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My car's engine comes with a Bosch hot film airflow mass meter. The probe inside the sensor's body features both the actual airflow sensor and what looks like a thermistor. I'm telling you that because, last week, i've cleaned both the actual sensor and the thermistor with quick drying circuit board cleaner - one evaporating rapidly and leaving no residue at all. Let the cleaner dry up and then fitted the MAF back into its place.

I've recently got a live data enabled OBD scanner in my possession and today i hooked it up and read some live data out of curiosity. What struck me most is that, while the ambient temperature was about 59F at the time, the ECU read about 74F as IAT value (IAT is sensed by the aforementioned thermistor located on the MAF's probe). And that was after a short ride, enough to bring the engine into closed loop fuel control mode so that i could also check the O2 sensors (engine coolant temperature was around 165F while i read the live data). I then opened the hood and touched the MAF's housing, and it was effectively warmer than ambient air to the touch, just like the air filter housing.

Is it normal for IAT to be higher than ambient temperature?

  • where was the ambient temperature measured? in the engine bay or somewhere else? – Solar Mike Oct 31 '18 at 9:58
  • @SolarMike Outside. By the ambient temperature sensor located in the radiator grille, the same location of the air intake. – Al_ Oct 31 '18 at 10:02
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I have a gauge that shows IAT and my car shows the ambient, outside temperature in the gauge cluster. IAT is always higher than ambient. IAT values that you get from the ODB-2 port are coming from somewhere near the throttle body or MAF (as mentioned above). This value is what they ECU uses to calculate fuel/timing/etc.

The path of the air to the throttle body runs across the engine, often in rather serpentine ducting. During it's travel, the air picks up a fair amount of heat.

I imagine the reasons for this are several, possibly including dampening intake noise, heating the air for better emissions at certain temperature ranges, smoothing and managing air flow and who knows what else. I'm sure there are more design considerations for the shape, length and route of the intake.

So, yes, it is normal for IAT to be higher than ambient.

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