Air mass too high A6 4F 3.0 TDI ASB

Hello everyone,

I have an experience that I can't explain.

First, regarding the car: An A6 4F Allroad 3.0 TDI, manufactured in 2008 with ASB (233 hp). 208,000 km. Chain tensioner and intake valves with control motors have been replaced as part of the recall. LMM (lambda sensor) replaced at 180,000 km. Injectors have been in place since 2011 and are well within tolerance.

Okay, now let's get to the current problem. In the spring, at 192,000 km, the pressure sensor from the AGR valve was defective. Since I still had one from a 2005 2.7 TDI BPP lying around, I installed it quickly. The pressure sensor, valve, and mechanism are identical, only the inner diameter of the cast part at the connection to the flexible pipe flange is slightly smaller (10%). I thought it couldn't hurt if a little less exhaust gas is returned to the engine.


Now, since the installation, the consumption has noticeably increased. In my search for the cause, I have checked all the usual suspects, including the valve components, and inspected all the valves in the intake path for oil mist and followed up on any findings.

In inland waterway navigation, two things are noticeable:
1.) The desired and actual loading pressure always match, in every operating condition. Besides the normal short overshoots after load changes, the values are as expected when connected (as I have experienced with no V6 TDI so far),
2.) The air mass values are not only 10% higher than expected, but in the lower load range, they are almost always TWICE as high as the target value. Even with higher loads, the actual value is significantly above the target value, but the difference decreases as the load increases. Only at idle (and up to approximately 1200 RPM) do the values completely equalize.

I replaced the magnet valve and pressure converter with one from another V6 TDI engine from the same engine family about 1500 km ago. No changes.

Previously, I thought this wouldn't be a problem; actually, more oxygen is always good for performance and fuel consumption. But could it be that some kind of interference is actually happening that's affecting me? Or is it just a coincidence, and I have to keep looking for other causes?

Greetings

Bo

Hello Bo,

If there is a leak in the intake path, you will see excessively high air mass values at higher boost pressures (for example, the intake hose is being forced inwards due to damage from rodents).
Something unusual would be the effect, as you said, in the medium load range. Do you have a log with air mass (Is), injection amount, engine speed, and target/actual boost pressure?

Best regards, Rainer

Here is the log and the view. Unfortunately, there wasn't much free space earlier. Hoping this helps nonetheless.


Except for a freak incident (briefly activated cruise control at 210 mph), everything is behaving as it has in the last few months.

Bild had not taken it...

[EDIT] Somehow, I can't seem to upload an image! [/EDIT]

Ste.Bo wrote:

He didn't take the picture... [EDIT] Somehow, I can't seem to upload an image! [/EDIT]

Below, under Posting/Edit [Select File +], then select and upload the file. Just tested. What is the image format?

Hi,

It actually looks quite good, except for the fact that the "as-is" value for the air mass in idle is too high. Specifically, it's exactly as high as if the AGR valve were simply too small. It's either broken or not being operated correctly.

Quote:

The AGR valve pressure sensor is defective.

Does it still have a pressure canister? My 2007 BRE already has an electric one. Why don't you take a photo of it?

"The engine control unit's fault memory is empty and the engine control unit software is original?"

Best regards, Rainer

dieselschrauber wrote:

Hi, It actually looks quite good, except for the fact that the "as-is" value for the air mass in idle is too high. Specifically, it's exactly as high as if the AGR valve were simply too small. It's either broken or not being operated correctly. Quote: The AGR valve pressure sensor is defective. Does it still have a pressure canister? My 2007 BRE already has an electric one. Why don't you take a photo of it? "The engine control unit's fault memory is empty and the engine control unit software is original?" Best regards, Rainer

In idle mode, at 777 RPM, it almost always comes down to 270/hub (see: (After the "red light" phase, approximately 260 seconds), but even slightly beyond that, things start to fall apart.

[EDIT] Ah, yes, error memory cleared, original software! [/EDIT]

Here are the images of the printer cartridge.

Thank you and best regards.

Bo

dieselschrauber wrote:

He didn't take the picture... Ste.Bo wrote: [EDIT] Somehow, I can't seem to upload an image! [/EDIT] Below, under Posting/Edit [Select File +], then select and upload the file. Just tested. What is the image format?

I've tried it multiple times. BMP is not what he wants.

Hello Bo,

The AGR valve is actually pneumatically operated on your ASB, while the swirl flap control seems to be electrically operated.
With the latter, a possible cause for your AGR differences is eliminated.

The pneumatic AGR valve operates with vacuum. No negative pressure: AGR is set to the maximum fresh air ratio for the specific operating condition, and the Lustmasse is at its maximum value.

Now, if the AGR is struggling to function and reduce emissions, the necessary vacuum at the AGR inlet will not be achieved. Or, there is a blockage in the AGR line from the turbocharger/exhaust manifold to the intake tract, and despite the open AGR valve, not enough exhaust gases cannot be returned.

Quote:

Since I still had a 2005 2.7 TDI BPP lying around, I installed it quickly. The pressure vessel, lid, and mechanism are identical, except for the inner diameter of the casting part at the connection to the flexible pipe flange, which is slightly smaller (10%).

"Maybe the "new" vacuum chamber has a different diameter and a different spring stiffness." Even a flow that is 10% smaller (approximately 21% less area) can be a cause for the insufficient removal of exhaust gases.

However, I doubt that the significantly increased consumption is due to what you say, unless you drive very frequently under partial load and the engine never gets fully warm. Then it could certainly have that effect.
I have no idea where and how you're driving, the density of speed cameras on your side of the lake is so extreme, and the roads are so heavily speed-restricted... perhaps the poor 3.0 TDI V6 can never really get its full power. What is your driving profile?

"Damage from marten bites in the intake manifold tubes also results in excessively high air mass values, because the air is first sucked in through the intake manifold and then escapes again before entering the intake tract."
However, the relatively normal air mass values at full throttle do not match.

Best regards, Rainer

Hello Rainer,

The AGR valve, throttle body, and intake manifold with stepped valves were cleaned when I replaced the AGR valve. The deposits were not significant.

dieselschrauber wrote:

... perhaps the poor 3.0 TDI V6 engine can never really reach its full potential. What is your driving profile?

The car is my wife's daily driver. Every morning, she drives 12km on a federal highway at around 80km/h, then 25km on the A7 at 160-180km/h, and returns in the afternoon. This is probably also the reason why the DPF only regenerates every 1300-1500km, even though there are also some short trips as a family car. Okay, that's the right driving profile.

I've already considered all the possibilities. Somehow, there's always one thing that negates a diagnosis. I haven't been as concerned about potential leaks in the vacuum system yet, because the control element diagnosis seems to be running smoothly. Need to take a closer look.

Nevertheless, I still don't know if the excessively low AGR rate can actually lead to increased consumption through any mechanisms, or whether it was simply a matter of timing with the AGR valve.

Thanks for your input and best regards.

Bo

Does the car rust when fully loaded? Sometimes, you can see quite well at night in the rearview mirror when someone is driving behind you.

lampenmann wrote:

Does the car rust when fully loaded? Sometimes, you can see quite well at night in the rearview mirror when someone is driving behind you.

I understand what you mean, but he doesn't actually.

I would suggest testing the AGR (Air-Gas Ratio) under pressure sensor and bellows with a large syringe to check for leaks.

Possibly, detach and reattach the hose multiple times, as the leakage can also depend on the position.

Please also make sure to test the rest of the under-pressure system. If a vacuum is lost elsewhere, it may no longer be sufficient for operating the AGR valve.

Best regards, Rainer

I have some free time on Thursday, so I'll go through everything then.

Greetings

Bo

Okay, on Thursday, I completely checked the entire vacuum system using a large syringe (also while the engine was running and with actuator diagnosis). Result: everything is sealed and functioning properly.

Found and fixed a small leak between the AGR valve and the AGR cooler flap housing, indicated by a slight exhaust odor.

Thought it was over, but no...

Today, on the drive from Lucerne to Ulm, there was still a lot of air resistance, and the fuel consumption was 11 liters instead of 7.5 liters at 120 km/h with the cruise control in Switzerland. In high-load areas and below 1300 RPM on country roads, the consumption is normal.

Loading pressure always good, rail pressure good, DPF differential pressure at 200km/h in 6th gear at 183mbar.

"I can't find the 'guilty party', except that the injection seems to be happening a bit too late." Since it's a consequence and not a cause, I'm really not making any progress.

Does anyone have any ideas about what else could be logged?

Greetings

Bo

Hi,

Quote:

In addition to the fact that the injection timing is quite late. Since this is a consequence

How do you come up with it?

The injection timing is essentially dependent on the coolant temperature. Check the in the engine control unit's measurement values for plausibility.
Would also explain the increased consumption.

Best regards, Rainer