Fluctuating LMM Values: Causes & Solutions

Hello,

After having struggled with the search function for quite a while, I'm asking again! I'm having a problem where the maximum speed is not being reached. Sometimes it's a maximum of 180 km/h, and recently I managed to reach 205 km/h with a lot of effort. Oh, and the engine is an AFN with 198,000 km.

I logged the values using VAG-Com.
At idle, the air mass was 430mg/stroke, which I think is okay!
At 3500 rpm, the desired value is 850, but the actual value is 525.

Daraufhin habe ich alle Schläuche gecheckt! As far as I'm concerned, everything's fine!
LMM replaced with a new part - slightly improved power output below 3000 RPM.
Today, I cleaned the EGR valve! There weren't any major deposits in the intake pipes. The valve itself was completely carbonized and wouldn't move. After a thorough cleaning, its function was restored!

Currently, the air mass at idle is 345 mg/hub, and above 3000 RPM, the value drops to 505 mg?! The value fluctuates quite a bit now, meaning it goes up to 890 and then drops again. Initially, it increases with the accelerator pedal pressure. However, at a constant RPM, it stabilizes around 505-525 mg! Nevertheless, the acceleration feels noticeably better! I haven't been able to test the top speed yet!

Does anyone still have any ideas about what might be causing this?

'The catalytic converter is okay, and according to VAG diagnostics, the boost pressure is within the normal range. The variable turbine geometry (VTG) is operating smoothly.'
The car drives approximately 2000 km per month on the highway, usually at speeds of around 120-130 km/h. It rarely uses full throttle continuously.

"Depending on how much you press the accelerator..." what are you actually measuring then?
In the partial load range, you can't learn much about the mass flow rate (MFR) based on the values.

Do you have full-throttle RPM values ready, specifically in the range of 2000-4000 RPM?

What do they look like?

Maybe you also have a loose connection at a plug.

I saw something similar recently on a colleague's ASV (Annual Status Verification), where the values even dropped to zero.

m;

Well,
Okay, so if I, let's say, give it full throttle at 3000 rpm, both the target and the actual mass airflow values increase steadily! In other words, the target and actual values are almost always the same.

However, if you maintain a constant RPM, for example 3250 or 3750 rpm, the value gradually decreases until it stabilizes at 505-525 mg/hub! The same applies to 4000 rpm.

If I release the accelerator and then press it down again, the same thing happens!

Hello Boror!

Quote:

However, if you maintain a constant RPM, for example 3250 or 3750 rpm, the value gradually decreases and stabilizes at 505 - 525 mg/hub! The same applies at 4000 rpm

.

If you maintain a constant speed, I assume you are not using full throttle; instead, you are reducing the load to the point where a steady state is established between the rolling resistance and the engine torque.
It's perfectly logical that your mass airflow (MAF) values are decreasing, as the boost pressure is also being reduced. The target values for boost pressure and MAF should also decrease proportionally.

You can best test your LMM (likely referring to a Mass Air Flow sensor) on a long incline. Start with full throttle in, for example, 3rd gear, just above idle speed, and hold the accelerator pedal in the same position while revving the engine up to approximately 4500 RPM. During this process, you should log the values.

Regards,
Alex.

Okay, Alex put it better.

That's what I meant when I asked about "full-throttle values between 2000-4000 RPM."

The whole thing will look something like this:



"In my car, the air mass always starts to drop around 3000 RPM, but the fuel injection volume is not yet being reduced."


m;

Yes, that sounds perfectly reasonable! I'll try to log that next time!


But why can't I reach the top speed anymore?! I could easily drive 210-215 km/h according to the speedometer before!

Thank you for the information.

But why can't I reach the top speed anymore?! I could easily drive 210-215 km/h according to the speedometer before?!

Without more detailed logs, we can only guess.

Crystal ball mode on:
- The EGR system is so difficult to operate that the engine consumes its own exhaust gases even at full throttle, preventing it from drawing in fresh air. Consequently, the mass airflow sensor (MAF) reading is too low.
- or the mass airflow sensor (MAF) is faulty, reporting such a low air mass at full throttle (over 3,000 RPM) that the engine control unit (ECU) significantly reduces the fuel injection to prevent the engine from running too rich. And without a high fuel injection rate, you won't get the full power, and therefore no maximum speed...
- Or not enough boost pressure, resulting in insufficient airflow, which then leads to fuel injection reduction.
/crystal ball mode off

Conclusion: Check out the technical articles; there's one on systematic troubleshooting using VAG-COM, and others on topics like the EGR valve, the mass airflow sensor, and so on.

If anything is still unclear, please ask and include the relevant log (ideally as an Excel chart) so we can help you more easily.


Regards,
Private.