LMM and Diodes: A Comprehensive Guide

Hello fellow enthusiasts,

Before I have to replace my AKN (V6) air mass sensor (MAF sensor) again and buy a new one (this is the third one), I tried the diode trick. So far, the results have been excellent, except for the RPM range around 1600 RPM. Within that range, it smokes like a Russian tank if you accelerate too quickly...
I installed a 1N41 diode. Currently, it's handling a continuous current of 1000mA per channel.

Is there a (feasible) way to make this diode trick speed-dependent?

If not, could you recommend a reliable boost pressure increase solution?


Thank you, René.

@rene_s: Are you basically talking about a voltage regulator that depends on the speed?

@all: Since I don't want to start a new thread, and it's relevant to the topic of LMM, what other ideas do you have when the LMM value is too high? (Is this even possible? Is it possible for an old LMM value to be too high?) The intake air path is sealed. (I was also thinking about a faulty boost pressure sensor).
The workshop definitely wants to replace the mass airflow sensor (MAF sensor) because of this.

(Oh yes, the value comes from a full-load measurement in the workshop at 3000 RPM - that's why the EGR value is low.)

1N4148... The smoke is likely coming from the EGR valve, which is desperately trying to reduce the air mass by mixing exhaust gases.
The diode is only for testing purposes and is not intended for continuous use.

Best regards, Rainer.

Hello Rainer,

...I forgot to mention, there's no AGR (Abgasrückführung). Is there really no alternative to either a diode or replacing the mass airflow sensor (MAF sensor) every 10th time you fill up the tank?

I can see why it's smoking.
The engine control unit (ECU) detects a large amount of air and releases the maximum fuel injection value allowed by the fuel map.
Because there isn't much air, there is smoke.

Increasing the LL (lambda) won't help much because the turbo simply can't generate enough pressure in that range (I've tested it myself).
'There's not much power gain to be had up to 1600 RPM (at least with the AFN engine!).'
Best regards,

Was my question intentionally overlooked, or does no one know anything about it?

Devilseye503 wrote:

Was my question intentionally ignored, or does no one know anything about it?

I suspect the latter. A high LMM (mass airflow measurement) value is usually only indicative of leaks in the charge air system, or of a charge pressure that is set too high (most likely in the mid-RPM range).

Or with any kind of exotic electronic problems... or if the airflow sensor (MAF) characteristic doesn't match the engine control unit (ECU), for example, if a MAF sensor designed for a less powerful engine is installed.

1n4148... the smoke is likely coming from the EGR valve, which is desperately trying to reduce the air mass by mixing exhaust gases.
The diode is only for testing purposes and is not intended for continuous use.

Greetings, Rainer
Hello,

'The soot still forms even when the EGR system is deactivated. If you look at the soot and torque limitations, which typically apply up to around 1500 RPM, you can see why soot formation is inevitable – because, with the diode engaged, the injection quantity exceeds the soot limit.'
The same applies when switching from partial to full load, because immediately after pressing the accelerator pedal, the maximum amount of fuel is injected, even though the cylinder filling is initially quite low due to the lack of built-up boost pressure.
The most soot is produced in my AVF (Abgas-Ventil-Freisetzung - exhaust valve release) system when I give it a sudden burst of gas at 1800 RPM. From this point on, the amount of fuel injected, which results from the torque limitation, is approximately 50 mg/stroke. At around 1500 RPM, it's only about 40 mg/stroke.

Consequently, the engine's performance at low RPM is better in my case. The lack of power when starting from a standstill is no longer as pronounced. The engine's responsiveness is almost on par with my old AZJ gasoline engine.

I consider the diode to be a perfectly viable long-term solution.

The problem is only with the error entries. The error entry 'implausible MAF sensor signal' is not a problem. However, this error entry usually appears after driving for a longer period. Occasionally, there's also an error indicating that the EGR (Exhaust Gas Recirculation) system is not working (which makes sense). This entry causes the warning light to illuminate in the instrument cluster, and I get the impression that this error doesn't occur if the MAF sensor error is already present in the error memory. I can provide more details later.

I am definitely interested in hearing about other people's experiences.

Best regards, Matthias.

Contrary to my previous assumption, the AGR error code also appears even when a mass airflow sensor (MAF) error is present in the memory. Prolonged operation with reduced power output, especially at highway speeds, seems to trigger this error. I'll see if I can narrow down the cause later.

Do you have any alternative ideas to using a diode? I really want to keep the significantly better response time and the reduced inrush current weakness that the diode provides.
The problem, of course, is the consistently high Mass Air Flow (MAF) sensor reading. Ideally, we would see a MAF value that plausibly changes during operation, but that is generally at a higher level than what is observed when the MAF sensor is being tested directly.

Best regards, Matthias.

Hello everyone,

'I've been driving for a while now with the AGR (exhaust gas recirculation) system deactivated, and it's also been coded to the lowest possible rate using VAG-COM, along with a diode. Especially on the highway, I frequently experienced the phenomenon where the car wouldn't accelerate properly. It felt like the mass airflow sensor (MAF) was disconnected, or like it was running with the emergency MAF value, which significantly reduces the fuel injection amount.'
I initially suspected a bad connection with the diode, which I can now rule out. The problem occurs after just a few seconds at a constant speed of 2300 rpm. When accelerating, almost nothing happens, and when reaching 2400 rpm, the full injection amount is present again. The problem doesn't seem to appear when driving at a constant speed of 2200 rpm.
Do you have any ideas about this? As soon as I have more time, I will try to see if I can gain more insights by analyzing the data blocks.
It seems that the only solutions are either adjusting the fuel map or improving the airflow to the mass airflow sensor (although removing the grid is supposedly bad for its lifespan).

A knowledgeable forum member informed me that he had a similar problem with an A4 equipped with a 96kW AWX engine. He suspects that the voltage level at the MSG is too close to the threshold that is interpreted as a short circuit.

'Initially, I only experienced this problem sporadically. Now, with a different diode, the problem occurs within seconds at 2300 min^-1. Therefore, the possibility of excessive voltage at the MSG seems quite plausible.'


Can you contribute to this?


Best regards, Matthias.

Update:

In a diode that causes a voltage drop of 0.48V across the LMM (likely referring to a mass flow meter), the power output decreases after only a few seconds, as described above, when operating at 2300 min^-1.
In another case, with a voltage drop of 0.56V, it only occurs very rarely.
So, I proceeded to check the maximum mass flow rate occurring at the LMM (i.e., kg/h). This occurs at a speed of slightly above 4000 RPM.
I've now entered the characteristic curve of the mass flow controller with the mass flow rate and determined that a voltage drop of 0.8V is sufficient. In my case, this corresponds to approximately 900 mg/hour at 4000 RPM. Let's see if there will be a drop in performance again.

Sure, here is the translation of the text from German to English:

'Another question:'
Has anyone ever tried reducing the heater voltage of the mass airflow sensor (MAF)? This could allow for higher MAF values across the entire RPM range. Unlike the diode method, where a constant mass airflow is set, the air mass per cylinder decreases with increasing RPM.