with development engineers. I became a Christian and I invest my time differently. Now and then I try to help people who have problems with tuned cars. If I get the tuning file by email, I can check it and tell what problem it is. You can suppress all error codes even though there is a problem. And if you do the software series and there is no error message even though the DPF is removed or empty, something is wrong.
 

Poor King

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May 20, 2020
Location
NY
TDI
92 Jetta, 03 Jetta
First thing I would do is reset it. Turn your key into 1/accesories. Hold down throttle pedal for 5 seconds, and then release.
 

Big Lebowski

Active member
Joined
Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
I come from Germany and had a tuning company. I did it for 15 years, writing files myself because I was in contact with development engineers. I became a Christian and I invest my time differently. Now and then I try to help people who have problems with tuned cars. If I get the tuning file by email, I can check it and tell what problem it is. You can suppress all error codes even though there is a problem. And if you do the software series and there is no error message even though the DPF is removed or empty, something is wrong.
That would be extremely nice of you. It appears this problem has been passed on from one owner to the next without being fixed. I assume I have to contact the last tuner and obtain a copy of the file so I can email it to you. I will let you know how I go. Thanks.
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
I don't think a tuner gives out its data. Don't you know anyone who has a flasher so that you can read out the ECU data?
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
......................snip........................ Now and then I try to help people who have problems with tuned cars. If I get the tuning file by email, I can check it and tell what problem it is. You can suppress all error codes even though there is a problem. And if you do the software series and there is no error message even though the DPF is removed or empty, something is wrong.
Most of us lack the knowledge to interface with the ECU in order to extract/insert (download/upload) the data. Do we simply need a cable and something similiar to the EDC15 suite?
 

tdi_my live

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Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
At first it is only about an ecu flasher. It has its own software and is self-explanatory. This only reads out or writes binary data in. Editing this data is a different matter.
 

Big Lebowski

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Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
I don't think a tuner gives out its data. Don't you know anyone who has a flasher so that you can read out the ECU data?
I am not aware of anyone who does. Is the flasher something relatively inexpensive that I can buy? Would there be a model that you recommend? Thanks
 

Big Lebowski

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Location
Australia
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Audi 4.2 TDI
Correct me if I am wrong here but if I only need to read the ECU to obtain a copy of the file then I could use a clone? Or would I ultimately need something that reads and writes? Thanks. Probably stupid questions but I am on a learning curve here.
 

tdi_my live

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Location
Germany
TDI
Skoda Fabia
No problem. It's not just about getting the control unit number and software version, as with VCDS. A flasher reads and writes files.
But you need VCDS to query certain values such as boost pressure or injection quantity and more.
 

tdi_my live

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Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
That will most likely be enough. What the flasher must be able to do in any case, to read and write both via K-Line and CAN. Just K-Line may not be enough.
 

Big Lebowski

Active member
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Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
Think I have found a clone that will be suitable. I will order it. The biggest issue may be the time required for it to arrive via post.
 

Big Lebowski

Active member
Joined
Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
Still waiting for the flasher to arrive but do have access to a VCDS at moment but no error codes showing despite still going into limp mode on acceleration after the car has reached operating temperature. While I am waiting for the flasher does anyone have any suggestions as to what to hunt for using the VCDS? Thanks
 

tdi_my live

Veteran Member
Joined
Jul 24, 2011
Location
Germany
TDI
Skoda Fabia
you can log all temperatures. in any case, exhaust gas temperature. boost pressure where Pressure-should and Pressure-is be seen. Rail pressure.
Injectiontime and start of injection
 

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
Maybe log RPM, MAF requested & actual and Boost requested & actual while reproducing the issue. Also check the various temperature sensors, cold, then hot.
 

Big Lebowski

Active member
Joined
Oct 1, 2020
Location
Australia
TDI
Audi 4.2 TDI
The issue is that VCDS says there is no ‘label file’ for engine – 01 and engine – 11, I get ‘Labels: REDIR FAIL!’.

If I am understanding this correctly then this is because the dual controllers on this car are not common and VCDS does not have a ‘label file’ created for these controllers.

As a result of this there is no functional capacity for the VCDS 'advanced measuring values' and that function is greyed out. The other problem is that while the ‘measuring blocks’ function is available, there is no reference as to what each measuring block relates to. Further, the VCDS is not sure what the data being returned within each block actually is a measurement of and can only have a guess.

I can input measuring block numbers into the ‘measuring blocks’ function and the VCDS will return 4 values but I am not sure what most of these relate to.

This makes it difficult then to go looking for the problem that is causing the engine to go into limp mode with no error codes being thrown. For example, if I want to go and find turbo boost values then I am not sure what measuring block number to input? And even if I did then the VCDS may not specify what the 4 data values being displayed are a measurement of – in other words, the VCDS may not display which data is specified boost and which is actual boost.

Any ideas?

Thanks
 
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