While replacing my air filter I was looking at the old one and wondering, "I can see this has debris on it, but how much was that affecting airflow to the engine?"

Obviously the more restrictive the air filter, the less air can get to the engine, and hence the less peak horsepower can be produced.

Then I thought, "Isn't a modern fuel-injection engine sort of a large computer for measuring how much air (or oxygen) mass is getting to the cylinders?"

Now I'm wondering:

  1. Is it true in principle that the ECU knows how much air is getting through the filter?

  2. If so, does it log that data anywhere that can be accessed – e.g., are there OBD codes that would provide that, or from which that can be deduced?

  3. If so, can I deduce how clogged my filter is with a single full-throttle run to peak RPMs, and compare the airflow data with those I get doing the same run with a new filter (at the same density altitude)?

  • This is a great question and I bet Zaid could answer it. He did the Great MAF Experiment a while back, which was a lot of fun. I'm thinking with a baseline reading from your ECU in grams/second with a clean air filter and percentage of throttle, you could figure it out. You'd just need subsequent readings after that to give you the skinny. Obviously, every car is going to be different and would need to be reset with each new air filter (to adjust for differences in filters), but yah, I'd think you could do it. Commented Sep 28, 2018 at 20:47
  • Air filters cost is so low it is not worth wondering about it. Commented Sep 29, 2018 at 15:25

1 Answer 1


I suppose it would be doable with an additional sensor to measure the ambient air pressure, assuming the engine has already an MAF and MAP sensor.

Intuition tells me that airflow resistance could be roughly calculated by dividing the pressure difference by the measured airflow (as in electrics, where R=U/I). (Intuition also tells me that real experts will laugh at me). That should result in the following formula: Rfilter = (Poutside-Pinside)/Airflow

Optimizations could include: Averaged measurements, minimizing the disturbances by turning off the EGR and maintaining a steady throttle.

AFAIK no car has such an sensor to measure the ambient air pressure.

To answer your specific questions:

  1. Yes, a modern engine knows how much air flows at the moment. It has a special sensor, the MAF.
  2. AFAIK there is no trouble code for a clogged air filter. The nearest would be some P10X code, indicating unexpected sensor values.
  3. And no, I don't think that is sufficient to measure the filter resistance as the flow depends on the filter resistance, "demand" (engine RPM * displacement / 2) and "supply" (ambient air pressure)
  • Note that I stipulated holding "density altitude" (effectively "ambient air pressure") constant. I was thinking MAF at peak engine power would make the "demand" factor constant.
    – feetwet
    Commented Sep 29, 2018 at 1:53
  • Hmm, my Subaru had an ambient air pressure sensor built into the ECU to compensate for elevation changes.
    – David Sain
    Commented Sep 29, 2018 at 5:26
  • Some cars have sufficient sensors to control the fuel and alter the delivery according to the fuel temperature coming from the tank - as it causes a change in density...
    – Solar Mike
    Commented Sep 29, 2018 at 6:57
  • @Solar Mike I cannot follow you. I understand that temperature changes affect fuel density. But how would that enable to measure the filter resistance?
    – Martin
    Commented Sep 29, 2018 at 7:51
  • 1
    if they can be bothered to have a sensor to measure fuel temperature, then they will have sensors for ambient air pressure and temperature....
    – Solar Mike
    Commented Sep 29, 2018 at 7:58

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