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Air temperature sensor under heat soak

Understanding AFR

Discussion and questions related to the course Understand AFR

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when the air temp sensor (or MAF) is under heat soak and read the temp to be higher than what it actually is, would it cause the engine to run richer(to cool it down) or leaner (because it thinks the less dense hot air requires less fuel due to the apparently dropped oxygen content)?

If the IAT reading is higher than what the actual inlet charge temperature is then the engine will run lean. This is because the calculated air density is less due to the falsely high IAT reading so the ECU compensates by supplying less fuel.

Is there a way to programme it such that it actually runs richer for the purpose of cooling down the engine if it stays above a particular temperature for a period of time?

Yes sure. All you would do is alter the IAT compensation table to achieve your desired aim. Generally we find a 2-3% change to the fuelling per 10 degree change in IAT will maintain a consistent AFR. If we want to run richer as the IAT increases we just need to make a smaller reduction in this table.


Can you confirm this is related to speed density? My understanding was the MAF reads the mass of the air so temperature has little affect on the reading.


P.S. Celsius or Fahrenheit?

Hi there,

Please correct me if i am wrong, but in my understanding, it's ~2.6%/10oF and ~4.7%/10oC, and I just found out that i have set my tables wrong :)


What you've graphed there is the absolute values for air density and hence your equation is for the change in absolute value. What we're interested in from the ECU's perspective is the percentage change from one point to another. This actually works out to be closer to 3% rather than the 2.5% I suggest when dealing with deg C but I find that this is usually a little optimistic and that across the board, 2.5% works pretty well.

Great. Thanks for the clarification. I agree that the 2.5% seems too be working well, but the charts got me wondering.

Now it's clear