LMM Filter: What is it and what does it do?

Hello everyone.

After my Mass Air Flow sensor (MAF) from Bosch, which was already quite old, started showing noticeable signs of malfunction on my AJM engine, I installed a new one today. It's a Pierburg unit with the part number...
722684.08.0. During my inspection, I noticed that there are two grids on the air filter housing - one made of plastic and one made of metal with very fine mesh. What is the purpose of these grids? Shouldn't foreign objects already be excluded by the filter itself?

I'm also interested in that.

The old Pierburg mass airflow sensor for the AFN engine either had none or a very coarse filter, and it lasted over 245,000 km (oh, that nasty Lu textile filter).

Bosch products also seem to have issues with grilles.

My new mass flow meter (ARL) deflects the airflow by approximately 45° using a plastic grid with very small openings (less than 1 mm²/opening). This is achieved through long, thick grid channels that are angled at 45°, effectively obscuring the view of the actual sensor. However, air can flow freely around the actual sensor tube within the mass flow meter housing!

This may have both advantages and disadvantages . (e.g., unfavorable inflow, or conversely, more favorable inflow/vorticity, or perhaps a reduction in the measured airflow to achieve more accurate measurements?).

I once placed a piece of the filter material from a range hood/air conditioner (that fiberglass stuff) in front of the mass airflow sensor (MAF sensor), and then reattached it to the housing. The goal was to measure, over a defined short period, whether the coarse textile filter would allow anything through. However, the car stopped working, likely due to very poor MAF sensor readings.


The only thing a paper filter lets through, as far as I can tell, is diesel soot from other smelly vehicles.

That's probably not the cause.

Why don't you try removing the grill and see if it becomes more responsive or if it starts smoking?
(At least in my case, the plastic grid is firmly embedded; otherwise, it would have already come loose.)

Foreign objects are, in fact, excluded by the filter.

Offtopic = ON

'On our 3.0 DCI Interstar, a gasket failed due to a manufacturing error near the mass airflow sensor (MAF). This allowed debris to bypass the turbocharger and end up in the intercooler. A mesh filter would have been helpful in this situation.'

We were still wondering why the turbo was making such a whistling sound .

Offtopic = OFF

Thank you for the responses. While browsing online, I found a BMW forum where users were discussing these grilles. In the photos posted, the grilles appeared almost as dense as curtains. Some users had temporarily removed the grilles in certain areas and reported improved throttle response and acceleration. I can definitely see how that would be the case with such tightly spaced elements.

If 60 liters per second are flowing through, then a grid like that will definitely cause some obstruction.
I think that in my case (with the fine mesh wire netting on both sides), the netting mainly serves to protect the delicate filament from transport and installation damage.
However, the above-mentioned points contradict this (one side is plastic, the other side is wire mesh... what's the purpose?).

A coarser mesh would have been sufficient in that case...
I'm going to throw mine away too, let's see what happens .

Please don't misunderstand. The order is as follows:

Air filter housing, plastic grid, metal grid, MAF sensor (Mass Air Flow sensor), no grid.

Ah, I see, I have a very fine grid on both sides...

It's understandable in your case if something were to come out of the air filter housing (sometimes a pine needle might get stuck on top when you change it yourself, but if someone else does it...).

Perhaps the whole thing also has a background related to fluid dynamics?

"So, the air mass sensors with the fine mesh inside the coarser mesh (on the air filter side) have better dirt and water resistance because the particles are directed around the sensor membrane."
This also allows a smaller amount of air to flow through the sensor, which means it must be more sensitive in order to achieve its normal operating characteristics.

If the protective grille is removed, too much air mass will be measured, which bypasses the turbidity limit -> resulting in potentially better airflow and a shorter lifespan for the mass flow meter.

If the protective grille is removed, then too much air mass is measured, which bypasses the turbidity limit -> resulting in potentially better airflow and an earlier failure of the mass flow meter.



...so get rid of that thing .

Since I have a uniform grid, the actual airflow is being measured.

Okay, so post your findings when that thing comes out.

Hey, so post your findings when that thing comes out.


He won't react any faster because, according to VAG data, my mass airflow sensor is already providing far too much air. Whether it's actually showing 860 instead of 840 when it should be 400 (hypothetically), that's just a matter of perspective.

The question is whether the turbocharger is noticeably relieved. Given the amount of air that's passing through, it probably is, but on the other hand, there's an air filter that tends to restrict the airflow.

It will hardly be noticeable that the grid has been removed.

Hello.

I couldn't resist, and I did it .

The grid is gone!

And lo and behold:
Previously, the acceleration during spontaneous 'kickdown' has gradually and smoothly increased, but now it occurs with a noticeably more forceful jolt (though not unpleasant), happening very spontaneously.
(Like the old AFN, when I would come off an Autobahn heating stage and then find myself in city traffic, it would be more responsive, with a noticeable jerk as it got going.)

I haven't yet been able to determine whether it now behaves like a CDI taxi in Munich, accelerating sharply and reaching full power with a prominent Russian flag displayed.

Overall, the performance has likely improved marginally, but the improvement is barely noticeable or measurable.

But it was worth it after all...

Let's see how long it lasts... I'll let you know.

'Hmm, that doesn't sound bad. I just wonder how to get this grill out. I removed it from my AQY (2.0, 115 hp) before, but a piece of the pipe where the grills are located broke off (at the edge). Is there a trick to it? Simply prying it out with a screwdriver won't work.'

Oh raw power, you're already there.

Before using the lathe, I was too apprehensive because of the dust.

I simply pushed all the edge stitches through with the awl and then removed the central piece. I then cleaned up the edges to avoid any loose plastic parts.

Is this the usual wooden hammer procedure, which is what my cars fear...?

but it reacts so beautifully and spontaneously to throttle inputs! And until the turbocharger kicks in, I can also wait, thanks to the lack of an EGR system.

I'm starting to like it, but now I just need to eliminate the sluggishness at over 3500 RPM -> next step: ULF (presumably referring to an upgrade or modification).
and then it's good... (but I wouldn't dare to do the latter myself).

So, the mass airflow sensors with the small, fine grid inside the coarser grid (on the air filter side) have better dirt and water resistance: the fluid is channeled past the sensor membrane.
This also allows a smaller amount of air to flow through the sensor, which means it must be more sensitive in order to achieve its normal operating characteristics.

If you remove the protective grille, then too much air mass is measured, which bypasses the turbidity limit -> resulting in potentially better airflow and an earlier failure of the LMM (Mass Air Flow sensor). This is precisely the main reason for the grilles, and that's why I would leave them as they are or stock up on some spare LMMs. Another reason is that the airflow is then directed 'definedly' past the LMM; in this case, a certain deterioration can be expected, but it will certainly remain within an imperceptible range.

Conclusion: You can remove the grilles, but the disadvantages that come with it are significant, so I would advise against it. There are more effective measures that offer greater benefits and don't cost much either.