Keep on reading if you have access to an OBD-II reader and a fuel-injected vehicle

As mentioned in this meta post, I'm on a mission to collect data for mass air flow values at idle. With sufficient data, it should be possible to post an informed, objective response to the volumetric efficiency rule-of-thumb question.

It should take no longer than 15 minutes to obtain the readings, fill out the questionnaire and send it over. All results will be aggregated on Google Forms.

Here is the questionnaire (Google Forms)

Here is the chat room dedicated to this endeavour


  1. Mechanics.SE Username (self-explanatory, optional)
  2. Vehicle Make, Model, Year (comma-separated values are fine)
  3. Engine Type (gasoline or diesel, naturally aspirated or forced induction)
  4. Engine Size (how big is the engine, in liters?)
  5. What was the OBD-II readout for idle RPM with vehicle stationary?
  6. Was the air-conditioning on?
  7. Do you know if the engine use a mass air flow (MAF) sensor or manifold absolute pressure (MAP) sensor to provide the OBD-II readout?
  8. What was the OBD-II readout for mass air flow?
  9. What were the units for the mass air flow OBD-II readout?
  10. How old is/are the MAF/MAP sensor(s)?

Don't forget to hit Submit and thank you for your time!

All information provided will be made public at the end of this survey.

Additional notes/clarifications

  • Please take the readings with the engine at operating temperature. If you are unable to do so for whatever reason, please mention it in the "Other comments/remarks" field
  • It is OK to post the expected values from an official service manual in lieu of logging data via OBD-II and the value(s) are in units of mass / time
  • a generic Bluetooth/Wi-Fi OBD-II reader should be sufficient for logging the requested information
  • This questionnaire is applicable for both gasoline and diesel engine vehicles
  • It is not mandatory to take RPM/MAF readings with the A/C both on and off, but if you do that counts for two datum points
  • Exact engine displacement is not critical, advertised engine displacement ought to be sufficient (4.991 L vs 5.0 L)
  • Is it good enough to give the expected values from the work shop manual? the WSM for my 98 mazda 626 2L has expected MAF values at idle both with and without A/C on. The wsm for my 99 Nissan Almera 1.6L might have also. Commented Dec 20, 2016 at 20:17
  • @RobertS.Barnes that should be fine as long as it is in the official workshop manual and the values are in mass flow per unit time
    – Zaid
    Commented Dec 20, 2016 at 20:19
  • At least for the Mazda the units are grams per second. Not sure about the Nissan, I'll have to check. Commented Dec 20, 2016 at 20:21
  • Very good, sir!
    – Jason C
    Commented Dec 20, 2016 at 20:38
  • @ZachMierzejewski yes, a generic Bluetooth based OBD-II reader will give you back this information
    – Zaid
    Commented Dec 20, 2016 at 20:39
  • @Zaid I deleted my comment because I found the answer implied in your question. But for posterity, it was: Can I find 8. ... OBD-II readout for mass air flow ... with a cheap, generic OBD-II scanner? Commented Dec 20, 2016 at 20:42
  • Are you interested in diesel values?
    – dlu
    Commented Dec 21, 2016 at 0:22
  • I will see if I can contribute with three cars this coming weekend ... as long as my reader will cooperate with the right values. Also, would you want stated displacement, or actual displacement. Remember, there was a question a few weeks ago which garnered a lot of attention for this ... A Ford 5.0L engine doesn't exactly displace 5.0L, but more like 4942 cc's. You may want to clarify this in your question. Commented Dec 21, 2016 at 0:50
  • @dlu absolutely interested in diesels! They're definitely in scope
    – Zaid
    Commented Dec 21, 2016 at 4:05
  • Say, I don't know if it is too late to change the form, but it might be worth adding fields for the method used to read the data. For example I would report something like "VCDS-Mobile N-M" (or however it is that they specify the group and item). Also, I don't have any idea how old my MAF sensor is (other than maybe 11 years), how do you want that reported?
    – dlu
    Commented Dec 21, 2016 at 4:11
  • Do you want us to get both A/C on and A/C off readings?
    – dlu
    Commented Dec 21, 2016 at 4:11
  • @dlu what you use to take the reading isn't really that critical in my opinion, but you mention it in the "Other Comments" section. Regarding MAF age, you can specify the "Never changed in my ownership" option
    – Zaid
    Commented Dec 21, 2016 at 4:41
  • @dlu if you can readings with both A/C on and off that would be great since it would give us two datums, but I don't want to impose it on others
    – Zaid
    Commented Dec 21, 2016 at 4:42
  • Easy enough to do, the hard part is getting myself to dig out the scanner and go out in the cold and dark :-) Do you care if the engine is up to operating temperature or not?
    – dlu
    Commented Dec 21, 2016 at 4:45
  • 1
    Block heaters on and engine is warming up, looking for the scan tool in my newly organized shop…
    – dlu
    Commented Dec 21, 2016 at 5:04

2 Answers 2


I think the same caveats exist that existed months ago.

I feel Intake air temperature is vital for this experiment. A so-called "MAF" really isn't. It's a guessmatic predicticator prognostitron (tm) of flow based on cooling of a hot wire, or hot film (better). This "voltage" or "frequency" rise may be non-linear, and further scaled by the ECU based on actual factory testing with all relevant plumbing in place.

I suggest you are making a number of assumptions about air temperature, humidity, density, as well as an assumed stoich at idle [maybe?] but I wonder if this is still true on GDI IC. I think there is a component of combustion that ultimately affects VE, albiet not directly. "Load" comes to mind.

Another issue is the associated plumbing before the MAF; it's a well-known caveat of installing aftermarket Cold Air Intake systems, which change the factory tested flow, and also change the response in the very narrow region of the hot wire or film, and can cause a vehicle to run lean and/or with high fuel trims.

For useful apples-apples comparisons, I think the only VE data that's useful is under signifigant load. Otherwise, who cares about VE at idle?

Besides the GDI issue, you might also want to restrict this to Normally Aspirated vehicles. VE goes out the window with any sort of supercharging. (In a good way, I admit.) There won't be any boost at idle, at least on turbosupercharged vehicles, including diesels.

There are simply too many variables, including ECU software implementations, calibrated intake plumbing, variable intake devices, and AFR strategies.

I don't want to discourage a noble effort, but I'm just not seeing the point. There's no way you can compare the VE of a Subaru turbo 2.5 EJ205 to a Honda VTEC 1.7 at Idle and have data that is useful. BOTH engines have characteristics (turbo, VTEC) that change VE based on rpm and/or load.

The only useful data would be an additional plot on a dyno chart showing MAF values vs. RPM, along with the intake air temp, density, humidty, and other appropos corrections.

Still, I'll run my 1996 and newer vehicles and see if they correlate to any real meaning. Most of my fleet is pre-OBD2, which is a shame because I'd love to know the VE baseline of my '72 BMW 2002tii M10 racing engine.

  • Thanks for supporting this initiative. I thought long and hard about whether to include IAT in the survey but decided against it for a few reasons
    – Zaid
    Commented Dec 22, 2016 at 12:54
  • Not saying that I'm 100% sure I don't need it, but I didn't want to burden the survey participants with Yet Another OBD2 Parameter™
    – Zaid
    Commented Dec 22, 2016 at 13:02

Here is a sample response for one of my cars

  1. Zaid
  2. BMW, M5, 1999
  3. Gasoline, naturally aspirated
  4. 5.0
  5. 613 RPM
  6. No
  7. MAF
  8. 19.00
  9. kg/h
  10. Less than 2 years old

Based on this screenshot

Sample OBD-II readout

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