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I've noticed that my air intake temp sensor gives the correct ambient temp reading in the morning when the engine is cold, but that it typically is giving me readings between 7 to 17 degrees Celsius above the ambient temperature. Is that normal and does it have any significant negative effect?

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Most of the time, intake air temperature is not the same as ambient air temperature.

This is because the air absorbs heat from the hot engine bay area before the intake air temperature sensor (IATS) measures its temperature.The heat-soak effect typically worsens on hot days and when the engine is under high loads.

Another factor to consider is that the IATS itself may suffer from heat soak, resulting in higher than expected readings.

Your observation in the morning also correlates with this; with the engine cold and no road heat (car parked overnight), it makes sense that the ambient air and the intake air temperatures match.

7-17 °C seems quite normal... one of my cars is notoriously bad - on really hot days the temperature difference can be as much as 40 °C!

  • So basically the air entering the engine really is that hot due to it basically being preheated by the heat coming off the engine bay? Another thing I noticed is that the IAT slowly drops from 20+ to about 7+ when I start out with with a hot engine. I started out this evening with ambient at 16, ECT at 48 and IAT at 39, and after about 15 minutes of driving I had ECT 79 and IAT at 21. That's a bit weird... – Robert S. Barnes Feb 2 '15 at 18:08
  • @RobertS.Barnes : It's not really that surprising. When you drive, the fresh air coming into the engine bay has a cooling effect, which reduces the amount of heat soak felt by the IATS – Zaid Feb 2 '15 at 18:14
  • The easiest way to check your IAT to ensure it is giving you the correct temperature is to test it. Every IAT has as set of values (range) which corresponds to the temperature. For instance, the Camaro I used to own has a resistance value of 177 ohms @ 100degC. It has 9420 ohms @ 0degC. The chart I am looking at has 15 steps between the two values, but I bet you could translate an exact value for each degree of temperature (whether C of F). I would bet most (if not all) IATs work the same way and have a corresponding chart which would make testing one pretty easy. – Pᴀᴜʟsᴛᴇʀ2 Feb 2 '15 at 23:11

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