Here is the flow-voltage mapping for the Bosch sensor utilized by my E39 M5.

I find it curious that readings below 0.9 V correspond to negative flows.

BMW E39 M5 MAF map


  • Is it possible that air can flow the other way (maybe through some sort of pressure wave that results from sudden throttle plate closure)?

  • Given that this is a hot-film MAF sensor (essentially a hot-wire anemometer), how does it detect change in flow direction?

  • Why should fuel management care about negative mass air flow? Assuming sudden throttle plate closure results in back flow, I would imagine that the throttle position sensor can be used to detect this scenario as well.

  • Have you considered the effect of reversion? Commented May 30, 2016 at 2:35
  • Reversion is not the norm. I would be surprised if 20-year old fuel management can detect reversion and make adjustments accordingly.
    – Zaid
    Commented May 30, 2016 at 2:42
  • Looking at the chart, the negative flow doesn't make up a large amount of it. To me, that means it won't happen very often. If there is reversion and the ECU can detect for it, this would tell the injectors not to fire. This would hopefully prevent fuel from going place it shouldn't and also prevent the engine from possibly running in reverse. Commented May 30, 2016 at 2:47
  • Reversion is common on turbo cars when snapping the throttle shut under boost as the compressed air must now go somewhere. Even with a BOV the air pressure will equalise eventually, this means it must go back out the intake and will be "reverse" flow.
    – rollsch
    Commented Apr 5, 2018 at 2:15

1 Answer 1


There is no such thing as detectable "reversion" airflow to a MAF. Especially on a non turbocharged car. The MAF has no clue about the direction of flow, only how much energy it takes to keep the wire or film heated.

I doubt that the "negative" aspect of your graph is ever seen. Even at idle, you shoud see 2-5 g/s. I wonder why the graph is kg/h... that seems like ugly units for a MAF that needs to respond instantly (hopefully). Potatos, Apples. Too lazy to convert. My pedantry, not yours.

Simply put, if your car is at idle, it must be using air. Snap closure of a throttle butterfly at speed might cause some backflow, but again a MAF has no idea about the direction. So this is meaningless. Without knowing the BMW E39 Engine Control Unit (ECU) maps, I am certain 0% Throttle Position Sensor (TPS) is a cell (or cells) on the ECU map. Cells which ignore all MAF information, <1.7v or not. Your IDC (injector duty cycle) should go to zero (no fuel injection). No air, no fuel.

The graph implies a scaling, but you should not assume that every point on the graph is obtainable in running conditions.

And everything I just typed is meaningless without tacit understanding of what the ECU does with this information. So to answer "why should fuel management care" I would say it probably doesn't. I would also say (again) that there is no such thing... google how hot film or hot wire works... there is no element of any type MAF (wire/film) that I am familiar with that senses direction of airflow.

And actually, this is the crux of the problem. MAFs are awesome, but limited. In a turbo application, even more so. The ultimate goal is to find the proper Air Fuel Ratio. But even that is wrong... we imagine a utopian sensor that would count free oxygen molecules (O2), not NOx or water, that we can use for go-fast combustion. It should be termed Oxygen/Fuel Ratio.(Even "fuel" isn't right; are we talking 91 or 93 pump, E85 (flexfuel/alchol)?? Gotta count every oxygen and every carbon) On a nice fast ride (actually anything), such a sensor doesn't exist--especially at 300-500 cfm. So whatever MAF technology the Bavarians in Munich decided to use, it's probably top-notch. I don't know where the graph came from, but "negative" airflow areas are not plausible. Show me a below 1.7v reading on your working MAF on your running E39 and I'll eat my hat. Yours too.

Not something to be concerned about.

On edit: Forget every single thing you just read. I am totally, utterly, completely wrong.

Eine elektronische Hybridschaltung wertet diese Messdaten aus und ermöglicht so die genaue Erfassung der durchströmten Luftmenge einschließlich der Strömungsrichtung. ... A hybrid electronic circuit evaluates the measurement data and enables the accurate detection of the air volume, including the direction of flow.

This is from Bosch MAF HFM5

If you look at the schematic you can see that the output signal operational amplifier is biased by multiple temperature-sensing resistors, in different places on the film. So it knows which part of the film cools first, in a way. It can sense the direction of slightly reversed airflow.

I've got hat sauce to pick out. As far as the "why", I can only speculate (gee, great track record so far Steve) that this MAF behavior might be useful in a turbocharged application. After eating much crow, I still stand by my assertion that there will be little to no reversion airflow on a Normally Aspirated engine.

On edit edit: A better [speculation] might be now you have capability for a closed loop electronic Idle Air Controller, able to deal with a wide range of operating conditions, yielding potentially lower idle emissions, even on an Open Loop cold start--which has traditionally been a very difficult task to do well.

  • My questions are more curiosity than concern. It would be so easy for Bosch to blank out those fields as zeros, yet they chose to assign negative values. Also, 0-5 V is the usable range for these sensors, so I find it puzzling that they should go through the trouble to assign negative values to the sensor's mapping. The curve in the question is what BMW programmed into the M5 ECU, so there's clearly a use for those numbers.
    – Zaid
    Commented May 30, 2016 at 12:12
  • 1
    Awesome answer... except for the use of mystery abbreviations. Okay to use them, but best practice is to ALWAYS define them when first used. ECU, MAF, E85, IDC, AFR. Anything else is just poor communication.
    – zipzit
    Commented May 30, 2016 at 15:52
  • Now I'm curious as well. Makes no sense. Why waste precious MAF sensor sweep range on a fairytale condition? My only guess is that is a base map, and additional information will drive those numbers positive. For instance (just guessing), maybe a -40 to +250F IAT is buried in the MAF as well, and your graph depicts "corrected" MAF values at a ludicrous 250F air temperature. More reasonable and realistic temperatures add to the cell values on your map as cooler air predicts denser charge. I dunno. Those numbers MUST be corrected somehow. Any airflow is "positive" airflow to a MAF.
    – SteveRacer
    Commented May 30, 2016 at 16:06
  • 1
    The HFM5 datasheet was what I wanted to share. Good job with the edit. You're fortunate that I'm not into hats, though I'm wondering if you should make one out of rice paper to keep your word :D
    – Zaid
    Commented May 30, 2016 at 18:00
  • 1
    So I'm gonna call in some favors. Despite my rusty German. I worked for both BMW and Bosch. Now this is bucket-list for me. There must be a good explanation. And I must know. Why does my breath smell of hats??
    – SteveRacer
    Commented Jun 1, 2016 at 5:20

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