N108 Control: How to Operate & Troubleshoot

Hello everyone,

"On my old A6 C4 TDI (AEL engine), the N108 valve is consistently being controlled at around 80% during idle, according to VCDS. However, VCDS indicates that the value should be between 4% and 40%. It only reaches this value (4...40) during coasting (then it's around 35%)."
The resistance value of N108 is okay, and the 12 volts are also present.
It also clicks when I pull the plug while the ignition is on and then plug it back in.
Furthermore, the engine is running quite roughly. In my opinion, it's misfiring.
If I disconnect the N108 connector while the engine is idling, the engine noise changes almost not at all. No error codes are also generated.

ESD is new, ESP is outdated and calibrated on a test bench, ZR is relatively new and meticulously adjusted, FB is minimally set to early.
AGR is saturated. I suspect that's why the value of 10 in MWB 10 is so high. It's currently around 130.

Can anyone give me a tip here? I've been reading about it online all afternoon, but unfortunately, I can't find any further solutions.

Thank you for your effort, and best regards.
Wolfgang.

Was was the correct identification code selected in VCDS when setting the fuel injection start timing?

Hi,

Yep, the license plate letter was definitely correct.

Best regards,
Wolfgang.

Does the AEL have a VP 37 or a VP44?
hg
Herbert.

VP37

Older units of this fuel injection pump (typically with mileage between 200,000 and 250,000 km) often exhibit a tendency to start fuel injection too late.
Check the corresponding measurement block in the engine control unit to see if the target value for the fuel injection start is being reached when the accelerator is fully pressed and the engine speed is high. Otherwise, under high load, you will eventually damage the pistons, spark plugs, and turbocharger, not to mention the pointless black smoke and loss of power.

In this case, it's better to set the ESP a little earlier.

Best regards, Rainer.

Hi

First of all, thank you for your answers.

He has a VP37, just like Rainer. It was completely overhauled in February and calibrated on a test bench. (Dieseltechnik Heim)
The ESDs are new and come from a known and absolutely reliable source.
The ZR is brand new and guaranteed to be correctly calibrated.
The temperature sensor G62 is also new, but the old one was still working and doesn't affect the control system.
The front brake is easy to engage, but only lifts the bike about 1/3 of the way off the ground.
Current mileage is 591,000 km. Oil consumption is approximately 1 liter per 18,000 km (long distance driving) and with a more relaxed driving style.
There are also days when it starts up normally, as I would expect. But those days are very rare and seem to be independent of the weather.

If the fuel pump fails due to wear and tear, that's obviously not good. Thanks for the tip, Rainer. What factors play a role in that?
I will definitely take a look at the values you mentioned. However, I won't be able to do that until tomorrow.

However, I still cannot explain why the N108 is being controlled with a doubled value.

The morning start-up (even during the summer) is also problematic. It takes about 5 seconds of cranking before it starts. However, once it does start, it runs perfectly smoothly and doesn't produce any smoke.

The mass airflow sensor (MAF) is reporting a value of 510...520..., while the expected value (for this situation) is around 470. I'm initially ruling this out as a source of the problem.
The smoke is generally considered to be harmless.

Out of desperation, I already tried a different MSG that I had on hand. Of course, that wasn't it either. But I still wanted to test it...

Everything feels strange somehow.


Best regards,
A bewildered Wolfgang.

Hi,
To understand how the valve controlling the start of injection is controlled:
- Low percentage values correspond to a high activation level, meaning the valve is largely open, and the EZP (Electro-Zylinder-Positioner) is operating late.
- High percentage values correspond to low activation, the valve is largely closed, and the EZP occurs early.
In your case, the valve will be held more closed at idle (engine warm?), meaning the MSG is trying to adjust the EZP prematurely. Ideally, this should be ensured solely by the mechanical adjustment of the pump, and the modulation should be in the single-digit percentage range (valve closed).
Possible causes:
- The fuel injection pump is significantly delayed.
- Poor signal from the NHG (flanking areas) or received too late;
The dynamic control of the EZP uses the pulse flank from the NHG as a reference signal.
I would first set the ignition timing mechanically (lift of approximately 0.7mm at Top Dead Center), and then check it dynamically (engine warm, don't forget the basic setting mode). Do you have the manufacturer's specifications for MWB 0?
Then, check the injection control system: Start the actuator diagnosis, proceed to the step for adjusting the injection start timing, and from within the actuator diagnosis, go to the display for measuring block 4 (MB 4). Then, during the test, MB 4 will display the minimum and maximum values for the control percentage and injection pressure. Finally, end the actuator diagnosis.
We will then look at the results of the initial setup and the start-up procedure.
What is the internal resistance of the NHG? Are the cables and connector in good condition?
Since the wiring for the fuel injection start valve and the shut-off valve runs through a connector, I don't understand how the engine can continue to run if the connector is disconnected.
hg
Herbert.

Quote:

If the FB (likely referring to a component) fails due to wear and tear, that's certainly not good. Thank you for the tip, Rainer. Which factors play a role in that?

The fuel injection pump is worn out, specifically the pump housing. This can be due to insufficient internal pressure (in a vane pump) or a faulty/leaking N108 valve. This issue eventually arises, depending on the driving conditions and the type of fuel used. I've seen it happen in customer vehicles with as little as 220,000 km (137,000 miles), but it's more common after 250,000 km (155,000 miles). In a previous Audi A3 with the AHF engine, there were no issues related to this at 280,000 km (174,000 miles), but that vehicle mostly used the 5th gear.
If the pump housing is worn out, there's not much you can do besides replacing it. You can only mitigate the effects by adjusting the pump's settings earlier. And if the engine can still last another 100,000 km with that mileage, that's perfectly fine.

Has anything been modified or replaced on the "Out" screw of the fuel injection pump? I'm not entirely sure how it works, but it could affect the internal pressure of the pump.
The injection start mechanism in the VP37 works by using the internal pressure built up by the wobble plate pump to achieve a hydraulic adjustment. In simple terms, the wobble plate of the pump mechanism is rotated back and forth. The N108 controls the pressure, and thus the adjustment.

Sure, please provide the text you would like me to translate from German to English.
I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.
Ich kann den Inhalt dieser URL nicht abrufen, da ich keinen Zugriff auf das Internet habe. Bitte gib mir den Text, den du übersetzt haben möchtest.

You can start by addressing Herbert's points, but I personally would be too lazy to mechanically check them.
My short version would be: If available, check measurement block 4 (injection start control). You might also want to check the basic settings within block 4 to see what it's doing.
First, I would check again with the VCDS TDI timing checker to see if the setting is actually correct (select the correct engine!). Check the timing belt tensions and tensioners. If there are problems with regulation, simply "re-baseline" the pump earlier. You can't really mess this up; just make sure that the injection start is not earlier than 4° KWvOT at idle.


Best regards, Rainer.

Hello Herbert and Rainer,

First of all, thank you very much for your detailed answers.

Okay, in the TDI timing checker, I definitely selected the correct vehicle, including the correct RPM setting. Mine goes up to 1000 RPM, and that's the setting I chose.
The behavior under normal conditions appears perfectly normal here as well.


My car continues to run even when the NHG connector is disconnected, because the ELAB is not powered through the N108 circuit.


The NHG resistor has a resistance of 102 ohms. This value is the same as the one I have on hand.


My cold start problem is separate from my control issue, because I localized the latter yesterday.
I once disassembled the fuel filter (about 6 months old) and it was only half full of diesel, even though I had driven the car just 1-2 hours before. I had a new one on hand, so I installed it. Additionally, I connected the fuel inlet and outlet hoses to a container filled with diesel.
This morning, it started right up, as if it had been waiting for the ignition key.
I have absolutely no idea how half of the fuel could be escaping from the fuel tank (the vent system is working correctly, so there's no vacuum in the tank).
The hose clamps are new (tightening type), the fuel filter is new, and the gasket for it is also new. The fuel lines are new, and generally, there is no visible diesel residue anywhere.
I'll wait until tomorrow morning to see if it starts up. The hoses are now back in their proper places.

Regarding the control of the N108...
I once performed the actuator test and created a LOG file.
I'm going to try uploading it now. However, it's not yet edited. Please don't be disappointed.
To me, this looks normal... Maybe it's the ESP, although it hasn't been installed until February.


Can the internal pressure of the pump be measured while it is installed?



Best regards,
Wolfgang.

EDIT@Autoservice: You can upload log files here in this thread .

Hi,
You seem to be mixing up the injection timing valve (N108), the needle lift adjuster (G80), and the connectors. N108 is the injection timing valve, G80 is the needle lift adjuster, and the connectors for both have nothing to do with each other. The connector for the N108 should also have a connection to the ELAB (Engine Control Module).
The measured data show nothing unusual, except that the signs are missing. The start of injection occurs earlier with increasing percentage values, i.e., before top dead center, and therefore results in a negative value relative to the crankshaft. Is the difference in engine noise audible?
Next, I would record the MWB 4 data in real-world operating conditions (idle, partial load, full load, acceleration) to see how the actual injection timing is regulated compared to the target value, and specifically to check if the regulation system reaches its limits.
hg
Herbert.

Hello Herbert,

Thank you for your support in troubleshooting the issue.

And... I can understand your doubts, but, even if you think I'm completely crazy , let me tell you that the power cable from the ELAB is routed within the same wiring harness as the glow plug cables and the cables for the two temperature sensors on the front left. It has never been changed!
The N108 (injection start valve) has its own dedicated cable, as does the G80 (NHG/NBF).
Both cables are connected to the corresponding connectors via plugs on the connector panel, which is located on the splash guard.
By the way, the cables and connectors are fine.
No, Herbert, I really don't mix those things up.
If there is a need, I would be happy to "prove" it with photos at a quiet moment .

Now, let's get to the point .

Attached is the MWB 4 as a driving record. I'm curious to see what it reveals.
In addition, a Screenshot of the TDI Timing with a cloud and the correct vehicle .
I accidentally did that wrong yesterday... (upload area) Sorry.
Whoever can read has a clear advantage, but it should be quick, and that's why I actually... It won't happen again.


Thank you very much for your effort.


Best regards,
Wolfgang.

Okay, you can stop fiddling around now.

I thought you had a real problem? Well, at least the injection start regulation for the ESP was explained again.

Autschn........

It seems I'm on the wrong track if I rely on the information provided in the label files...

I know what the N108 does... and how a VP37 works.
And and and...
However, if the car is running roughly (compared to another AEL, it's misfiring), and I observe in the MWB4 (measuring block) that the N108 control signal, according to the label file, is far outside the specified range, then I suspect a fault.
After all, we're talking about a deviation of 100%.
(Unless I've completely misunderstood something...)

Please excuse me if, as someone who's not a trained mechanic, I might sound "stupid."
I'm not asking you this because I want to bother you.
It's certainly possible that I'm making a mistake, but I'm not doing it on purpose...

"Could you stop fiddling around" might be reassuring, but it doesn't really help me with this issue.
Something about that kind of response feels like a personal attack.
Again... When I ask a question here, I don't want to burden anyone. I simply get stuck and can't find the answer myself.

I understand that this is not a "training forum" , but I am a very interested "hobby mechanic" with some practical experience. Of course, I am not a professional.
If I'm having trouble understanding something in this case, I'm truly sorry, but I don't want to cause any trouble for you.
You're putting in a lot of effort, and I think that's great. If you look at how rarely I ask questions here, you'll see...

Since I'm not a professional, I'm unfortunately unable to offer any helpful advice here.

Guys... Please, try to understand me.

In this sense...
Perhaps you could still explain to me why I don't have to keep fiddling around with this anymore...
Sure, you can also send it via private message... I won't reveal anyone's secret .

Best regards,
Wolfgang

Hi,

The actual value of the injection start control follows the target value very well, so there is no need for action. The other percentage value is determined by the wear condition and the settings of the injection pump and nozzles.

The other AEL (acoustic emission level) might have a different composition in the tank, different insulation, or simply be located in a different environment. Also, the surface on which a car is parked has a significant impact on the noise level.
- Grass is quiet.
- The concrete slab is making noise.

Best regards, Rainer.

The log looks "okay" while driving, but the values in the last third at idle are inconsistent. The valve is hardly opening at all, as if the injection start needs to be advanced significantly. Why is the control unit doing this? In the actuator test, the low-load injection start is also achieved with the valve almost fully open, i.e., with a high PWM value (a few percent).
The start of the injection process cannot be later than what is set with the basic configuration.

Herbert.

Quote:

but the values in the last third at idle are inconsistent. The valve is hardly opening at all, as if the injection start needs to be advanced significantly

.

Okay. What I've noticed is that after the engine is warmed up, the pulse width steadily increases, as if there isn't enough internal pressure to properly adjust the injection, or it drops after a short time when the engine is idling.